Leaching Results in the Assessement of Slag and...
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DOCTORAL THESIS Luleå University of Technology Department of Chemical Engineering and Geosciences Division of Mineral Processing 2005:44|:402-544|: - -- 05⁄44-- 2005:44 Leaching Results in the Assessment of Slag and Rock Materials as Construction Material Mia Tossavainen
Transcript of Leaching Results in the Assessement of Slag and...
DOCTORA L T H E S I SDOCTORA L T H E S I S
Luleå University of TechnologyDepartment of Chemical Engineering and Geosciences
Division of Mineral Processing
2005:44|: 402-544|: - -- 05⁄44--
2005:44
Leaching Results in the Assessment of Slag and Rock Materials as Construction Material
Mia Tossavainen
Leaching Results in the Assessment of Slag and Rock Materials as Construction Material
Mia Tossavainen
Department of Chemical Engineering and Geosciences Division of Mineral Processing Luleå University of Technology
S-971 87 Luleå
Luleå 2005
Abstract
Extraction of rock material and ore for construction and metal production involves large
quantities of wastes and by-products. Iron- and steelmaking slag has durability qualities and
latent cementitious properties which are positive in construction.
The leaching of so-called hazardous metals is important in the evaluation of secondary
materials for utilization, while knowledge of the leaching behaviour is not required for rock
materials used in road making. Iron- and steelmaking slags have varying content of glass and
puzzolanic minerals that hydrate in contact with water. The influence of the puzzolanic
properties in leaching tests is not considered.
Swedish rock materials have been investigated with the availability test, NT ENVIR
003, in order to form a basis for comparison of soluble trace elements. A long-term leaching
test, NVN 7347, has been used to determine whether diffusion is the dominating leaching
mechanism in a quenched, amorphous BF slag. Four different slag types, modified by rapid
cooling, have been investigated with the compliance test, EN 12457-2.
Overall, the solubility of metals from the rock materials was low but a substantial part of
sulphide-bound elements were released under oxidizing conditions, and compared to
metallurgical slags, the released amounts of some elements were larger. The major phase had an
important influence on the solubility of trace elements. An amorphous slag without puzzolanic
properties has low solubility and prevents leaching of enclosed trace elements. For a
cementitious slag, both dissolution and stabilizing reactions take place during a leaching test
and the matrix may be dissolved to a large degree. Quenching of a slag for increased glass
content implies more phase transformations, and equilibrium will not be reached in a short-
term test. The reactions of the puzzolanic minerals hamper the evaluation of leaching tests
results. For a proper assessment, knowledge of material properties and leaching tests is essential.
The solubility of the trace elements in the original and modified steelmaking slags was
low and a reduction is difficult without better control of the major phases. Rapid cooling
results in a more homogeneous slag with few phases and the control of the properties are
thereby enhanced.
Results from leaching tests of both rock materials and slags should be used in a wider
context in order to give the test results a reasonable importance in the evaluation of the
materials.
Key words: Leaching test, rock material, BF slag, steelmaking slag, assessment, environmental
impact, construction, diffusion, amorphous, mineralogy, reactivity, glass
II
Acknowledgements
This thesis study has been carried out at the Division of Mineral Processing, Department of
Chemical Engineering and Geosciences at Luleå University of Technology (Ltu). The projects
have been part of the research programme of MiMeR (Minerals and Metals Recycling
Research Centre).
The work has been financed largely by MiMeR, with additional funding from the
Swedish Waste Research Council (AFR), the Swedish Foundation for International
Cooperation in Research and Higher Education (STINT) and the County Administrative
Board of Norrbotten, whose support is gratefully acknowledged.
I would like to thank Dr Ann-Marie Fällman, Swedish EPA, for supervising the
licentiate thesis, for helping me plan the continuation and for her encouraging interest in my
work. For subsequent supervision, I wish to thank Professor Eric Forssberg at Ltu and
particularly Dr Lotta Lind, AB Sandvik Materials Technology, for support, discussions and
advice.
I had the opportunity to study at the Indian Institute of Science, Bangalore, India, during
a period, which was most enriching, and I am very grateful to Professor K. A. Nataranjan and
Professor S. Subramaniam and the students at the Department of Metallurgy for supervision
and for making my stay so fulfilling.
Many thanks to my colleagues at Ltu with whom I have worked and discussed the
results. I also wish to thank all the member companies of MiMeR who have provided the
material, but most of all, for the discussions and their interest in the work.
Finally, I am grateful to my family and friends for cheering me on and encouraging me
throughout this work.
Mia Tossavainen
Luleå, Octobre 2005
III
List of publications
This thesis is based on the work contained in the following papers, which are referred to by
roman numerals in the text.
I. The potential leachability from natural road construction materials
Mia Tossavainen and Eric Forssberg
The Science of the Total Environment 239 (1999)31-47, Elsevier Science B. V.
II. Studies of the leaching behaviour of rock material and slag used in road
construction: A mineralogical interpretation
Mia Tossavainen and Eric Forssberg
Steel Research 71 (2000) 442-448, Verlag Stahleisen GmbH
III. The potential leaching of road-making material and potential influences on the
leaching behaviour
Mia Tossavainen
Conference proceedings at Recycling and Waste Treatment in Mineral and Metal
Processing: Technical and Economical Aspects, 16-20 June 2002, Luleå, Sweden
IV. Leaching results of reactive materials
Mia Tossavainen and Lotta Lind
Submitted to Construction and Building Materials, September 2005
V. Characteristics of modified steel slags for use in construction
M. Tossavainen, Q. Yang, F. Engström and N. Menad
Submitted to Waste Management, May 2005
VI. Stability of modified steel slags
Mia Tossavainen, Fredrik Engström, Nourreddine Menad, and Qixing Yang
Conference proceedings at 4th European Slag Conference in Oulu, 20-21 June 2005
IV
Contents
Abstract II
Acknowledgements III
List of publications IV
Contents
1 Introduction 1
1.1 Background 1
1.2 Scope of the work 3
1.3 Disposition 4
2 Secondary materials in construction 4
2.1 General 4
2.2 Regulations of slag – aggregate or waste? 5
3 Materials 6
3.1 Rock materials 6
3.2 Slags 7
3.2.1 Blast furnace slag 7
3.2.2 Steelmaking slags 8
3.2.3 Other slags 10
4 Leaching tests 10
4.1 Availability test 10
4.2 Long-term leaching test 11
4.3 Compliance test 12
5 Results 12
5.1 Leaching of rock materials 12
5.2 Leaching mechanism in air-cooled BF slag and granulated BF slag 14
5.3 Modified steelmaking slags 15
5.3.1 Characterization and solubility 15
5.3.2 Volume stability 16
V
6 Discussion 18
6.1 Factors to consider in the use of leaching test results 18
6.2 Mineralogy and solid condition 19
6.2.1 Cooling condition and mineral formation 19
6.2.2 Liberation of trace minerals 22
6.3 Puzzolanic properties and solubility 24
6.3.1 Phases in the matrix 24
6.3.2 Leaching of puzzolanic materials 24
6.4 Significance of the test results 30
6.4.1 Sampling 31
6.4.2 Potential surface changes 31
6.4.3 Comparison with limit values 31
7 Summarized conclusions 31
8 Reflections and suggestions for further work 33
9 References 35
10 Standards and Directives 38
Appendices: Publications and conference papers I - VI
I The potential leachability from natural road construction materials
II Studies of the leaching behaviour of rock material and slag used in road
construction: A mineralogical interpretation
III The potential leaching of road-making material and potential influences on the
leaching behaviour
IV Leaching results of reactive materials
V Characteristics of modified steel slags for use in construction
VI Stability of modified steel slags
VI
1 Introduction
1.1 Background
Rock material, an important natural resource, is extracted in large quantities for use in the
production of metals and for construction. In Sweden the production of ore and metals has
been around 45 million tonnes/year for the last 30 years (www.sgu.se). The refining of rock
materials results in both products and by-products, and in 2002 the mining industry produced
54 million tonnes of by-products, 95% of which was deposited (Naturvårdsverket, 2004a).
Different kinds of slag constitute the major part of by-products from the iron and steel
industry. At an integrated steel plant the production of one tonne steel results in about half a
tonne of by-products.
The amount of extracted rock materials was 77 million tonnes in 2004 (SGU, 2005) and
57% was used in road construction. The quantity of produced gravel was 20.3 million tonnes.
Despite good resources of high quality rock materials in Sweden, there is a shortage,
particularly of gravel, in densely populated areas.
The Swedish parliament in 1999 adapted environmental quality objectives regarding the
status of the environment (air, soil, water, etc.) and the use of resources. The goal is to achieve
these objectives by the year 2020 (http://miljomal.nu). One of the interim targets is that by
the year 2010 at least 15% of the aggregates used will consist of recycled materials and the
extraction of gravel shall not exceed 12 million tonnes. By 2005, the amount of materials
landfilled shall be reduced by at least 50% of what it was in 1994. The extraction of gravel has
decreased substantially since 1995, when 44.6 million tonnes were produced, the year before a
tax was introduced. The target is expected to be met.
The extraction of pristine materials and the production of wastes must be managed in a
way that is sustainable for urban and rural society; i.e., to ensure the preservation of clean air,
water and soil and protection of the natural environment. The quantities of excavated materials
and solid wastes presented above are indicative of the need for reuse and recycling of extracted
materials, particularly with respect to the large increase in extraction of such resources in
countries that strive towards a higher standard of living. Society must make development of
conditions that encourage successful recycling a high priority. Under such conditions,
environmental, technical and economic aspects are jointly considered.
Regarding landfilled materials, an overall target is that final deposition should be avoided
if possible. Primarily, the material should be reused in the same process and, secondarily,
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utilized in other products or processes. The European directive regarding landfills
(1999/31/EC) and the rules for the acceptance of waste at landfills (2003/33/EC) place a
primary economic pressure on the producers to make slag into products.
Materials that are to be used in construction need to fulfil demands on both mechanical
and environmental properties for the specific use. Regarding the mechanical properties, one
major obstacle for use of slag from steelmaking processes is low volume-stability due to
expanding or disintegrating minerals/phases. Another hindrance for the use of recycled
materials is the content of soluble trace elements, e.g., chromium, molybdenum, barium, that
in high concentration or certain ion bonding may be hazardous to human health and the
environment.
Leaching tests are preformed on materials under controlled conditions and the leaching
parameters and the material properties give rise to different amounts of dissolved elements. The
results of leaching tests have a great bearing on the assessment of slag for utilization in e.g.,
road construction. Leaching is a reaction that takes place in all kinds of road-making materials.
It has an environmental impact and is one aspect among many others associated with the life
cycle of the material that must be evaluated. In this perspective, it is likely that other factors
that might be of great importance are exposed.
Slags from iron- and steelmaking have strength and durability qualities that make them
suitable as materials in construction. Crystalline steel slag has properties similar to e.g., basalt
and its high durability makes it suitable as an aggregate in asphalt concrete. Some slag types also
have a potential for use as a complement to cement due to the presence of a glassy phase that
displays puzzolanic properties (Ionescu et al., 1998, and Shi, 2004). By quenching (water
cooling) the slag from the melt, the amount of glassy phase can be enhanced. Blast furnace slag
(BF slag) has a suitable chemical composition for glass formation and is frequently used as a
cementitious material. Steel making slags, on the other hand, generally does not have enough
important puzzolanic minerals for formation of a glass phase and use as a cement complement.
In 2004, all BF slag produced in Sweden was used in road-making or cement, while only
12% of the steel slag (total 774 kt) was sold as external products (source: personal
communications). These products were mainly used in test roads, i.e., roads that are built for
which there is a scheme for investigation of primarily the environmental impact of the slag.
The leaching of chromium in such a test road has been studied in laboratory and field
tests on electric arc furnace (EAF) steel slag, by Fällman (2000). The Swedish Road
Administration has investigated a smaller test road with low traffic load during four years with
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respect to durability and solubility. The road was constructed with sectors of a crushed
concrete, a blast furnace slag, a fuming slag from a copper smelter and a crushed rock in the
sub-base of the road, Vägverket (2002) and Vägverket (2003). The physical properties of
ferrochrome slag (not included in the steel slag mentioned above) have been extensively tested
with good results and the slag is attractive as a construction material. Environmental concerns
were raised about the leaching behaviour of particularly chromium and a test road with
ferrochromium slag has been investigated by Lind et al., (2001).
A test road is usually sampled during a period of only a few years, which is a shortcoming
in such an investigation as many factors have an influence under field conditions, and it is
difficult to control the factors of interest. Another problem is that the amount of sampled
leachate in field tests is often small. The lack of similar investigations of rock materials
complicates the evaluation of the test roads with slag.
1.2 Scope of the work
A better knowledge of leaching tests and results, as well as of material properties, is very
important in the assessment of slag for use in e.g., road construction. The aim of this work has
been to investigate leaching of primary and secondary road-making materials and study factors
that may have an influence on the release of trace elements such as chromium, molybdenum,
zinc and vanadium that are regarded as pollutants.
In the first part, leaching tests of rock materials were preformed in order to obtain
reference values regarding the release of trace elements to be used for a comparison with slags
evaluated for utilization. The results have been presented in a licentiate thesis (Tossavainen,
2000).
The findings with the rock materials led to further investigation as to whether a granulated,
amorphous slag is less soluble than a crystalline one with a similar chemical composition. The
chosen test material was BF slag as it is a well-known material that has been thoroughly
investigated for construction purposes. Finally, different types of iron- and steelmaking slags
have been studied to determine whether rapid cooling is a suitable method for enhancing the
glass content and amorphous state and thereby reducing the release of trace elements. The
behaviour of the matrix that is often the host of the trace elements has been of major interest.
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1.3 Disposition
Chapter 2 of this thesis describes the situation regarding the regulations and use of slag in
construction in Sweden with respect to environmental impact. The materials and the methods
that are used in this work are described in Chapters 3 and 4. In Chapter 5 the results are
presented and in Chapter 6 the behaviour of the rock materials and the slag types in leaching
tests are discussed. The conclusions regarding the use of leaching results in the assessment of
slag and rock materials are summarized in Chapter 7. Suggestions for further work are
presented in Chapter 8.
2 Secondary materials in construction
2.1 General
The possibilities for realizing the goal of eliminating landfilling depend on many factors and
vary for different materials. If the material can be recycled within the process chain, e.g., as slag
from the basic oxygen furnace is used in the blast furnace, or reused by the producer, as
aggregates from old roads are used in new roads, the conditions for fulfilment of the targets are
optimal. If, on the other hand, the material is introduced as a new product on the market the
conditions are more complicated. In the latter case, a new product has to compete with
materials that are well known and established on the market, and tools for testing and a smooth
and rapid assessment are required. A new product needs to have equal or higher quality, lower
price, shorter deliveries, etc. for acceptance on the market. Apart from regulations there are
also traditions, preconceived ideas and reluctance about the quality of recycled material.
When a slag is assessed for use, the impact on the environment, particularly with respect
to leaching of metals, is considered very important. A similar assessment is not done when a
rock material is considered for use, which implies that the rock is regarded as a non-leaching
material. The petrography is investigated during production (each third year) in order to detect
minerals that are prone to weathering, such as sulphides and mica, and may cause a reduction
of the durability of the material. Metals such as zinc, lead, copper, mercury and nickel, which
are considered road pollutants, are often bound or associated to sulphides. For a reasonable
evaluation of the impact of the release of such metals it is important to have knowledge of the
leaching behaviour of both primary and secondary materials, but few values are available for
rock materials. For that reason, SP (the Swedish National Testing and Research Institute), is at
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present (2005) testing the leaching behaviour of 40 samples of rock material on behalf of the
Swedish Steel Producers Association.
As mentioned above, the extraction of rock materials has a major impact on the
environment. The use of natural materials was regarded as one of the most important
environmental loads in an expert assessment of the life-cycle impacts of by-products and
natural materials used in road construction (Mroueh et al., 2001). For slags and other by-
products, the impact from production is not as significant, as the material is extracted for
another purpose e.g., steelmaking. This essential fact is usually not exposed or discussed.
These very important differences in the conditions regarding testing and assessment of
primary and secondary materials complicate and delay a sustainable use of extracted rock
materials and ore.
2.2 Regulations of slag - aggregate or waste?
Slags that are to be used in road-making have to fulfil the requirements of mechanical and
environmental properties according to standards. The national standards for aggregates and
road materials in the member states of the European Union have been replaced by European
standards in June 2004. The aggregate standards are drawn up within the technical committee
TC 154 of the European Committee for Standardisation (CEN, Comité Européen de
Normalisation).
The methods for testing the mechanical properties of residues according to the new
European standards have been studied by Arm (2003), who states that some of the European
methods are more or less unsuitable for residues and recycled materials and that new methods
need to be developed. Among the materials studied, crushed concrete and air-cooled blast
furnace slag are assessed to give road application properties as good as, or even better than,
those of crushed granite. Togerö (2004) has investigated leaching of substances and metals from
a reference concrete, a concrete of granulated BF slag and a concrete with fly ash. She states
that the concrete type does not change the leaching behaviour significantly in short-term tests
but that much more research of the long-term binding of substances in cement-based materials
is needed. Lind (2002) has shown that BF slag, aside from the fact that it leaches less than rock
materials, reduces the impact on the environment by sorption of metals on the slag.
Standards for the use of BF slag in Swedish road construction will soon be included in
the standards set by the Swedish Road Administration (Vägverket, 2004). Blast furnace slag,
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slags from the basic oxygen furnace, the electric arc furnace, ferrochromium slag as well as a
slag from the fuming of a copper slag are presently being evaluated (2005) for product
certification by SP.
Regarding the chemical properties of aggregates there are standards for chemical analysis,
EN 1744-1 (CEN, 1998) and for preparation of eluates by leaching, EN 1744-3 (CEN,
2002a). There are no standardized European guidelines or limit values linked to the leaching
test.
Slags that are to be landfilled are classified according to the standards for wastes which are
drawn up by the CEN TC 292 and used in the directive regarding landfills, 2002/33/EU.
The standardized tests for leaching behaviour are not the same for wastes as for aggregates.
A slag has to be characterized, no matter whether the material will be utilized or
landfilled. In Sweden, were the major part of steelmaking slags has been landfilled, slags are in
general tested and assessed according to the methods drawn up for the characterization of
wastes, even though the intention is to use the slag as a material.
The key issue in the assessments of slags is the definition of waste within the European
Union. The European Court of Justice has published several judgements dealing with the
question as to whether substances and materials are waste and when waste ceases to be waste.
For the time being, slags are classified as wastes and there is a discussion of the definition of
end-of-waste (Euroslag, 2005). The fact that a slag is defined as a waste complicates the
evaluation, the marketing and the use of slag as a product in e.g., construction.
3 Materials
3.1 Rock materials
The nine rock materials; seven types of crushed rock and two types of crushed gravel used in
this work were selected to represent typical materials of high quality road-making materials
produced in some quantities for use in base or sub-base layer. Mafic rock materials, gabbro and
gabbro-diorite, as well as felsic rocks, e.g., granite and gneiss, with varying content of major
elements were sampled. The content of trace elements was in general very low, as shown in
Table 1.
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Table 1. The content of major components and minor elements in the nine rock materials.
Component total content element total conten% mg/kg
Al2O3 13.2 - 16.1 Arsenic 0.30 - 104CaO 1.35 - 12.3 Barium 368 - 857Fe2O3 2.43 - 12.1 Cadmium 0.06 - 0.28K2O 0.28 - 4.22 Chromium 44 - 331MgO 0.58 - 7.80 Copper 1.4 - 62MnO2 0.04 - 0.18 Mercury nd - 0.15
t
Na2O 2.42 - 4.00 Nickel 2.8 - 38.8P2O3 0.08 - 0.80 Lead 3.3 - 10.7SiO2 46.4 - 74.0 Strontium 72.1 - 760TiO2 2.05 - 0.25 Sulphur nd - 2000
Vanadium 17 - 282Zinc 38.5 - 114
nd = not detected
The crushed rocks and gravels were sampled in quantities of 60 – 80 kg of each material from
quarries or gravel pits during production or from stocks. The sampled materials were analyzed
with respect to mineral composition, total concentration of elements, buffering capacity and
natural pH. More data about the rock materials are found in Papers I to III.
3.2 Slags
3.2.1. Blast furnace slag
For the long-term leaching test described in Paper IV, samples of air-cooled BF slag and
granulated BF slag with a similar chemical composition were used. The major difference
between the two materials was the content of amorphous material; 96% in the granulated slag
and 12% in the air-cooled.
The air-cooled BF slag is a mixture of rapidly cooled glassy material at the surfaces and
crystallised material in the inner parts. Major elements in BF slag are silicon, aluminium,
calcium and magnesium, and the air-cooled slag consists largely of ternary compounds of theses
elements, of which the most common is melilite, a series of solid solutions from akermanite
(2CaO·MgO·2SiO2) to gehlinite (2CaO·Al2O3·SiO2). By quenching (water-granulation) the
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puzzolanic reactivity of these minerals is enhanced and the material is used e.g., as a
complement to cement. The chemical compositions of the two slag types were as expected
almost identical (see Paper IV).
The air-cooled slag was successively crushed and sieved so as to obtain a sample closely
approximating the particle size distribution of the amorphous slag.
The slag samples were analyzed for particle properties such as specific surface area and
compact density.
3.2.2. Steelmaking slags
Four different types of steelmaking slags were selected for the investigations regarding
modifications by rapid cooling. The slags, described in Papers V and VI, were sampled at
steelmaking plants in Sweden. The aim was to investigate slags with significant differences in
the chemical composition. A disintegrated ladle slag, a basic oxygen furnace slag (BOF slag)
and two types of electric arc furnace slags (EAF slag) were selected as test materials. The
chemical compositions of the slag types are shown in Table 2.
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Table 2. Chemical composition and basicity, Mb1, of the four types of steelmaking slags.
Component Ladle slag BOF-slag EAF-slag 1 EAF-slag 2
Fe2O3 % 1.10 10.9 0.98 20.3FeO % 0.50 10.7 3.25 5.60Fe met. % 0.44 2.3 0.06 0.63Al2O3 % 22.9 1.92 3.72 6.67CaO % 42.5 45.0 45.5 38.8MgO % 12.6 9.57 5.22 3.94MnO % 0.23 3.14 2.01 4.99SiO2 % 14.2 12.1 32.2 14.1Cr mg/kg 2700 480 32700 26800Mo mg/kg 280 50 500 70Zn mg/kg 370 50 130 260Ni mg/kg 70 25 3180 90Cu mg/kg 20 8 140 160K mg/kg 80 220 590 <20Na mg/kg <20 <10 150 <20P mg/kg <50 2270 <50 2000Ti mg/kg 840 8270 7910 2400V mg/kg 280 14800 310 1700
Mb 1.5 3.9 1.4 2.1
The materials were modified with the aim of reducing the solubility and enhancing the
volume stability. The slags were modified in two different ways by varying the cooling
conditions, described in Paper V:
re-melted in an induction furnace and water granulated (rapid cooling)
re-melted, as above, and left to cool in the crucible (semi-rapid cooling)
The ladle slag was only modified by rapid cooling. The slag samples were analyzed for
chemical composition and particle properties. The composition of the phases was investigated
using X-ray diffraction, optical microscopy and scanning electron microscopy (SEM). The
volume stability was determined by use of the steam test according to EN 1744-1, (CEN,
1998).
1 Mb =(CaO+MgO)/(SiO2 + Al2O3)
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3.2.3. Other slags
The slags that were used for a comparison with the rock materials in Papers I to III were air-
cooled BF slag from the two integrated steelmaking plants in Sweden and a slag fuming slag
from a copper smelter. These slags have been investigated in other studies by Fällman (1993),
Rogbeck and Elander (1995) and Fällman and Carling (1998).
The content of trace elements in the BF slag is generally low, except for sulphur and
vanadium, which in the slags used in this comparison, was in the range 0.9%-1.4% and
0.035%-0.053%, respectively.
The slag fuming slag (Fayalite slag) was an amorphous slag with a chemical composition
almost identical with the mineral fayalite (Fe2SiO4). The content of trace elements, such as
zinc, chromium and copper, was high; 1.32%, 0.16% and 0.48%, respectively.
4 Leaching tests
The leaching tests used in this work are standardized methods and one pre-standard.
4.1 Availability test
The rock materials were investigated with the availability test, NT ENVIR 003 (Nordtest,
1995a). This is a method for distinguishing between soluble and non-soluble parts of the
material under certain conditions in a geological time-frame. Optimal leaching conditions
regarding diffusion, buffer capacity and the chemical concentration are controlled. The batch
test was performed in two steps at L/S (liquid/solid) ratio 100: three hours at neutral pH and
18 hours at pH 4 (controlling the pH using 0.5M nitric acid). The two leachates were
combined and analyzed for dissolved elements and redox conditions. Controlled oxidized
conditions were achieved by addition of H2O2 (Nordtest, 1999).
The investigation of the rock materials is presented in Papers I to III and investigation of
the granulated and the air-cooled blast furnace slags is presented in Paper IV.
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4.2 Long - term leaching test
The air-cooled and the granulated BF slags were investigated with a long-term test based on
the pre-standard NVN 7347 (1994) for compacted granular materials in order to investigate if
the leaching in each material was controlled by dissolution or diffusion.
The material was placed in a wide-necked flask, covered with a layer of glass beads and
dionized water. The flask was sealed to prevent prolonged contact with air. The free volume
of water was exchanged, without disturbance of the material, after 0.25, 1, 2, 4, 8, 16, 32 and
58 days. The leachates were analyzed for pH, redox potential and the dissolved elements of
interest. The liquid/solid ratio in the test in this work was lower than that specified in NVN
7347 (1994).
The inert elements2 (van der Sloot et al., 1997) potassium and sodium were used for the
calculation of leaching mechanism. By plotting the relation cumulative release (mg/m2) against
the time (logarithmic scale), the slope of the line was used to identify the dominating
mechanism. A slope of 0.5 ± 0.1 indicates diffusion controlled release and a slope of >0.8
dissolution from the surface. Surface wash off is indicated by an initial slope of <0.4.
If diffusion is the dominating leaching mechanism, the quantity of a component leached
at time t is calculated according to an equation based on Fick’s second law of diffusion (Crank
1975):
m
tDUdAU
e
t
2/1
02
where:
Ut is the released quantity in mg/kg, U0 the potential leachable amount of the component in
mg/kg, De the effective diffusion coefficient (m2/s), A the surface area of the product in m2, m
the weight of the product in kg, d the bulk density of the product in kg/m3 and t the time in
seconds (s).
The investigation is presented in Paper IV.
2 non-reactive towards the matrix
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4.3 Compliance test
The four samples of steelmaking slags, as well as the BF slag in air-cooled and granulated state,
were tested according to the short-term compliance tests in two steps, EN 12457-3 (CEN,
2002b) or one step, EN 12457-2 (CEN, 2002c). The standardized test is used to determine
whether a material complies with a previous basic characterization of the leaching behaviour
according to the European directive regarding landfills, 2000/33/EC. The amount that can be
dissolved, assuming the system reaches equilibrium or semi-equilibrium, is determined. The
material was leached in two steps at L/S ratio 2 and 8 or in one step at L/S 10. These
investigations are described in Papers IV to VI.
All the leaching tests were done in duplicate and the results are presented as a mean
value, except for the long-term test. The leachates were analyzed by the accredited laboratory
Analytica (www.analytica.se) using ICP-AES and ICP-MS (Inductively Coupled Plasma
Emission Spectroscopy and Mass Spectroscopy). The water used for the granulation was not
analyzed.
5. Results
5.1 Leaching of rock materials
The major minerals in the mafic rock materials were the rock-forming silicates such as
plagioclase, amphibole, biotite mica and pyroxene, and in the felsic rocks and gravels quartz,
alkali feldspar and biotite mica were predominant. The rock materials had a buffering capacity
owing to the silicates, which dissolve very slowly.
The release of the major elements was low, as the silicates generally have low disposition to
weathering and low dissolution rate (Aastrup et al., 1995, Strömberg and Banwart, 1999).
Calcium and potassium were the most soluble elements and silicon was released in very low
amounts in spite of the high total content (see Table 3). No pure calcium mineral was
identified and calcium was most likely bound in the silicates.
- 12 -
Table 3. Release of major elements (mg/kg) from the nine rock materials. Results from the
ordinary availability test (av.) and oxidized availability test (ox. av.).
Release of major elements (mg/kg)
av. ox.av.
Aluminium 74 - 173 124 - 198Calcium 570 - 2230 800 - 2000Iron 163 - 894 3.6 - 7.5Potassium 381 - 1828 635 - 2500Manganese 18.7 - 71.4 12.9 - 37.1Magnesium 71.8 - 429 113 - 160Sodium 42.5 - 158 71 - 210Silicon 172 - 503 173 - 266
Overall, the abundance of trace minerals and the total concentration of sulphur and elements
often considered as typical highway pollutants such as zinc, copper, nickel, chromium,
cadmium and mercury in the rock materials was low. The opaque minerals that were identified
with optical microscopy in the nine materials are listed in Table 4.
Table 4. Identified minerals in the nine rock materials.
Oxides Formula Sulphides Formula Others FormulaMagnetite Fe 3 O 4 Pyrrhotite Fe 1-x S Fe-hydroxidesIlmenite FeTiO 3 Pyrite FeS 2 Transformed FeSHematite Fe 2 O 3 Chalcopyrithe CuFeS 2 Graphite C
Sphalerite ZnSPentlandite (FeNi) 9 S 8
The most frequently observed opaque minerals were the iron-containing oxides. Several
sulphides were identified as well.
The amount of soluble trace elements was generally low. The trace elements present or
associated with sulphides, such as zinc, copper, nickel and cadmium, showed the highest
releases of the analyzed elements; particularly under oxidized conditions (Table 5).
- 13 -
Table 5. Release of trace elements (mg/kg) from the nine rock materials. Results from the
ordinary availability test (av.) and oxidized availability test (ox. av.).
Release of trace elements (mg/kg)
av. ox. av.
Arsenic nd - 3.29 nd - 0.07Barium 4.1 - 52.5 10.7 - 33.9Cadmium nd - 0.04 0.03 - 0.09Chromium 0.10 - 0.33 0.25 - 0.29Copper 0.16 - 0.76 4.03 - 9.11Mercury nd - 0.18 nd - 0.08Nickel 0.4 - 2.0 0.5 - 3.4Lead 0.03 - 1.82 0.26 - 0.41Strontium 0.9 - 8.3 1.8 - 4.7Sulphur nd - 92.9 58.2 - 248Vanadium nd nd - 2.9Zinc 2.4 - 8.0 7.6 - 17.8nd = not detected
The release of chromium and vanadium was very low, in spite of high concentrations.
The complete results of the rock materials are presented in Papers I, II and III.
5.2. Leaching mechanism in air-cooled BF slag and granulated BF slag
This investigation was based on the hypothesis that an amorphous slag has lower solubility than
a crystallized slag. A material in which the leaching is controlled by diffusion is likely to be
more stable than if dissolution is the dominating leaching mechanism. The measured
cumulative release, indicated that this was the case for the BF slag, as all the major elements in
the long-term test, except magnesium and silicon, were more soluble from the air-cooled BF
slag than from the granulated BF slag, Paper IV.
However, the releases of the inert elements (potassium and sodium) developed in
different directions for the two types of slag during the test period. The solubility of the air-
cooled slag decreased towards the end of the test and in the granulated slag it increased (Figure
1).
- 14 -
The values of the measured and the calculated release of sodium or potassium were not equal
for either of the materials, which should be the case if leaching was controlled by diffusion
(NVN, 1994).
Figure 1. Release plot for sodium (Na) from air-cooled and granulated BF slag.
10
100
1000
10000
1 10 100 1000 10000Time (hour)
Cum
ulat
ive
rele
ase
(mg/
m2)
Measured Na GranulatedCalculated Na GranulatedMeasured Na Air-cooledCalculated Na Air-cooled
The slope of the curve, >0.6, indicated that the dominating leaching mechanism was
dissolution for both materials in this long term test. The leaching time was not sufficient to
determine if and when diffusion occurs.
These findings were supported by a comparison with results of the compliance test,
which is designed to determine what can be dissolved, assuming the system reaches
equilibrium or semi-equilibrium during the test. The releases of potassium and sodium were in
the same range in both tests. Both the air-cooled and the granulated BF slag were mainly
leached by dissolution.
- 15 -
5.3 Modified steelmaking slags
5.3.1. Characterization and solubility
The investigation of the steel slags was performed as a screening test to determine if rapid
cooling would result in a matrix with high stability and qualities to prevent the trace elements
from leaching. The trace elements of special interest in these slags were chromium (Cr),
molybdenum (Mo) and vanadium (V). The results regarding the phase composition (X-ray
diffraction, SEM and optical microscopy) and leaching with the compliance test are presented
and discussed in Papers V and VI.
The X-ray diffraction spectra showed that all the slag samples, except the granulated ladle
slag, had complex phase compositions. All four slag types were basic and the ladle slag, with a
high content of aluminium, was the only one that became almost completely amorphous by
the rapid cooling. Few phases could be identified with high confidence in the slag samples due
to the presence of glassy material and solid solutions with complex compositions resulting in
broad and overlapping peaks. The best-fit phases are presented in Paper V. Some phases were
not likely. The major phase in each material is listed in Table 6.
Table 6. The major crystal phase identified with X-ray diffraction in the original and modified
slag samples.
Ladle slag BOF-slag EAF-slag 1 EAF-slag 2Slag sample
Original Ca12 Al14O33 (Mayenite) Ca2SiO4 (Larnite) Ca3Mg(SiO4)2(Merwinite) Ca2SiO4
Semi-rapid cooling Ca2SiO4 Ca3Mg(SiO4)2 Ca2SiO4
Rapid cooling MgO (Periclase) Ca3SiO5 Ca3Mg(SiO4)2 Ca2SiO4
The solubility of both major and minor elements was very low in the ten investigated slag
samples (see Table 7).
- 16 -
Table 7. Leaching results of ladle slag, BOF slag, EAF slag 1 and EAF slag 2 with the
compliance test.
Ca Mg Fe Si Al Cr Mo VSlag sample
mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kgLadle slagrapid cooling 1140 nd 0.37 15.6 299 0.08 0.008 0.20
BOF slagoriginal 7095 nd 0.14 4.9 2.63 0.03 0.21 0.26semi rapid cooling 4404 nd 0.07 14.9 19.2 nd nd 0.70rapid cooling 2070 nd nd 62.5 1.6 nd 0.07 7.71
EAF slag 1original 1145 nd 0.04 37.4 139 0.73 3.90 0.29semi rapid cooling 647 2.2 nd 141 5.12 0.82 0.11 2.76rapid cooling 457 4.3 nd 132 2.73 0.93 0.07 0.31
EAF slag 2original 1545 nd 0.17 3.5 636 5.81 0.76 0.26semi rapid cooling 2505 nd 0.07 1.1 426 0.01 0.02 0.02rapid cooling 684 1.2 0.05 50.4 45.6 3.83 0.36 2.51
nd = not detected
The table shows that a more rapid cooling resulted in a changed solubility of major and minor
elements, but the change in phase composition and glass formation was not sufficient to
prevent the trace elements from leaching. The leachate of the original ladle slag was not possible
to filtrate, due to some phase formation in contact with the water, which is why no analysis
was performed.
5.3.2. Volume stability
The volume stability was analyzed on the original slag samples, except the ladle slag, which was
not possible to test in the original state due to the fine-grained material. The results are shown
in Table 8.
- 17 -
Table 8. Volume expansion (%) of the original BOF slag, EAF slag 1, EAF slag 2 and the
granulated ladle slag.
Slag sample Ladle slag BOF slag EAF slag 1 EAF slag 2% % % %
Original n.a. 8.5 0.7 0.8Granulated 0 n.a. n.a. n.a.n.a = not analyzed
The ladle slag was the only one that was tested in the granulated state, as the other slags were
not modified in sufficient amounts for the stability test. The volume stability of the modified
slag samples was enhanced, as discussed in Paper V and Paper VI.
6. Discussion
6.1 Factors to consider in the use of leaching test results
22 different materials have been investigated in this study. Nine materials were crushed rock
and gravel and 13 materials were the original or a modified form of five slag types.
Comparisons were done with two slags from other studies.
Rock materials and slag are from the same source, the bedrock; but the chemical
compositions of major and trace elements differ. The mineralogy/phase composition, the
content of the minor elements as well as the distribution can vary substantially in both material
types, which results in different leaching behaviours.
The characterization of these so-called primary and secondary materials has shown that
there are some important properties of the materials that need to be considered for an
appropriate use of the tests and the results. The amounts of dissolved trace elements are
measured and used in decision-making, but the influence of the major phase as well as the
significance of the values must be recognized.
The factors that are assessed as having a major influence on the leaching results are:
Mineralogy
Solid condition
Reactivity
- 18 -
These are closely connected, and the properties of the major elements have an important
function.
6.2 Mineralogy and solid condition
6.2.1 Cooling condition and mineral formation
The formation of minerals/phases in both rock materials and slag, and whether they will be in
a crystalline or an amorphous state, depends on the conditions during cooling and the chemical
composition. The mineralogy and the solid state determine material properties such as the
durability, the solubility and the reactivity. Generally, an amorphous material is less soluble
than a crystallized phase with a similar chemical composition, due to the very slow kinetics for
a re-crystallization and reactions with the surrounding liquid media (White, 1986).
However, rapid cooling (quenching) and enhancing the amount of amorphous material
is also a process that increases the reactivity of a material with puzzolanic properties, such as
iron- and steelmaking slag (Daugherty et al., 1983, Murphy et al., 1997, and Shi, 2004). Rapid
cooling is also a means of reducing the content of unstable silicates, as well as free CaO and
periclase, MgO, that expand at phase transformation (Juckes, 2003).
The materials studied can be divided in three groups regarding the solid conditions:
Solid state Material
1. Crystal crushed rock and gravel
2. Amorphous granulated BF slag, granulated ladle slag,
fayalite slag
3. Mixture air-cooled BF-slag, original and
(crystal - amorphous, solid solutions) modified steel slags (10 materials)
From a liquid metal mixture different minerals/phases are crystallized at specific temperatures
and the cooling rate has an influence on how well the crystals will be formed.
In a crystal material the atoms are arranged with both short-range and long-range order.
At rapid cooling the liquid matrix, with the atoms in short-range order, is frozen in an
amorphous state, and no definitive lattice pattern is developed; i.e., the long-range order is
absent.
- 19 -
Glasses are the most typical amorphous substances and compact and homogeneous
materials are characteristic. Substances that can form glass consist of long-chained, or large,
irregular molecules, e.g., silicates. Such melts have high viscosity and have difficulty organizing
into crystals at cooling, particularly at rapid cooling. The granulated slags in this investigation
that were almost completely amorphous had high content of silicon or/and aluminium, which
are glass forming elements.
A rock-like material is the result when the melted material has a viscosity low enough for
developed mineral formation during the cooling. Small molecules are more easily arranged into
developed lattice patterns due to a higher mobility, and a long cooling time is favourable. The
rock materials in this study consisted largely of silicates, but the cooling time has been very long
(primary rock formation and metamorphous) and well developed major and trace minerals
have been formed.
The phase composition of the four steelmaking slags was more complex. The original air-
cooled materials sampled at the plants included both glass, silicates and undeveloped silicates as
well as oxides in solid solutions located in between the silicate matter.
The influence of the cooling time is shown in the number of phases that were identified
with optical microscopy in the rock materials and differentiated in the slag samples (see Table
9). The numbers for the slags shall be used as an overview, as the transparent phases
differentiated by the colour in the slag samples were fragments which consisted of glass,
silicates, glassy material and opaque phases of which the major part contained iron. Rock-
forming and trace minerals that probably were sampling errors (at the plant) were observed, as
well.
Table 9. The number of transparent and opaque phases identified or differentiated with optical
microscopy in the nine rock materials and the four different slag types in original, semi-rapidly
(S-R) and rapidly (R) cooled state. The numbers within brackets include the traces of rock
minerals.
Crushed rock Gravel Ladle slag BOF-slag EAF-slag 1 EAF-slag 2transp. opaque transp. opaque transp. opaque transp. opaque transp. opaque transp. opaque
Slag sample
Original 7-10 5-9 9 7 - 9 3 (10) 6 2 6 4 (10) 6 4 (6) 5 (13)S-R cool. 2 4 1 4 2 5R cool. 1 (3) 1 2 3 2 2 1 2
- 20 -
In the rock materials, several transparent and opaque phases were identified. The transparent
phases were mainly different rock-forming silicates, such as feldspar, pyroxene and amphibole.
The silicates in the rock materials most likely have a higher degree of perfection than the
silicates formed in the slags. Several ore and trace minerals were identified (see Table 4). The
composition of minerals in the gravels varied more compared to that of the rock materials, as
the origin of the gravels varies.
Due to the relatively short cooling time (compared to the rock materials) for the original
steelmaking slags, few developed minerals were formed. The matrix included a variety of crystal
and glassy silicate phases. Iron-rich solid solutions were a substantial part of the material,
particularly in the BOF slag and the EAF slag 2. Some trace minerals were differentiated and
spinell phases were observed both in the silicate matrix and in the solid solution.
The cooling time was shorter and fewer phases were formed in the semi-rapidly cooled
samples compared to the original samples. The content of glass and glassy matrix was thus
higher in the semi-rapidly and the rapidly cooled materials and the solid solutions also had a
different appearance.
The low content of glass-forming elements limited the formation of glass in the BOF
slag and the two types of EAF slag. The changes in glass- and phase formation of the slags were
observed with SEM, as well. The rapidly cooled materials were more homogeneous and had a
more uniform appearance.
The differences in phase composition between original, semi-rapidly and rapidly cooled
slag samples showed how susceptible the slag is to the cooling rate, and it is possible to change
the composition substantially by a limited change in the cooling conditions. A rapid cooling
reduces the number of phases and an acid slag is more responsive to a reduction of the time,
due to a higher content of the glass-forming elements silicon and/or aluminium. The silicates
are the phases that are solidified first in steelmaking slags, followed in order by the solid
solutions containing ferrite phases and RO phase (FeO, MnO, MgO and CaO), (Monaco and
Lu, 1996). It is therefore more difficult to control the phase formation by cooling of a slag
with high content of solid solutions, such as EAF slag 2, than slag with more glass-forming
elements.
The BF slag and the fayalite slag were not investigated in a similar way regarding the
composition of minerals/phases, but the almost completely amorphous state of the granulated
slags was identified with other methods (Paper II and Paper IV).
- 21 -
6.2.2 Liberation of trace minerals
The release of a trace element is possible if the trace mineral is in contact with the leachate.
The precondition is either that:
a) the mineral is liberated from other minerals or from the major phase by crushing/grinding
or
b) the major phase enclosing the mineral is dissolved.
For the leaching tests used in this investigation, the materials were crushed to <4mm, or
crushed and ground to 95% <125 m. The grinding results in different degrees of liberation of
trace minerals depending on the solid state of the material.
The crystal major phase of the rock materials breaks along the mineral grain boundaries
and thus promotes liberation of the trace minerals at grinding, while an amorphous material,
such as the fayalite slag, breaks randomly. In the rock materials the ore- and trace minerals
shown in Table 4 were liberated. The trace elements in the amorphous slag samples i.e., fayalite,
ladle slag and granulated BF-slag, were on the other hand largely enclosed in the amorphous
major phase.
The grinding of the original and modified steelmaking slags resulted in a mainly random
fragmentation of the glassy matrix (see Paper V). The degree of liberation of trace minerals at
grinding varied due to the mixture of solid solution, rock-like and glassy material. The air-
cooled BF slag was crushed along both mineral grain boundaries and randomly, as the slag
consists of both rock-like and amorphous material.
In Table 10 the solubility of rock materials, fayalite slag and air-cooled BF slag show the
influence of the properties of the matrix and the grinding. The test used is the availability test,
in both the ordinary and the oxidized form.
- 22 -
Table 10. Solubility (%) with the Availability test (L/S 100) of the major elements and some
trace elements in rock materials, fayalite slag and air-cooled BF slag.
Rock materials BF slag Fayalite
av. % ox.av. % av.% ox.av.% av.% ox.av.%
Aluminium 0.10 - 0.20 0.11 - 0.24 12.3 10.5 0.19 0.10Calcium 2.08 - 23.2 2.91 - 6.52 34.1 30.0 0.94 0.43Iron 0.25 - 2.42 nd - 3.45 20.0 nd* 0.41 0.00Magnesium 0.40 - 2.07 0.52 - 3.77 27.3 24.2 0.49 ndSilicon 0.05 - 0.19 0.06 - 0.08 20.9 19.2 0.21 0.03
Cadmium nd - 33.2 33 - 76 4.17 68.8 nd ndChromium nd - 0.61 0.16 - 0.22 0.86 0.36 0.08 0.02Copper 0.26 - 6.56 10.7 - 23.3 nd 10.6 2.36 6.36Nickel 2.0 - 24.0 3.1 - 11.7 nd nd 4.58 3.33Lead 0.44 - 22.0 3.00 - 5.43 6.92 8.83 0.75 1.86Sulphur nd - 9.3 nd - 24.8 21.4 35.7 nd 0.91Vanadium nd nd - 3.5 6.5 18.0Zinc 3.1 - 17.6 9.6 - 30.3 48.8 91.8 0.61 0.18* not detected
The solubility of the major elements is very low in the rock materials as well as in the amorphous
fayalite slag, and the liberated trace minerals in the rock materials were more or less soluble in
the leachate. The release of sulphide-bound elements, such as zinc, nickel and copper, was
substantial, particularly under oxidizing conditions. Vanadium and chromium, in contrast,
present in relatively high total content in the materials, were not soluble as they were
substituted ions in the lattice of the oxides, such as magnetite and ilmenite (Deer et al., 1962).
The low solubility of the amorphous fayalite slag implied that a high content of trace
elements such as zinc, copper and chromium were immobilized in a glass phase.
The soluble matrix of the air-cooled BF-slag, on the other hand, allowed release of
minor and trace elements. Besides the properties of the matrix, the trace mineral and the total
concentration had an influence on the dissolved amount of the metal element.
The fayalite slag contained 13200 mg/kg zinc but only 80 mg/kg was soluble, while 18
of 59 mg/kg of the zinc in gabbro-diorite was dissolved due to much higher solubility, 30%.
More than 90% of the zinc was released from the BF slag, but the amount was negligible.
- 23 -
6.3 Puzzolanic properties and solubility
6.3.1 Phases in the matrix
The comparison of rock materials and the slags in Table 10 shows that, in the availability test,
the matrix of air-cooled BF slag was largely dissolved. The air-cooled BF slag contained
amorphous material (12%) and the major phases have latent puzzolanic properties, which
implies that the slag to some degree reacts with water and eventually will stabilize. The
availability test is developed to optimize leaching and the puzzolanic reactions of the air-cooled
BF slag had little influence in this test.
The 22 materials studied, contained minerals with varying degrees of reactivity in
aqueous solutions (and leaching tests), due to e.g., puzzolanic properties, glass, and expansive
phases. The most influential reactivity was most likely the puzzolanic properties but expansion
due to phase transformation or hydration cannot be ruled out.
The major elements in steelmaking slag: silicon (Si), magnesium (Mg), calcium (Ca),
aluminium (Al) and iron (Fe) are generally present in several minerals of which some have
puzzolanic properties, such as larnite ( -C2S), alite (C3S), dicalcium ferrite (C2F) and
brownmillerite (C4AF). The volume stability of the slag may be affected by phase
transformations during cooling, such as when C2S is transformed to C2S and increases 10-
12% in volume. Hydration reactions of free CaO or periclase, MgO, also result in expansion.
Several reactive phases were present in the original and modified samples of the ladle
slag, BOF slag, EAF slag 1 and EAF slag 2 (Paper V). A quantification of the phases or analysis
of the reactivity has not been performed.
The puzzolanic properties in the air-cooled BF slag were enhanced substantially by
quenching (water granulation), while the amorphous fayalite slag was resistant in aqueous
solutions and, according to Borell (2005), can be regarded as an inert construction material,
with respect to leaching.
Regarding puzzolanic properties, the rock materials and the faylite slag had none or very
low reactivity compared to that of the iron- and steelmaking slags, and the granulated BF slag
was probably the most reactive. The original ladle slag had hydraulic properties, which made it
impossible to test for leaching properties.
6.3.2 Leaching of puzzolanic materials
The influence of both the solid state (amorphous – crystalline) and the puzzolanic phases was
shown in the results with the compliance test. In Table 11, the solubility of the steel slag in
- 24 -
- 25 -
original and modified conditions, the fayalite slag, the BF slag in air-cooled and granulated state
and one rock material, gabbro, is shown. The release is presented as % of the total content
(Paper V).
- 26
-
Tab
le 1
1. S
olub
ility
with
the
com
plia
nce
test
of t
he s
teel
mak
ing
slags
, the
BF
slag
in a
ir-c
oole
d an
d gr
anul
ated
form
, the
faya
lite
slag
and
Slag
sam
ple
Ca
Mg
FeSi
Al
Na
KS
ZnC
rM
oV
%%
%%
%%
%%
%%
%%
Ladl
e sl
agra
pid
cool
ing
0.35
nd0.
020.
030.
20nd
nd0.
91nd
0.00
80.
040.
14
BOF
slag
orig
inal
2.20
nd0.
000.
010.
03nd
4.06
7.52
0.09
0.01
0.54
0.00
sem
i rap
id c
oolin
g1.
310.
00nd
0.03
0.17
ndnd
0.99
ndnd
0.19
0.01
rapi
d co
olin
g0.
63nd
nd0.
120.
02nd
nd1.
40nd
nd0.
170.
05
EAF
slag
1or
igin
al0.
35nd
0.00
0.02
0.70
4.44
1.43
3.41
0.01
0.00
20.
780.
09se
mi r
apid
coo
ling
0.21
0.01
nd0.
100.
020.
55nd
0.77
nd0.
002
0.02
0.85
rapi
d co
olin
g0.
150.
01nd
0.09
0.02
ndnd
0.39
nd0.
002
0.02
0.09
EAF
slag
2or
igin
al0.
56nd
0.00
0.01
1.80
ndnd
3.60
0.01
0.02
21.
140.
02se
mi r
apid
coo
ling
0.87
nd0.
000.
001.
10nd
14.6
0.31
0.18
0.00
00.
030.
00ra
pid
cool
ing
0.24
0.00
0.00
0.08
0.13
ndnd
1.56
nd0.
012
0.61
0.14
Faya
lite
slag
rapi
d co
olin
g0.
080.
020.
000.
000.
000.
270.
110.
080.
040.
001
0.01
na
Rock
mat
eria
lG
abbr
o B
0.07
0.02
0.00
na0.
010.
120.
901.
120.
180.
003
0.03
na
BF sl
ag
air-
cool
ed0.
820.
010.
010.
030.
033.
125.
720.
27nd
ndna
0.05
gran
ulat
ed0.
130.
030.
010.
080.
001.
460.
710.
02nd
ndna
0.08
gabb
ro B
.
Overall, the leachability of the major elements was very low and significantly lower in the
amorphous fayalite slag and the rock material compared to the iron- and steelmaking slags, which
contain reactive phases. Table 11 shows a reduction of the release of calcium and aluminium in
the rapidly cooled materials, but also a similar increase in the release of silicon as of both silicon
and magnesium for the quenched BF slag in the three leaching tests discussed in Paper IV. The
enhanced solubility was explained as an enhancement of the reactivity and greater extent of
phase transformations in an amorphous material with puzzolanic properties. The results in
Table 11 support that explanation.
The BOF slag was the only slag type that expanded substantially in the original state,
8.5%. The release of calcium was higher in the original BOF slag compared to the other slags
and most likely it was an effect of the hydration and carbonation of free CaO and a release of
Ca and its hydrates in contact with the leachant. The expansion was reduced in the granulated
materials (see Papers V and VI).
According to the conductivity measurements of the leachants, the solubility decreased
when the cooling was more rapid, which implies that on the whole, the granulated material
was the most insoluble. However, the granulated materials were washed during cooling and
the presented results do not include the surface wash off.
The release of the trace elements was very low in all the materials and there is no general
correlation to the solubility of the major elements. The varying release was due to the complex
mineralogy. The semi-rapidly cooled EAF slag 2 showed a completely different result
compared to the other slags, and there is no obvious explanation for that, which is why the
results need to be treated with caution. The properties of EAF slag 2 are, as mentioned above,
difficult to control by cooling, due the high content of solid solutions. Overall, based on these
results, speculation about the influences of the cooling conditions on the behaviour of the trace
elements in the steelmaking slags is not advisable.
The content of sulphur is quite high in BF slag, but as shown in Table 11, the sulphur in
the rock material was more soluble than in the BF slags and the percentage of released zinc
from the gabbro was higher compared to all the original slag samples. A liberated trace mineral
in the rock materials is not influenced by the behaviour of the major phase, but the leaching of
trace elements in the slags is, on the other hand, largely dependent on the behaviour of the
major phases.
The big difference between the three material types exposed in Table 10 is less obvious
in Table 11. Both test types were percolation tests with relatively short testing time ( 24h) but
varied a lot regarded e.g., L/S ratio (100 and 10), particle size and Ph condition. This
- 27 -
elucidates the influence of leaching test factors and that results from only one test type may
cause misinterpretation and are insufficient, particularly for knowledge about the materials with
cementitious properties and long-term behaviour.
The complicated behaviour of puzzolanic materials in leaching tests was obvious in the
investigation of the dominating leaching mechanism in amorphous and crystal BF slag. The
results of the long-term test, Figure 1, showed that dissolution was the controlling leaching
mechanism in both material types. The granulated BF slag was washed at quenching, and the
release was small from the beginning but increased substantially towards the end of the test,
while it was reduced for the air-cooled BF slag. The testing time was not sufficient to
determine if either of the materials would stabilize and diffusion would eventually be the
dominating release mechanism.
A quenched slag with high reactivity leached according to the availability test, with
optimal leaching conditions, has ample conditions to undergo several phase transformations
resulting in both dissolution and stabilizing of elements. Tests of the same material will thus
give different results if equilibrium is not reached. Figure 3 shows the deviation from the mean
value of a duplicate test of the availability test for BF slag in air-cooled, weathered air-cooled
and granulated state (Paper IV). Both elements present in cement-forming minerals and the
inert elements potassium and sodium are presented. Results of three of the nine investigated
rock materials (Papers I, II and III) are included as a comparison with materials with very low
reactivity and no puzzolanic properties.
The size (range) of the deviation from the mean value (in percentage) of the duplicate
test may reflect the reactivity and indicates in that case that the granulated slag had high
reactivity, and that the weathered slag had become more reactive as well. Probably, all
materials with puzzolanic properties, to some extent react with moisture, CO2, etc. during
storage and this has an influence on the leaching result.
The rock materials generally had lower deviation compared to the slags, due to a low
reactivity in aqueous solutions.
- 28 -
Figure 3. Deviation from the mean value of dissolved element, Na, K, Al, Ca, Mg, S and Si
(only in rock materials) in the duplicate of the Availability test, expressed in percentage (%).
0
5
10
15
20
25
30
35
40
45
50
Granulated BFslag
fresh AC BFslag
weathered ACBF slag
Gabbro B Gravel A Gabbro-diorite
devi
atio
n fro
m m
ean
valu
e (%
)
K Na Al Ca Mg S Si
In the investigations presented in this study, the stabilizing reactions of the puzzolanic minerals
in the slags most likely have only partially taken place, and the systems are not in equilibrium
in that respect. The liquid/solid ratio is quite high in the tests used; 10 or 100, which is one
reason why the dissolving reactions have dominated. For cement, a w/c3 ratio of 0.4 - 1 is
common and the hydration reactions start directly in contact with the water, which is not the
case for the slags, which needs an activator. The content of C3S and C2S, the most important
minerals in cement, is lower (or lacking) for iron- and steelmaking slags (Ionecue et al., 1998)
and the influence of the latent puzzolanic reactions during leaching varies substantially for
different slag types. The stabilizing reactions would probably have a greater impact if the
materials were processed for use as binder in a similar manner that is used for granulated BF
slag, i.e. finely ground, and with addition of an alkaline activator.
The impact of the puzzolanic properties and amorphous materials with and without such
properties seems to be obvious when comparing the results with the availability test of the BF
slag in both the granulated and the air-cooled condition (Paper IV) with results of two
materials without puzzolanic properties: the amorphous fayalite slag and the crystalline gabbro-
diorite (Paper II). The solubility of Ca and Mg was 45% and 42% in the granulated BF slag,
3 w/c = water/cement ration
- 29 -
and 23% and 15% in the air-cooled BF slag. The release from the gabbro-diorite was 3.9% and
1.6%, and from the fuming slag 0.9 % and 0.5%, of Ca and Mg, respectively.
The fayalite slag remains very stable during the availability test, while both types of the BF slag
react with the water and are dissolved to a high degree. The stabilizing reactions of the BF slag
seem to have little impact in this test with high liquid/solid ratio. This very important property
of the slag complicates the evaluation of leaching results if the reactions have only partially
taken place. This effect is not as enhanced in all leaching tests.
In addition to the difficulties in evaluating the test results of reactive materials the
puzzolanic properties can also make the testing of such materials complicated or impossible.
Column leaching of steelmaking slag has resulted in a stabilized material at the very low
liquid/solid ratio set by standards (Nordtest, 1995b, and SIS, 2004). Testing of the leaching
properties of the stabilized material in a crushed condition is meant to be an alternative, but in
such cases the sample preparation needs to be developed. The properties of a stabilized slag are
dependent on the time factor, and the leaching behaviour of a crushed material differs from
that of a monolithic material.
This example raises the question as to whether it is feasible to determine the leaching
properties of a puzzolanic material with a test, where, in some cases, the stabilizing reactions
dominate.
6.4 Significance of the test results
6.4.1 Sampling
At a steel making plant, the influence of the cooling conditions and chemistry means that each
batch of a slag differs with respect to phase composition. The variation, in a larger scale, is
probably not very big, but as the mineralogy in most cases is complex and the material consists
of particles in a great range of sizes, the sampling will be crucial, particularly with respect to the
trace elements.
The outcome of the cooling, i.e., minerals and phases formed, is more unpredictable in
an air-cooled slag compared to a quenched slag, as the cooling at the surfaces is much more
rapid than in the inner regions of the batch. In spite of standardized sampling, the material of
two samplings will vary and consequently, so will the test results.
- 30 -
The rock materials used in road construction in Sweden are generally homogeneous and
selected to include low content of sulphides and mica. Sampling is generally performed at a
continuous production line. As the chemical composition is not usually analyzed during
production, the rock materials contain varying content of e.g., sulphides.
6.4.2 Potential influences on the release of elements
The low total content of trace elements in the material and in the leachates of both rock
materials and slags implied that the accuracy of the analysis was low, as discussed in Paper I,
and the materials are thus sensitive to surface changes that might affect the leaching results.
Influences of bacterial leaching, weathering, sample preparation and storage (Paper III,
Tossavainen, 2000) probably have little influence on leaching compared to those of the
puzzolanic phases and the cooling conditions. However, they must not be neglected in tests
with conditions less robust compared to the availability test, and as the values of release that are
discussed regarding utilization are very low. Zevenberger (1994) has shown that secondary
minerals formed during storage of glassy combustion ashes have resulted in changes in the
mobility of elements.
6.4.3 Comparison with limit values
When steel slags in Sweden are assessed for construction purposes the results of leaching tests
may be used for a comparison with limit values according to NFS 2004:10 (Naturvårdsverket,
2004b) corresponding to the European directive 2003/33/EC for the different landfill classes.
With knowledge of sampling conditions and with respect to the low accuracy and
particularly the influence of the puzzolanic properties of iron- and steelmaking slag, it seems
realistic that the use of limit values should not be applied strictly in an evaluation.
7. Summarized conclusions
Leaching tests are used in the characterization and evaluation of slag for use in construction.
This study regarding characterization, including leaching tests, of rock materials and different
slag types have shown that there are important properties of the materials that need to be taken
into consideration in the use of the leaching test results. The major conclusions are summarized
below:
- 31 -
- Rock materials include so-called hazardous elements that are more or less soluble
depending on the mineral to which they are bound. Sulphides were the most soluble
minerals in the investigated rock materials.
- The released amounts (mg/kg) of trace elements from the nine rock materials were small
and can be regarded as a negligible environmental burden. Nevertheless, 20 - 30% of the
total content of elements such as zinc, copper and nickel, bound to sulphides, can be
dissolved under oxidizing conditions. Neither the chemical composition nor the leaching
behaviour is included in the investigations of rock materials for use in construction. This
means that, occasionally, materials with high solubility of hazardous trace elements may
be used.
- The soluble amounts of some elements from the rock materials were greater compared to
that from the BF slag and the fayalite slag.
- The leaching tests in this study are designed to investigate the release of elements under
specific conditions. Materials with puzzolanic properties, such as many iron- and
steelmaking slags, react with water and both dissolution and stabilization reactions in the
major phases take place during leaching.
- Quenching of a slag for increased glass content and reactivity implies a greater extent of
phase transformation in contact with water and equilibrium will not be reached in a
short-term test. The stabilizing reactions have only partially taken place in the tests used
in this investigation.
- The reactions of the cement-forming minerals in slag hamper the evaluations of leaching
results and should therefore be considered. The impact of the reactivity is enhanced in
the availability test in which the leaching conditions are optimal.
- Slag with puzzolanic properties will probably react with moisture, CO2, etc. during
storage and to some degree undergo phase transformations, which may result in leaching
results that differ from those of freshly produced slag. Amorphous slag without cement-
- 32 -
forming minerals, on the other hand, shows low solubility and a leaching behaviour
much different from the iron-and steelmaking slags investigated here.
- The significance of leaching results of materials with puzzolanic phases is questionable.
- Each batch of a steelmaking slag has a phase composition different from others. The
formation of glass, major and minor phases and their distribution depend on the
chemistry and the cooling conditions.
- The solubility of the trace elements in the four investigated steelmaking slags was very
low and a better control of the release is difficult without an enhanced control of the
major phases.
- A rapid cooling e.g., by water granulation, results in a more homogeneous slag with few
phases and the control of the properties is thereby enhanced.
8. Reflections and suggestions for further work
It is important to put the leaching test results of both rock materials and slags in a wider
context in order to give the results a reasonable importance in the evaluation of the materials,
with respect to environmental impact. A screening inventory of impact factors during the life
cycle of the materials is recommendable.
- The big impact from the production/extraction of both ore and rock materials is not
usually recognized. The use of pristine bedrock and the production of emissions and
wastes during the production stage motivate further studies for enhanced utilization of
the extracted materials.
- The fact that rock materials includes soluble elements, such as zinc, nickel and copper is
important and should be acknowledged in the discussion and assessment of secondary
materials, for which the leachability is always discussed, no matter the amount of the
release and the significance of the result.
- 33 -
- For a proper evaluation of leaching test results, it is necessary to have good knowledge of
both the material properties and the leaching tests.
The present characterization of secondary materials such as slag needs to be complemented
with:
- The question: how appropriate is the leaching test for the material? The appropriateness
is related to the reactivity.
- Investigations of the influence of puzzolanic phases and other reactive phases in different
leaching tests.
- Investigations of the puzzolanic properties on the long-term behaviour.
For enhanced control of the trace elements in slags, it is recommendable to have a better
control of the major phases and investigate, e.g.:
- Mineral formation during cooling for different slag types.
- Rapid cooling and addition of glass-forming elements e.g., combustion ashes, for a more
homogeneous glassy material and fewer phases.
- 34 -
9 References
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experimental tests on MSWI bottom ash, crushed concrete and blast furnace slag. Doctoral
thesis. THRITA-LWR PhD 1007. ISSN: 1650-8602, ISBN: 91-7283-562-1.
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kemi i Sverige, (Groundwater Chemistry in Sweden). Rapport 4415, Swedish EPA, In
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Borell, M. (2005). Slag – a resource in the sustainable society. Securing the future 2005.
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Crank, J. (1975). The mathematics of diffusion. Oxford University Press, Bristol, England
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ISBN:0-582-30093-2. Longman, London.
Daugherty, K.E., Saad, B., Weirich, C. and Eberendu, A.(1983). The Glass Content of Slag
and Hydraulic Activity. Silicates Industries. 4-5, pp 107-110.
Euroslag (2005). Legal Status of Slags. Position Paper. June 2005. The European Slag
Association – EUROSLAG. Duisburg. Germany.
Fällman, A-M. SGI (1993). Utlakningsegenskaper hos hyttsot och hyttsand (Leaching
Characteristics of Blast Furnace Slag and Dust). Svenskt Stål Tunnplåt AB, DNR 2-462/92. In
Swedish.
Fällman, A-M. (2000). Leaching of chromium and barium from steel slag in laboratory and
field tests – a solubility controlled process? Waste Management, 20. Elsevier, pp 149-154.
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Fällman, A.-M., and Carling, M. (1998). Miljömässig karaktärisering av järnsand,
(Environmental characteristics for slag from copper smelter). Boliden MineralAB, In Swedish.
http://www.sgu.se/sgu/sv/naturresurs/malm_metall.html (2005-10-21). Internet home-page
http://www.miljomal.nu/index.php (2005-06-17). Internet home-page.
http://www.analytica.com (2005-10-06). Internet home page.
Ionescu, D., Meadowcroft, T.R. and Barr, P.V. (1998). Hydration potential of high iron level
glasses: Criteria for the recycling of steel slag as a Portland cement additive. ICTI/Ironmaking
conference proceedings, pp1245-1254.
Juckes, L. M. (2003). The volume stability of modern steelmaking slags, Mineral Processing
and Extractive Metallurgy (Trans. Inst. Min. Metal. C), December 2003, Vol. 112, pp177-197.
Lind, B.B., Fällman, A.-M., Larsson, L.B. (2001). Environmental impact of ferrochromium
slag in road construction. Waste Management, 21. Elsevier, pp 255-264.
Lind, L. (2002). Conference proceedings. Recycling and Waste Treatment in Mineral
Metal Processing: Technical and Economical Aspects, 16-20 June 2002, pp 392-299, Luleå,
Sweden
Monaco, A., and W-K., Lu, (1996). The properties of steel slag aggregates and their
dependence on the melt shop practice, Steelmaking conference proceedings, Vol. 79, pp 701-
711, A publication of the Iron & Steel Society, 1996, Pittsburgh
Mroueh, U.-M., Eskola, P., Laine-Ylijoki J., (2001). Life-cycle impacts of the use of industrial
by-products in road and earth construction. Waste Management, 21, Elsevier, pp. 271-277.
Murphy, J.N., Meadowcroft, T.R. and Barr, P.V. (1997). Enhancement of the cementitious
properties of steelmaking slag. Canadian Metallurgical Quarterly, Vol 36, No 5, pp315-331.
Naturvårdsverket (2004a). Industrins avfall 2002 (Industrial waste 2002). Report 5371, ISSN
0282-7298, Swedish EPA
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Naturvårdsverket (2004b). NFS 2004:10. Naturvårdsverkets föreskrifter om deponering,
kriterier och förfarande för mottagning av avfall vid anläggningar för deponering av avfall.
(Implementation of criteria and procedures for the acceptance of waste at landfills pursuant to Article 16 of
and Annex II to Directive 1999/31/EC). ISSN 1403-8234. In Swedish.
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användning av hyttsten (Road E4, Nyköpingsbro-Jönåker, Use of Blast Furnace slag and
impact on the environment). SSAB Merox AB. In Swedish.
SGU (2005). Aggregates. Production and resources 2004. Per. Publ. 2005:3. Geological Survey
of Sweden, ISSN 0283-2038
Shi, C.(2004). Steel slag – its Production, Processing, Characteristics, and Cementitious
Properties. Journal of Materials in Civil Engineering. pp 230-236. ASCE. May/June 2004.
van der Sloot, H.A., Heasman, L., Quevauviller, Ph. (1997). Harmonization of
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Strömberg, B., Banwart, S., (1999). Weathering kinetics of waste rock from Aitik copper
mine, Sweden. Journal of Contaminant Hydrology, 39, pp 59-89.
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Gothenburg, Sweden
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used in Road Construction. Licentiate thesis. 2000:23, ISSN: 1402-1757. LTU.SE
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Vägverket (2003). Swedish Road Administration. Årsrapport 2002. Utförande av
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Kapitel A. Gemensamma förutsättningar. In Swedish.
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10 Standards and directives
CEN (1998). EN 1744-1. Tests for chemical properties of aggregates – Part 1: Chemical
analysis.
CEN (2002a). EN 1744-3. Tests for chemical properties of aggregates – Part 3: Properties of
eluates by leaching of aggregates.
CEN (2002b). Final draft prEN 12457-3, Characterization of waste-Leaching-Compliance test
of leaching of granular waste material and sludges-Part 3: Two stage batch test at a liquid to
solid ration of 2 l/kg and 8 l/kg for materials with high solid content and with particle size
below 4 mm (with or without particle reduction).
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solid ration of 10 l/kg for materials with particle size below 4 mm (with or without particle
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Nordtest (1995b). NT ENVIR 002. Solid waste, granular inorganic material; column test.
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– Uppströms perkolationstest (under bestämda förhållanden). (Characterisation of waste-
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- 39 -
Paper I
Ž .The Science of the Total Environment 239 1999 31�47
The potential leachability from natural roadconstruction materials
Mia Tossavainen�, Eric ForssbergDi�ision of Mineral Processing, Lulea Technical Uni�ersity, S-971 87 Lulea, Sweden˚ ˚
Received 4 December 1998; accepted 11 June 1999
Abstract
Leaching characteristics are used for the evaluation of waste materials in road construction. Few leaching testshave been performed on natural rock materials which implies that there is a lack of data to be used in comparisonwith waste materials. In order to form a basis for comparison of the leachability, nine natural road constructionmaterials in Sweden were investigated using the availability test NT ENVIR 003. The results show that the leachableamounts of heavy metals and sulphur generally are very small, but under oxidising conditions the solubility ofsulphide bound elements increases notably. Vanadium and chromium are probably present as ionic substitutes forother elements in mineral lattices and show very low leachability. The leachable amounts of some heavy elements,e.g. zinc, nickel and copper are higher in the rock materials and gravels than in the blast furnace slags. � 1999Elsevier Science B.V. All rights reserved.
Keywords: Leaching; Road material; Rock; Blast furnace slag; Heavy metals
1. Introduction
Approximately 40 million tons of crushed rock,gravel and sand were used in Sweden for road
Ž .construction in 1996 Anon., 1997 . Rock materi-
� Corresponding author. Tel.: 46-920-91841; fax: 46-920-97364.
Ž .E-mail address: [email protected] M. Tossavainen
als used for road construction in Sweden are ofhigh quality. They are mainly hard and stablerocks of primary formation including granite andgneiss with a low mica content and diabase,
Ž .quartzite, porphyry, and lepite Vagverket, 1994 .¨These materials have a high disposition for resis-tance to weathering and the content of heavymetal containing minerals, e.g. sulphides is low.
In many countries slags from iron and steelproduction are in general use as an alternative
0048-9697 99 $ - see front matter � 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 8 - 9 6 9 7 9 9 0 0 2 8 3 - 1
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�4732
Ž .roadmaking material OECD, 1997 . In Sweden,however, the use of blast furnace slag as well assteel-making slag is limited, not only due to lackof regulations regarding the environmentalproperties for utilisation, but also because of rela-tively abundant resources of high-quality rockmaterial for construction purposes. Stricter regu-lations for environmentally safe landfilling and ahigher rate of recycling and utilisation of sec-ondary materials, as well as an ambition to reducethe use of high-quality virgin material and land-filling areas have increased the interest to useslag as aggregates in roadmaking.
When resource depletion and the environmen-tal impact from construction materials are evalu-ated, it is pertinent to investigate the life cycle ofthe material from ‘cradle to grave’ with a life
Ž .cycle assessment LCA , which is a tool for de-scribing environmental impacts. For naturally oc-curring rock materials the most severe impact onthe environment probably arises from activities atthe quarry.
When secondary materials such as slag are tobe evaluated the focus of attention is on theirleaching characteristics. Leaching tests are usedto determine the availability and actual release ofelements that can intrude and contaminate the
Žgroundwater Mulder, 1991; Van Houdt et al.,.1991; Fallman and Hartlen, 1996 .¨ ´
Rock materials and gravels contain many heavymetal elements and sulphur but their leachingproperties are usually not investigated before theyare applied to road construction. This implies thatthere are few figures that can be used as refer-ences when alternative materials are evaluatedand there may also be natural materials in usethat have notably high values of leachability ofcertain elements.
Ž .It has been shown by Lindgren 1996 thatstone is probably the major source of releasedmetals in asphalt, while bitumen is the minorsource.
This paper presents the results from an investi-gation of the total content and the leachablefractions of elements from commonly used natu-ral road construction materials in Sweden. Theresults from leaching tests performed on ninematerials of gravel and crushed rock, with main
emphasis on some heavy metals, are discussed.The objective of the investigation was to obtaindata to be used as references regarding leachingcharacteristics.
A comparison is made with results from similarleaching tests performed on blast furnace slag.
2. Materials and methods
2.1. In�entory and sampling
Nine natural road construction materials wereselected among crushed rocks and gravels in threeadministrative provinces in Sweden. In theseprovinces alternative road construction materialsare available, e.g. blast furnace slag from SwedishSteel Strip Products in Lulea and Swedish Steel˚
Ž .AB in Oxelosund and fayalite copper slag from¨the copper smelter at Boliden Mineral AB inSkelleftehamn. The fayalite, in this context, is aslag product consisting of iron, originating fromthe copper ore, and silica added as slag former.These alternative materials are to some extentused for road construction.
In total, 22 samples of crushed rock and 12samples of crushed gravel were collected fromquarries and gravel pits. The aim was in each caseto collect a representative sample of non-weathered material from the production or stockof material for base or sub-base layer. This im-plies that the sampling methods varied dependingon the actual state at the quarry or gravel pit.
During production, sampling was made with afront-end loader. The bucket was filled with ma-terial from the conveyor belt, 3�4 times, andemptied in a string on even ground from whichsamples were collected.
Stocks were sampled with a loader or in somecases manually with a shovel. When a loader wasused the sampling was performed according tothe above description, after removal of the sur-face layer. At manual sampling the surface layer,10�20 cm, was removed before excavating mate-rial at different places in the stock. In total,60�80 kg of each material was collected aslaboratory samples.
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�47 33
2.2. Sample pre-treatment and selection of testmaterials
The laboratory samples were divided into 1�2kg sub-samples by riffling. The split material wascrushed in two steps, firstly with a jaw crusher,Retsch BB3, and secondly with a roll crusher,Humboldt Wedag WDG G9. The final maximumparticle size was 1 mm. The crushed material wasground in an agate mortar to a particle size of95% 125 �m.
The criteria for selection of test materials werethat they should represent: a material producedin some quantities; typical products of theprovince; and a considerable distribution in min-eralogy and analysis. The producers’ demands fora good road-making material, i.e. strength quali-ties, were included in the criteria.
The nine finally selected materials were seventypes of crushed rock and two types of gravelŽ .Table 1 .
2.3. Sample characterisation
The total composition of each material wasŽanalysed with ICP-AES and with ICP-MS Induc-
tively Coupled Plasma Emission Spectroscopy and.Mass Spectroscopy according to the EPA meth-
ods 200.7 and 200.8 by SGAB, licensed by TheSwedish Board for Technical AccreditationŽ .SWEDAC . Two different digestion methodswere used: lithium borate melt for main compo-nents and sulphuric acid in a Teflon bomb andmicrowave oven for certain trace elements. The
Ž .detection limit mg kg for the instrument is ac-Ž . Ž .cording to the following: arsenic 0.1 , barium 2 ,
Ž . Ž . Ž .cadmium 0.01 , cobalt 0.01 , chromium 10 ,Ž . Ž . Ž .copper 0.1 , mercury 0.04 , nickel 0.08 , lead
Ž . Ž . Ž .0.1 , vanadium 2 and zinc 0.7 .The analytical quality was regularly calibrated
with certified rock materials and sediments.Sulphur was analysed with an Carlo-Erba ele-
mentary analyser.The particle size distribution was analysed with
a Cilas Granolumetre 1064 in the interval 0.1�500�m. The analytical method is based on laserbeam diffraction caused by the particles.
The specific surface area was measured accord-ing to the BET-method with a MicromereticsFlowsorb 2300. Density was measured with a Mi-cromeretics Multivolume Pycnometer 1305.
2.4. Leaching tests
The actual leaching from a road varies depend-ing on the specific conditions at the site and thetime period, which motivates an investigation ofthe potentially leachable fraction of the material.The leaching test chosen for this work were based
Ž .on the availability test NEN 7341 1992 . It is atest to distinguish between leachable and non-leachable fractions. Optimal leaching conditionsregarding diffusion, buffer capacity and thechemical concentration build-up are controlled.The availability test methods used in this studyare designed to obtain 99% depletion of a subs-tance with an effective diffusion in the material ifpD 13, or 80% depletion if pD 15, wheree e
Table 1The nine selected materials of crushed rock and gravel
Sample Type Origin
No 1 Gabbro�diorite Svalget, Lulea municipality˚˚No 2 Gabbro Akerberg, Skelleftea municipality˚
No 3 Gneiss Hagnesta, Nykoping municipality¨No 4 Gabbro Kallax, Lulea municipality˚No 5 Gravel Langviken, Skelleftea municipality˚ ˚No 6 Gravel Larslund, Nykoping municipality¨No 7 Granite Kopparnas, Pitea municipality¨ ˚No 8 Lepite Finnforsfallet, Skelleftea municipality˚No 9 Gneiss Balsta, Eskilstuna municipality
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�4734
pD is the negative logarithm of the efficienteŽ 2 . Ž .diffusion coefficient m s Fallman, 1997 .¨
The leaching tests were carried out by theSwedish Geotechnical Institute, SGI. 8.00 0.01g dried material, ground to 95% 125 �mŽaccording to measurement with a Cilas Gra-
.nolumetre 1064 was leached in two steps, at pH7.0 for 3 h and at pH 4 for 4 h, at a liquid to solid
Ž .ratio L S of 100 in each step. The pH was heldŽconstant with a titrator controlling the pH using
.0.5 M nitric acid . The two leachates were com-Ž .bined before analysis NEN 7341, 1992 . The
method was modified and standardised in 1997Ž .NT ENVIR 003 . To increase the diffusion-basedleaching the second step leaching is performed at
Ž .pH 4 during 18 h instead of 4 h Fallman, 1997 .¨ŽThe oxidised availability test NT ENVIR 006,
.1999 was in this study carried out according tothe protocol for the ordinary availability test butat fully oxidising conditions controlled with addi-tion of H O .2 2
Six materials, Nos. 1�6 in Table 1, were testedaccording to the ordinary availability test for 4 hat pH 4 in the second step, and three materials,
ŽNos. 7�9, for 18 h in the second step NT ENVIR.003 . Four materials, Nos. 1, 2, 5 and 9, were
tested according to the ordinary test procedure aswell as the oxidised availability test procedure.The same test time were used for the second stepin the oxidised availability test as in the ordinarytest.
All tests were made in duplicate. pH, conduc-tivity and redox potential were measured in thecombined leachate after the test. The redox po-
tential was measured with separate Pt and calomelelectrodes as E .ŽKCl.
The metal content was analysed by SGAB withICP-AES and ICP-MS. Atomic fluorescenceŽ .AFS was used for mercury. The analyses wereperformed according to the EPA methods 200.7and 200.8.
3. Results
3.1. Starting materials
The total composition of the nine materials areshown in Appendix A.
The content of sulphur and heavy metal ele-ments was low in all materials, and the highestcontent of sulphur, 0.2%, was noted in the basic
Ž .rock materials Nos. 2 and 4 . The total contentof mercury and cadmium was very low or belowdetection limit for the instrument in all materials.
The particle size distributions of the nine mate-rials are presented in Fig. 1. The density, specificsurface area and particle size at 95% passing arelisted in Table 2. The specific surface areas areexpressed both as m2 g and m2 cm3 to includethe influence of the differences in density. Twomaterials, Nos. 6 and 9, had coarser maximumsize than specified in the leaching test standards.
3.2. Ordinary a�ailability test
pH and redox potential in the combinedleachates are presented in Table 3. Redox condi-
Table 2Particle size at 95% passing, density and specific surface area of the investigated materials
Material 95% Surface area Density Surface area2 3. 2 3.Ž . Ž . Ž Ž�m m g g cm m cm
1. Gabbro�diorite 120 1.96 2.87 5.632. Gabbro A 102 1.38 2.90 4.003. Gneiss A 106 1.28 2.65 3.394. Gabbro B 125 1.13 2.91 3.295. Gravel A 109 1.94 2.78 5.396. Gravel B 160 1.44 2.69 3.877. Granite 124 1.27 2.66 3.388. Lepite 118 1.50 2.74 4.119. Gneiss B 150 1.37 2.72 3.73
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�47 35
Fig. 1. The particle size distributions of the investigated materials analysed with a Cilas Granulometre 1064.
Table 3Ž .pH, redox Eh mV and pH pe in the combined leachates
Material Ordinary availability test Oxidised availability test
pH Eh pH pe pH Eh pH peŽ . Ž .mV mV
1. Gabbro�diorite 5.0 444 12.4 4.9 584 14.82. Gabbro A 5.0 488 13.2 4.6 602 14.73. Gneiss A 4.5 543 13.74. Gabbro B 4.4 590 14.45. Gravel A 4.6 594 14.6 4.4 624 15.06. Gravel B 5.0 601 15.1
a7. Granite 4.4 619 14.9a8. Lepite 5.1 468 13.0
a9. Gneiss B 4.6 574 14.3 5.0 582 14.8
a18 h at pH 4.
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�4736
Tab
le4
Ž.
The
leac
habi
lity
ofm
ajor
elem
ents
expr
esse
das
%of
the
tota
lco
nten
tth
edu
plic
ate
valu
esar
epr
esen
ted
aa
Ele
men
tU
nit
1.G
abbr
o2.
Gab
bro
3.G
neis
s4.
Gab
bro
5.G
rave
l6.
Gra
vel
Gra
nite
8.L
epite
9.G
neis
sa
�di
orite
AA
BA
BB
Al
%0.
21,0
.19
0.09
,0.1
00.
20,0
.20
0.14
,0.1
30.
09,0
.06
0.13
,0.1
10.
13,0
.11
0.12
,0.1
00.
13,0
.11
Ca
%3.
95,3
.85
2.91
,3.0
88.
58,8
.72
2.09
,2.0
63.
38,3
.33
22.4
,23.
92.
40,2
.08
4.44
,4.4
16.
23,6
.32
Fe
%1.
29,1
.32
0.54
,0.5
71.
53,1
.49
0.25
,0.2
60.
97,1
.00
0.83
,1.0
11.
33,1
.14
2.53
,2.3
01.
20,1
.23
K%
11.2
,10.
72.
63,2
.76
1.32
,1.4
925
.5,2
5.4
4.25
,4.3
81.
89,1
.79
5.18
,5.3
27.
65,8
.75
5.63
,4.8
1M
g%
1.52
,1.5
90.
52,0
.58
2.10
,2.0
40.
40,0
.40
0.82
,0.8
21.
42,1
.38
0.76
,0.7
01.
55,1
.45
1.07
,1.0
4N
a%
0.49
,0.5
40.
38,0
.46
0.43
,0.4
40.
55,1
.22
0.34
,0.3
30.
22,0
.28
0.31
,0.2
20.
29,0
.32
0.19
,0.1
4Si
%0.
19,0
.19
0.07
,0.0
70.
05,0
.05
0.13
,0.1
20.
06,0
.06
0.07
,0.0
60.
06,0
.06
0.06
,0.0
50.
06,0
.06
a 18h
atpH
4.
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�47 37
Tab
le5
aT
hele
acha
bilit
yof
trac
eel
emen
tsin
%of
the
tota
lco
nten
t
bb
Ele
men
tU
nit
1.G
abbr
o�
2.G
abbr
o3.
Gne
iss
4.G
abbr
o5.
Gra
vel
6.G
rave
l7.
Gra
nite
8.L
epite
9.G
neis
sb
dior
iteA
AB
AB
B
As
%2.
67,3
.06
2.78
,3.5
4N
DN
D1.
00,1
.44
4.15
,�1.
71,1
.65
2.65
,2.1
8N
DB
a%
7.83
,7.3
11.
88,2
.16
0.60
,0.6
14.
14,4
.08
2.42
,2.6
31.
16,1
.17
2.39
,2.3
50.
93,0
.72
1.26
,1.3
3C
d%
22.9
,13.
26.
08,8
.84
17.1
,20.
7N
D14
.9,1
2.0
30.5
,26.
019
.6,1
2.2
8.85
,9.7
335
.1,3
1.4
Co
%3.
19,2
.95
2.24
,2.5
43.
87,3
.44
1.4,
1.58
6.64
,6.9
14.
83,5
.02
5.47
,5.8
85.
73,4
.15
2.54
,2.6
1C
r%
0.31
,0.2
30.
19,0
.19
0.63
,0.5
90.
04,0
.05
0.16
,0.1
80.
27,�
0.27
,0.2
60.
36,0
.28
0.19
,0.0
9C
u%
0.59
,0.5
20.
40,0
.31
7.18
,5.9
50.
45,0
.36
0.67
,0.4
91.
41,1
.42
0.25
,0.2
81.
03,0
.61
2.57
,1.8
3H
g%
ND
ND
ND
�,1
.5N
DN
DN
DN
DN
DM
n%
4.41
,4.3
33.
33,3
.54
6.62
,6.4
61.
36,1
.23
4.93
,4.7
57.
72,7
.96
6.83
,6.2
412
.0,1
1.0
2.89
,3.3
0N
i%
4.00
,3.7
86.
25,7
.52
11.5
,10.
72.
02,1
.94
7.50
,7.5
76.
72,5
.64
7.22
,6.3
426
.0,2
2.1
3.41
,3.4
4Pb
%0.
93,0
.70
0.43
,0.4
53.
18,3
.03
2.64
,1.3
11.
36,1
.71
19.9
,24.
14.
47,1
.14
1.05
,0.8
93.
64,2
.49
S%
2.7,
2.8
2.4,
2.2
ND
�,1
.45.
8,5.
54.
5,5.
18.
5,10
.1N
DN
DSr
%0.
98,0
.97
0.81
,0.9
11.
25,1
.30
1.11
,1.0
80.
90,0
.88
2.23
,2.4
0.64
,0.5
50.
72,0
.70
0.89
,0.8
8V
%N
DN
DN
DN
DN
DN
DN
DN
DN
DZ
n%
7.05
,7.2
43.
55,3
.20
4.68
,4.3
45.
83,6
.50
5.01
,4.1
815
.4,1
9.8
4.30
,3.7
03.
37,2
.95
3.60
,3.3
4
aN
Dno
tde
tect
ed.
b 18h
atpH
4.
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�4738
tions are also expressed as pH pe as suggestedŽ .by Lindsay 1979 as a convenient single-term
expression for defining the status of aqueous sys-tems.
The leached amounts in the ordinary availabil-ity test on nine materials are presented as meanvalues of the duplicates in Appendix A.
3.2.1. Major elementsThe leachability of aluminium, calcium, iron,
potassium, magnesium, sodium and silicon for theduplicates are listed as the percentage of the totalcontent of each specific element in Table 4. Cal-cium and potassium had the highest solubility inall materials. The duplicates agreed well.
3.2.2. Trace elementsThe leachability of trace elements, heavy met-
als and sulphur for the duplicates is presented inTable 5 as the percentage of the total content.The total content of cadmium was below 0.3mg kg in all of the nine materials, and between12% and 35%, were soluble in six of them. Thetotal content of arsenic varied a great deal amongthe nine materials, from 0.30 to 104 mg kg, andthe leachability was below 4% in all materials.
3.2.3. Chromium and �anadiumThe total content and the leachability of
chromium were both low in all of the nine materi-als. The basic rock material gabbro B had thehighest total content, 331 mg kg, but only 0.05%was leachable. The total content of vanadiumvaried between 17 and 282 mg kg, but vanadium
( )in the leachate was not detectable 0.005 � g l forany material.
3.2.4. Zinc, lead, copper and nickelBoth the total content and the leachability var-
ied between all the nine materials. The totalcontent of zinc varied between 38 and 114 mg kg.Eight of the materials had leachable fractionsbetween 3% and 8% and in gravel B the solubilitywas 17.6% as a mean value for the duplicate.
Between 2% and 12% of the nickel was solublein eight of the materials and in lepite the meanvalue for the duplicate was 24.0%. The totalcontent of nickel was less than 40 mg kg in all of
the nine materials.The copper content varied between 11 mg kg
in gneiss A and 63 mg kg in lepite, and between0.3% and 7.2% was leachable.
The total content of lead was below 11 mg kgin all nine materials and the solubility was low,0.4�4.5%, with one exception, gravel B, with asolubility of 22% as a mean value.
3.2.5. SulphurAs shown in Table 5 the major part of the
sulphur remained in the material during leaching.
3.3. Oxidised a�ailability test
Four materials: gabbro�diorite, gabbro A,gravel A and gneiss B, were tested according tothe oxidised availability test with stable oxidisingconditions. pH decreased and the redox potentialincreased in combined leachates compared to the
Ž .ordinary availability tests Table 3 .The results from the analyses of leachates are
presented as mean values of the duplicates inAppendix A and for the duplicates for both theordinary and the oxidised availability tests in Table6.
The major elements were generally slightly in-fluenced by the oxidation, except for iron, were
Ž 2 3 .the oxidation Fe to Fe reduced the solubil-ity with approximately two orders of magnitude ofthe value in the ordinary availability test. Thesolubility of arsenic decreased under oxidisingconditions for the investigated materials.
Chromium and vanadium were not significantlyinfluenced by the oxidation, but the trace ele-ments mercury and cadmium showed a strikingincrease in leachability under stable oxidisingconditions.
The leachability of sulphur, copper, zinc andlead increased obviously in the test performed atstable oxidising conditions.
4. Discussion
The materials tested were sampled from quar-ries and gravel pits in operation and are repre-senting the quality which was produced at the
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�47 39
Tab
le6
aŽ
.Ž
.T
heav
aila
bilit
yfo
rle
achi
ngin
ordi
nary
Av.
and
oxid
ised
avai
labi
lity
test
Ox.
av.
expr
esse
das
perc
enta
geof
tota
lco
nten
t
cE
lem
ent
Uni
t1.
Gab
bro
�D
iori
te2.
Gab
bro
A5.
Gra
velA
9.G
neis
sB
bA
v.O
x.av
.A
v.O
x.av
.A
v.O
x.av
.A
v.O
x.av
.
Ca
%3.
95,3
.85
4.07
2.91
,3.0
82.
86,2
.96
3.38
,3.3
33.
40,3
.11
6.23
,6.3
26.
48,6
.55
Fe
%1.
29,1
.32
0.01
0.54
,0.5
70.
000.
97,1
.00
0.01
,0.0
11.
20,1
.23
0.02
,0.0
2K
%11
.2,1
0.7
18.9
2.63
,2.7
63.
98,5
.02
4.25
,4.3
83.
92,3
.61
5.63
,4.8
13.
10,3
.81
Mg
%1.
52,1
.59
0.52
0.52
,0.5
80.
53,0
.57
0.82
,0.8
20.
73,0
.74
1.07
,1.0
41.
24,1
.23
Na
%0.
49,0
.54
0.24
0.38
,0.4
60.
76,0
.79
0.34
,0.3
30.
97,0
.71
0.19
,0.1
40.
80,0
.64
Si%
0.19
,0.1
90.
080.
07,0
.07
0.08
,0.0
80.
06,0
.06
0.05
,0.0
60.
06,0
.06
0.09
,0.0
9A
l%
0.21
,0.1
90.
110.
09,0
.10
0.15
,0.1
70.
12,0
.13
0.18
,0.1
60.
13,0
.11
0.24
,0.2
5A
s%
2.67
,3.0
6N
D2.
78,3
.54
0.71
,0.5
01.
00,1
.44
0.28
,�N
DN
DC
d%
22.9
,13.
267
.36.
08,8
.84
40.3
,26.
514
.9,1
2.0
34.4
,32.
635
.1,3
1.4
38.7
,33.
9C
r%
0.31
,0.2
30.
220.
19,0
.19
0.17
,0.1
40.
16,0
.18
0.20
,0.1
90.
19,0
.09
0.23
,0.2
1C
u%
0.59
,0.5
223
.10.
40,0
.31
12.3
,9.1
50,
67,0
.49
13.4
,13.
02.
57,1
.83
23.8
,22.
8H
g%
ND
ND
ND
49.6
,�N
D89
.2,5
2.5
ND
56.9
,49.
0N
i%
4.00
,3.7
87.
096.
25,7
.52
12.0
,11.
37.
50,7
.57
9.93
,9.0
83.
41,3
.44
3.26
,2.9
1Pb
%0.
93,0
.70
3.79
0.43
,0.4
55.
17,4
.47
1.36
,1.7
13.
07,2
.92
3.64
,2.4
95.
61,5
.25
S%
2.68
,2.7
89.
502.
37,2
.20
10.3
,9.3
35.
80,5
.50
25.0
,24.
6N
DN
DV
%N
DN
DN
DN
DN
DN
DN
D4.
36,2
.66
Zn
%7.
05,7
.24
30.3
3.55
,3.2
016
.3,1
6.6
5.01
,4.1
816
.1,1
3.5
3.60
,3.3
49.
66,9
.58
aN
Dno
tde
tect
ed.
bSi
ngle
expe
rim
ent.
c 18h
atpH
4.
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�4740
sampling time or was in stocks for sale. Rockmaterials vary in assays and new samples willmost probably show a large similarity to the pre-sented ones, but there could also be big differ-ences for some elements.
The nine materials were selected to representtypical Swedish rock material used in road-mak-ing as base or sub-base material. These materialsfulfil the producers’ demands on a material withgood strength qualities, including a low content ofminerals that are prone to weathering, e.g. mica,and have a low content of sulphides.
Swedish blast furnace slag has been evaluatedfor use as road-making material and examined bythe Swedish Geotechnical Institute, SGI, regard-
Žing leaching properties Fallman, 1993; Rogbeck¨.and Elander, 1995 .
At comparison of the analyses of the sampledrock materials with the analyses of Swedish blastfurnace slag the most obvious difference is in themain components. Expressed as oxide, the con-tent of CaO is between 30 and 40% in the blastfurnace slag and ranges between 1.4% and 12.3%in the rock materials. This implies that the buf-fering capacity is higher in the slag. The total
Ž .content of silica in the two gravels 70% istwice the total content of silica in the slag. Ex-pressed as Fe O the total content of iron is2 3,below 1% in the blast furnace slag and between10% and 12% in the basic rock materials. Thetotal content of aluminium in the rock materialsis in the same range as for the blast furnace slag.
The amount of heavy metals is very low in therock materials as well as in the blast furnace slag.The total content of many elements are in thesame range or higher in the rock materials, e.g.copper, zinc, arsenic, chromium, nickel and lead.The amount of each element varies in a widerange between different rocks, but also betweenthe same kind of rock, but from different locali-ties. Certain elements are commonly associated
Ž . Žwith different rock qualities Table 7 Ekelund et.al. 1993 .
Basic rock materials, e.g. gabbro and gabbro�diorite, normally have higher total content ofchromium and vanadium compared to acid rocks,e.g. granite and gneiss. In this investigation theyalso have a higher content of sulphur. Conse-
quently it can be expected that the total contentof sulphide-bound elements, e.g. zinc, lead andcopper, is higher compared to the acid rocks. Forthe investigated nine rock materials there was nosuch obvious correspondence.
The total content of sulphur, as well as theamount of vanadium, is higher in the blast fur-nace slag compared to all the nine rock materials.The total sulphur content in both slags is 1.4%,compared to 0.2% in the basic rock materialsstudied.
As the rock materials in this study have verylow contents of heavy metals the analyses aresensitive to possible contamination in the samplepreparation and the precision and accuracy of theanalyses have great significance for the validity ofthe result. In many cases the analysed values areclose to the detection limit and, consequently, the
Ž .precision is low high standard deviation . Forsome materials with very low total contents of the
Želements chromium, arsenic and cadmium e.g.37.8 mg kg chromium, 0.32 mg kg arsenic, 0.08
.mg kg cadmium the relative standard deviationis between 10% and 30%. At higher total content,e.g. 0.13 mg kg cadmium in gravel B, the relativestandard deviation is 3%. The analysis of cadmiumin the leachate, in some cases, has a standarddeviation of 10�20%. The duplicate in most casesagree relatively well as is shown in Tables 4�6.The digestion methods used are developed to
Table 7Important element associations in different types of rocksŽ .Ekelund et al., 1993
Rock Increased contents Low contents
Granites Mo, Sn, W, K, Pb Co, Cr, Ni
Acid volcanic As, Cu, Pb, Znrocks Cd, Ag, Hg, Se
Basic rocks Cr, Co, Ni, Cu, Ti, V
Slates Ag, As, Au, Cd, Mo, NiPb, Zn, Co, U, Cu, Se
Sandstones No specific associationsLimestones of importance for geo-
chemical interpretationsare reported
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�47 41
preserve the volatile matter of elements, e.g. As,Cd, Pb, and are good for analyses of rock mate-rial.
A very small quantity, 8 g, is used for theleaching test, which makes it sensitive to varia-tions in the composition.
An availability test is designed to give an an-swer to what the potential leachable fraction of acertain material amounts to. Both solubility anddiffusion controlled leaching should be almostcomplete following the stipulated test conditions.The test is de�eloped for in�estigation of residuesŽ .NEN 7341, 1992 .
In order to increase the diffusion based leach-ing, the availability test method used in this studywas modified so that leaching at pH 4 was per-formed for 18 h instead of 4 h. The complete testshould result in 80% depletion of a material if
Ž .pD 15 Fallman, 1997 .¨e
Three of the nine natural materials in thisstudy were tested according to the modified testand six with the original test.
Ž .The diffusion coefficient D for elements innatural rock material varies within a wide rangedepending on the mineral composition and fac-tors like interactions with other ions in the envi-ronment in which diffusion takes place. For ex-ample, the diffusion coefficient for sodium inquartz is 1.1�10 19 m2 s and for calcium inquartz 1.4�10 49 m2 s. Potassium in feldsparhas a diffusion coefficient of 1.6�10 53 m2 sŽ .Faure, 1991 .
The elements in the rock materials and gravelsin this study have probably diffusion coefficientsin a wide range and leaching by diffusion is notachieved to the same level for all elements. Themodified test with prolonged leaching at pH 4 for
Ž .three materials Nos. 7�9 does not result in anyobvious increase in solubility compared to the
Ž .other six materials Tables 5 and 6 .Compared to the blast furnace slags, the avail-
ability for leaching of many elements, expressedas percentage of the total content, in most cases
Ž .is lower in rock materials Table 8 . The lowersolubility in the rock materials, quite likely, de-pends to a great extent on the fact that thediffusion leaching is more incomplete compared
to the waste materials due to lower diffusioncoefficients.
In rock materials their resistance to weatheringhas a large influence on the availability for leach-ing. Different minerals are more or less prone toweathering and quartz is a mineral with very low,and calcite very high, disposition. Between thesetwo minerals there are many different silicateminerals, among which alkali-rich feldspars are
Žless prone to weathering than dark minerals con-. Žtaining calcium magnesium and iron Aastrup et
.al., 1995 . Apart from the mineral content, thetexture and structure affect the ageing of rockmaterials. The sulphur content indicates a poten-tial existence of sulphide minerals, which in-creases the disposition to weathering.
The rock materials used for road constructionin Sweden are to a great extent crystalline rocks,e.g. mica-poor granites and gneiss, quartzite, dia-
Ž .base, porphyry and lepite Anon., 1992 . Theserocks are hard, solid and resistant to atmosphericchanges. The materials investigated here to alarge extent consisted of the minerals alkalifeldspar, plagioclase, pyroxene, quartz and to aminor extent of minerals like olivine, hornblende,
Ž .epidote and biotite data to be published . Most ofthese minerals have a low disposition for weather-ing.
Zinc, nickel, copper and lead are often boundto sulphides and under oxidising conditions theseelements are dissolved. As shown in Table 3,there were oxidising conditions during the wholeor major part of the ordinary availability test. Inspite of the fact that the content of sulphur waslow or very low in all the nine materials, theheavy metal elements often bound, or associated,to sulphides were partly dissolved during the testŽ .Table 5 . Both the total content and the leach-ability of these elements varied between the ninematerials, however, zinc and nickel seem to bemore leachable than lead and copper in thesenine investigated materials.
Cadmium with a total content below 0.3 mg kgin all nine materials showed a notable high leach-ability. The highest values were recorded forgravel B, 30.5%, and for gneiss B, 35.1%. Thehighest total content of arsenic and a leachable
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�4742
Tab
le8
aŽ
.T
heav
aila
bilit
yfo
rle
achi
ngin
four
natu
ral
mat
eria
lsan
din
two
blas
tfu
rnac
esl
ags
Fal
lman
,199
3;R
ogbe
ckan
dE
land
er,1
995
¨
bE
lem
ent
Uni
t1.
Gab
bro
�di
orite
2.G
abbr
oA
5.G
rave
lA9.
Gne
iss
BB
Fa
BF
b
Ava
il.O
x.av
.A
vail.
Ox.
av.
Ava
il.O
x.av
.A
vail.
Ox.
av.
Ava
il.O
x.av
.A
vail.
Ox.
av.
S%
2.70
9.50
2.30
9.80
5.70
24.8
0N
D14
.50
21.4
035
.00
47.2
059
.80
V%
ND
ND
ND
ND
ND
ND
ND
3.51
6.48
18.0
04.
807.
72C
r%
0.27
0.22
0.19
0.16
0.17
0.19
0.14
0.22
0.86
0.36
0.42
2.18
Cu
%0.
6023
.10
0.40
12.6
00.
6013
.20
2.20
23.3
0N
D10
.60
ND
7.12
Ni
%3.
907.
106.
9011
.70
7.50
9.50
3.40
3.10
ND
ND
21.7
932
.10
cPb
%0.
803.
800.
404.
801.
503.
003.
105.
406.
908.
8316
.68
cc
Zn
%7.
1030
.30
3.40
16.4
04.
6014
.80
3.50
9.60
48.8
091
.80
aN
Dno
tde
tect
ed.
b 18h
atpH
4.cIn
corr
ect
anal
ysis
.
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�47 43
fraction of 3.54%, were analysed in gabbro A, amaterial quarried in connection to a sulphidemine. Both arsenic and cadmium are often associ-ated to sulphides.
The availability for leaching of a certain metalvaries depending on if it exists as a major part ofa pure mineral or only as a trace, substitutingions in a mineral. In the rock materials, metalssuch as zinc, lead, copper, nickel and cadmium,bound or associated to minerals which are un-stable under oxidising conditions, were releasedto a higher percentage and also in quantity, com-pared to chromium and vanadium with much
Ž .higher total contents Appendix A, Table 6 .Inspection by optical microscopy was per-
formed on thin sections of six materials, Nos. 1�6,in order to identify heavy metal containing miner-als. No pure chromium mineral was observed inany of the materials. Chromium is, however, acommon substitute in minerals as magnetite, il-menite and pyroxene and these minerals wereidentified in the basic rock materials, gabbro�di-orite, gabbro A and gabbro B. Magnetite andilmenite both were also identified in the two
Žgravels, and magnetite in Gneiss A data to be.published . Vanadium is a common substitute inŽ .magnetite, Deer et al., 1996, p. 428 . Most likely,
vanadium and chromium in the investigated rockmaterials, to a large extent, were present as ionicsubstitutes in other crystals and this can explainthe low leachability, compared to elements pre-sent in pure minerals. This shows that, regardingthe leachability of rock materials, it is decisiveand of greater importance how the element ispresent in the mineral, than the total content.
Two types of gravel and seven rock materialswere examined. From this limited investigation itis not possible to draw any far-reaching conclu-sions regarding differences in the leachability forrock materials and gravels. However, the leach-ability of zinc and lead in gravel B were notablymuch higher than in the other materials, Table 5.In gravel B 20% of both lead and zinc werereleased compared to 7.2% zinc and 4.5% lead asmaximum values in the seven rock materials. Re-garding the other investigated heavy metals, theleachability in the gravels compared to the rock
materials was in most cases in the mean or upperŽ .range Appendix A, Table 5 .
At inspection by optical microscopy of bothtypes of gravel and four of the rock materials thesame kind of heavy metal containing mineralswere identified in the two gravels and in the fourrock materials. As the source of gravel is frommany different rocks from different localities, thecomposition of gravels varies and they also mightcontain a variety of minerals, some with highcontent of heavy metals and some with high dis-position for leaching. A higher leachability ofelements in the gravel compared to the rockmaterial might be explained by the fact that thegravel has been weathered and exposed to bacte-ria in the environment.
Compared with the ordinary test, the test withcontrolled positive potential resulted in an obvi-ous increase in the leachability of elements boundto minerals unstable in an oxidising environmentŽ .Table 6 .
Sulphides are unstable at oxidising conditions,and the elements zinc, copper, lead, nickel,cadmium and mercury, usually bound to or asso-ciated to sulphides, increased in leachability inthe oxidised availability test.
In the ordinary test, the concentration of mer-cury in the leachates was not detectable for anyof the four tested materials, but in a controlledoxidising environment between 50% and 70% wassoluble in three of the materials, expressed asmean values for the duplicates. The solubility ofcadmium increased from 7.5% to �30% ingabbro A, and from 13.5% to �30% in gravel A.These two materials have a relatively high totalcontent of cadmium, 0.18% and 0.28%, respec-tively.
As presented in Table 3, the redox potentialsmeasured in the combined leachates in the ordi-nary tests were positive in all cases, 444�619 mV,and indicated oxidising conditions during the ma-jor part of the test. The changes in pH and redoxpotentials at a controlled oxidising state weresmall, particularly for No. 5, gravel A, and No. 9,gneiss B.
Nevertheless, in the oxidised test the solubilityof sulphide bound elements increased as de-
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�4744
scribed above, and the solubility of iron decreasedŽ 2 3 .due to the oxidising Fe to Fe with approxi-
mately two orders of magnitude of the value inthe ordinary availability test. It shows that theconditions partly have been reducing during theordinary test. This implies that as the redox po-tential is varying during the test, the analysis ofcombined leachates gives limited information anda higher rate of oxidation has an obvious influ-ence on the leachability of sulphide bound heavymetal elements in rock material.
The total content of sulphur and vanadium inthe Swedish blast furnace slag is notably high.When the material is evaluated as road construc-tion material it is of special interest to investigatethe leachable fraction of these elements. Com-pared to the rock materials, the leachable frac-tion of both sulphur and vanadium in the slagswas higher according to the availability test, ex-pressed both as percentage of the total contentand in mg kg.
The content of vanadium varied from 17 to 282mg kg in the natural materials, and the two blast
furnace slags both contained 350 mg kg. Onlyone of the nine natural materials had a leachablefraction, 2.9 mg kg, which it was possible todetect, but from the slag 63.5 mg kg was re-leased. The total content of chromium variedfrom 20 to 331 mg kg in the rock materials whichwas high compared to 27 and 30 mg kg in theblast furnace slags, but the leachable fraction wasbelow 0.3 mg kg in all nine rock materials com-pared to 0.6 mg kg in one of the slags. As dis-cussed above, vanadium and chromium in therock materials most probably are present as ionicsubstitutes in other minerals, e.g. magnetite.
The investigated natural materials containhigher total contents of the heavy metals copper,nickel and zinc, and the fractions that are avail-able for leaching, in mg kg, are higher than inthe slags, especially when the conditions are oxi-dising. Fig. 2 shows the total content and theleachable fraction of copper expressed as mg kgin the natural materials and in the two blastfurnace slags. The mean values from the dupli-cate tests are used. Expressed as a percentage of
ŽFig. 2. The total content and the available fraction of Cu in four natural materials and two blast furnace slags Fallman, 1993;¨.Rogbeck and Elander, 1995 .
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�47 45
the total content the availability for many ele-ments is lower in the rock materials than in theslags, as is shown in Table 8.
This shows that in spite of low contents ofsulphur and heavy metal element in the rockmaterial, the available amount of many elementsis higher, especially if conditions are oxidising,than for the blast furnace slag, because the totalcontent of heavy metal elements is very low in theslag.
The particle size distribution and the specificsurface area of the investigated materials bothhave influence on the leaching process, in theaspect that an increased particle size results in aprolonged time for the diffusion of ions and anincreased specific surface enhances the solubilitycontrolled leaching.
The specific surface area after crushing andgrinding varied between the materials dependingon the original particle size distribution, the min-eral composition, the grindability of the mineraland the grinding performance. The amount offines has a large influence on the value of thespecific surface area.
As can be seen from Table 2 and Fig. 1 boththe median size and the specific surface areavaried between the materials with the same maxi-
Ž .mum size expressed as 95% . Gravel B with95% 160 �m and gneiss B with 95% 150 �m,contained the coarsest particles, but had a speci-fic surface area in the same range as five of theother materials prepared according to the testspecifications. As discussed above, the diffusionconstant for many elements in the rock material
Ž .is low Faure, 1991 , and a prolonged durationtime in the second step for three materials didnot result in an obvious increase in leachabilitycompared to the other six. Probably the recordeddifferences in particle sizes have a minor effecton the leachability for the tested materials.
As shown in Table 5 the leachability of, e.g.cadmium and lead is high for the coarsest materi-als, gravel B and gneiss B, compared to the othermaterials.
5. Conclusions
The availability for leaching of heavy metals
and sulphur in rock material and gravel dependson the mineral composition and the environment.How the elements are attached to the mineral ismore decisive than the total content of the ele-ments.
Heavy metals, such as zinc, lead, copper andcadmium which are bound or associated to sul-phides in rock materials, show a relatively highavailability for leaching, especially if stable oxidis-ing conditions are maintained. These elementsare the most leachable ones, expressed as per-centage, among the discussed heavy metals in theinvestigated rock materials.
Compared to blast furnace slag, the releasedamount of many heavy metals is higher in rockmaterials, because the total content of these ele-ments in blast furnace slag is very low. However,the leached amounts are �ery small. Expressed aspercentage of the total content, the availability forleaching of many elements is generally lower inthe rock materials.
Gravels seem to be more unpredictable regard-ing both total content and leachability of ele-ments compared to rock materials. The total con-tent and the soluble fraction of heavy metals bothcan be high.
The potentially leachable amount of heavymetals in rock materials and gravels can be higherand vary a great deal, compared to blast furnaceslag, as rock materials and gravels vary in compo-sition. Blast furnace slag has an even compositionwith small variations and the leaching behaviouris by now relatively well investigated as comparedto natural road-making materials.
When the environmental impact of natural andalternative road construction materials is evalu-ated, it is pertinent to investigate the materialswith an LCA, including the production of thenatural material at the quarry, as well as the useof a waste material which result in less landfilling.The investigated leachability of the materials isone factor of impact but in many cases not aserious one.
Acknowledgements
This paper contains results from a researchproject focusing on environmental impact from
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�4746
natural roadmaking materials. The financial sup-port was provided by MIMER, Minerals and Met-als Recycling Research Centre. The authors thank
Dr Lotta Lind, SSAB Merox AB, for helpfuldiscussions and comments during the work.
Appendix A: Total content and leached amounts in mg kg for the tested materials
1. Gabbro�diorite 2. Gabbro A 3. Gneiss A 4. Gabbro B
Total Avail. Ox. Avail. Total Avail. Ox. Avail. Total Avail. Total Avail.
Ca mg kg 49 100 1910 2000 43 800 1310 1280 9900 850 87 900 1830Fe mg kg 6600 894 5.2 74 800 414 3.6 17 000 257 85 000 216K mg kg 13 300 1460 2500 14 100 381 635 34 700 446 2300 588Mg mg kg 27 600 429 142 28 200 154 154 3500 71.8 47 000 188Na mg kg 29 700 152 71.0 24 600 103 190 400 116 18 000 158Si mg kg 261 300 503 201 258 900 184 208 340 000 172 217 000 277
Al mg kg 85 200 173 97 78 900 74 126 73 600 145 83 600 109aAs mg kg 5.46 0.16 ND 104 3.29 0.63 0.32 ND 1.28 ND
Ba mg kg 693 52.5 33.9 386 7.8 10.7 653 4.1 368 15.1Cd mg kg 0.08 0.02 0.06 0.18 0.01 0.06 0.08 0.02 0.06 NDCo mg kg 16.9 0.5 1.8 19.2 0.5 2.1 3.7 0.1 22.5 0.3Cr mg kg 121 0.33 0.26 161 0.30 0.25 20.3 0.12 331 0.15Cu mg kg 39.4 0.22 9.11 56.3 0.20 7.07 11.5 0.76 61.8 0.25Hg mg kg ND ND ND 0.153 ND 0.076 ND ND 0.198 0.003Mn mg kg 973 52.2 37.1 1150 39.8 12.9 400 25.9 1450 18.7Ni mg kg 29.5 1.1 2.1 29.2 2.0 3.4 3.8 0.4 38.8 0.8Pb mg kg 10.7 0.09 0.41 6.3 0.03 0.30 10.3 0.32 3.3 0.06Sr mg kg 588 5.7 4.7 367 3.2 2.6 72.1 0.9 760 8.3S mg kg 1400 38.2 133 2000 45.6 196 86.5 ND 2000 27.8V mg kg 177 ND ND 282 ND ND 17 ND 233 NDZn mg kg 58.9 4.2 17.8 75.7 2.6 12.4 53.9 2.4 38.5 2.4
b b b5. Gravel A 6. Gravel B 7. Granite 8. Lepite 9. Gneiss B
Total Avail. Ox. Avail. Total Avail. Total Avail. Total Avail. Total Avail. Ox. Avail.
Ca mg kg 24 700 830 800 9600 2230 25 400 570 20 400 900 20 600 1290 1340Fe mg kg 39 400 389 4.2 17 700 163 38 700 478 25 500 616 34 100 414 7.5K mg kg 19 600 845 738 32 800 603 24 600 1290 20 300 1661 35 000 1828 1210Mg mg kg 15 300 126 113 6200 86.2 14 800 108 7400 110 12 900 137 160Na mg kg 25 400 83.1 210 240 54.0 28 700 75.5 28 300 86.2 25 700 42.5 186Si mg kg 315 000 189 173 346 000 224 303 800 177 330 000 181 309 900 194 266
Al mg kg 74 600 93 124 69 900 55 77 800 93 73 600 81 81 500 98 198As mg kg 24.6 0.30 0.07 1.93 0.08 11.1 0.19 10.2 0.25 0.30 ND NDBa mg kg 628 15.9 13.2 573 6.7 744 17.6 622 5.1 857 11.1 12.1Cd mg kg 0.28 0.04 0.09 0.13 0.04 0.23 0.04 0.23 0.02 0.08 0.03 0.03Co mg kg 9.0 0.6 0.9 6.3 0.3 9.7 0.6 5.0 0.2 9.3 0.2 0.3Cr mg kg 151 0.26 0.29 37.8 0.10 107 0.29 44.3 0.14 114 0.16 0.25Cu mg kg 33.2 0.19 4.39 17.5 0.25 61.6 0.16 62.8 0.51 17.3 0.38 4.03Hg mg kg ND 0.179 0.125 0.053 ND ND ND ND ND 0.063 ND 0.033Mn mg kg 752 36.4 16.9 240 19.1 690 45.0 620 71.4 651 20.2 14.3Ni mg kg 24.0 1.8 2.3 9.6 0.6 22.7 1.5 2.8 0.7 16.9 0.6 0.5Pb mg kg 8.8 0.14 0.26 8.3 1.82 9.7 0.35 7.5 0.07 6.4 0.20 0.35Sr mg kg 235 2.1 1.8 98.8 2.3 400 2.4 236 1.7 235 2.1 2.2S mg kg 1000 56.6 248 500 24.1 1000 92.9 ND ND ND ND 58.2V mg kg 95.2 ND ND 32.8 ND 115 ND 37.6 ND 83.4 ND 2.9Zn mg kg 84.5 3.9 12.5 45.2 8.0 69.7 2.8 114 3.6 79.5 2.8 7.6
a ND, not detected.b Eighteen hours at pH 4.
( )M. Tossa�ainen, E. Forssberg The Science of the Total En�ironment 239 1999 31�47 47
References
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.chemistry in Sweden . Rapport 4415, Naturvardsverket,˚ISBN 91-620-4415-X, 1995 in Swedish .
¨Anon. Inventering av naturgrus och bergtillgangar inom Alvsby˚Žkommun Inventory of the resources of rock material and
¨ .gravel within Alvsby municipality . Lansstyrelsen i Norrbot-¨tens lan, Miljoenheten, 1992 in Swedish .¨ ¨
Anon. Grus, sand och industrimineral. Produktion ochŽtillgangar. 1996 Aggregates and industrial minerals. Pro-˚
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in Swedish, used by permission .Fallman A-M. Performance and design of the availability test¨
for measurement of potentially leachable amounts fromwaste materials. Environ Sci Technol 1997;31:735�744.
Fallman A-M, Hartlen J. Utilisation of electric arc furnace¨ ´slag in road construction. In: Kamon, editor. Environmen-tal geotechnics. ISBN 90 54108487. Rotterdam: Balkema,1996.
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NEN 7341. Leaching characteristics of building and solidwaste materials � Determination of the availability ofinorganic components for leaching. Delft: Netherlands
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OECD. Road transport research, Recycling strategies for roadworks, ISBN 92-64-15461-2. OECD, 1997.
Rogbeck J, Elander P. SGI, Vag E4, Nykopingbro-Jonaker,¨ ¨ ¨ ˚ŽMiljokonsekvenser vid anvandning av hyttsten Road E4,¨ ¨
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Paper II
Paper III
Paper IV
Leaching results of reactive materials
Mia Tossavainen1a, Lotta Lindb
aDivision of Mineral Processing, Luleå University of Technology, SE-971 87 Luleå, Sweden
bAB Sandviks Material Technology, SE-811 81 Sandviken, Sweden
Submitted to Construction and Building Materials, September 2005
1 Corresponding author., tel.;+46 920 491841; fax; +46 920 97364 E-mail address: [email protected]
Abstract
Leaching results are used in the assessment of slag for use in construction. The stability of the
major phase in iron- and steelmaking slags has a direct influence on the leaching of trace
elements. Steelmaking slags have varying content of glass and puzzolanic minerals, and water
granulation is a means of enhancing the glass content. A crystalline and an amorphous blast
furnace slag have been investigated with a long-term test to determine if diffusion or
dissolution was the controlling leaching mechanisms for each slag type. Both slag types were
leached by dissolution, though the duration of the long-term test was insufficient for
determining the subsequent progression of the release. Leaching of slags with puzzolanic
phases implies that reactions take place during the test, resulting in both dissolution and
stabilization of elements. The phase transformations probably hamper the evaluation of
leaching results and should be considered.
Keywords: Leaching mechanism, crystalline, amorphous, long-term, blast furnace, puzzolanic,
cement-forming
2
1. Introduction
1.1 Background and Aim
Rapid cooling by water granulation is a method of transforming a material into a glassy matrix
with properties different from the crystallized material. In general, an amorphous phase has
lower solubility compared to a crystallized phase with similar chemical composition.
Glass matrices with high stability are developed and used for solidification of nuclear waste,
for which long-term stability is crucial. With respect to kinetics, the amorphous phase has a
stability that resists dissolution in an aqueous solution. On the other hand, the glass phase is a
metastable solid with higher free energy than crystals of the same composition and has a
tendency to form more stable hydrated phases in aqueous solutions. According to White [1], the
resulting leaching behaviour and the final product stability vary dramatically with the chemical
composition of the glass phase and the interactions with the surrounding aqueous solution.
When a material, such as slag, is assessed for use in construction the solubility of the
material is important and the focus is on trace elements. However, as Yan and Neretnieks [2]
states is the case for combustion ashes, the glass phase is generally considered the host of the
toxic elements and the dissolution of the silicate matrix thus have a major influence on the
leaching of the trace elements.
Similar discussions regarding the leaching of trace elements in the case of nuclear waste
glass and combustion ashes may be relevant in the case of slags from iron- and steelmaking, as
minerals with puzzolanic properties and glass are often major components of these products.
Long-term investigations are necessary if we are to gain a better knowledge of the stability of
the major phase of slags processed for use in construction, and it is important that the
mechanisms that are likely to control the release of the matrix of the slag are identified.
Leaching results of both long- and short term tests are of major importance when slags are
evaluated for utilization and in such decisions it is important to understand in what way the
reactivity of the slag may influence the results.
The investigation presented here is a long-term leaching test of crystallized and amorphous
blast furnace slag. The aim was to investigate whether a granulated, amorphous slag is less
soluble than a crystallized slag with the same chemical composition. The hypothesis is that the
major leaching mechanism in the glass phase is diffusion. Short-term leaching tests have been
used for comparison and evaluation of the leaching behaviour.
3
1.2 Theory
Important mechanisms dominating the leaching of granular material are dissolution, surface
wash-off and diffusion through the material to the water-product interface [3]. Whether the
liquid percolates through the material or flows around it is generally determined by the
permeability of the material. The leaching mechanisms for a monolithic material and a granular
material compacted to low permeability are thus similar. Dissolution of a constituent takes
place until the solid is in equilibrium with the liquid phase in the leaching system (solubility
controlled) or until the constituent is completely dissolved (availability controlled). Diffusion
comprises a transfer of matter within the matrix towards the surrounding media. Where the
permeability of the material is relatively large, it is likely that diffusion dominates the release
rate [4].
The ion flux through a unit surface area of a particle under diffusion-controlled conditions is
described by using Flick’s second law of diffusion [5]:
2
2
xCD
tC
e
where C is the available concentration of the diffusing ion in the species, t (s) the leaching
time, x (m) the distance, and De (m2/s) the effective diffusion coefficient.
A leaching test for compacted granular material has been developed by Kosson et al. [6] to
determine if diffusion is the controlling mechanism in granular material and to provide
parameters regarding the release rate. If diffusion leaching is verified, the parameters that can
be determined are: the effective diffusion coefficient, De, the physical retention factor or
turtousity, , which reflects the extended path length due to the pore structure, and the chemical
retention factor, R, [3].
The effective diffusion coefficient relates to the physical and chemical retention factors as:
RDD x
e
where Dx is the diffusion coefficient for the component x in water (m2/s). For a component
considered inert (not reactive towards the matrix), R =1.
Sodium, chloride and potassium are assessed to be inert [4] and are often used for
determination of leaching rate parameters in diffusion-controlled systems.
4
2. Materials
Blast furnace slag (BF slag) is derived from the slag-forming materials, mainly limestone,
during pig iron production in the blast furnace. After tapping, the molten slag, which is very
similar to molten glass, can be cooled in two different ways: either slowly cooled in air or
semi-quenched by water granulation. The slow cooling results in a half-crystallized, rock-like
material, whereas the water granulated slag is amorphous and glassy.
For the investigations, the granulated slag was not pre-treated in any way, but was used in
the sand-like, 0-4-mm sized form in which it was produced.
The crystalline air-cooled slag, on the other hand was successively crushed and sieved so as
to obtain a sample closely approximating the particle size distribution of the amorphous slag.
The initial material had a grain size of 0-32 mm and was crushed in a Reich jaw crusher
followed by sieving on a 3.36-mm sieve-cloth.
The material used in the long-term test and the compliance test in this study was freshly
produced. For the availability test, the air-cooled BF slag was tested in two forms: freshly
produced and weathered by storing outdoors for one year.
3. Methods
3.1 Materials characterization
Total chemical analysis of the slag was performed by ICP in combination with X-ray
fluorescence. The amounts of Femet and FeO were determined by titration, and for C and S
contents determination, melt extraction in combination of infrared spectroscopy (IR) was used.
Grain size distribution was determined by sieving and the compact density investigations
were performed using a Micromeretics Multivolume Pycnometer 1305. The specific surface
area (BET) was determined with a Micromeretics Flowsorb 2300.
For the determination of the glass content of the solid slag, a representative sample was
crushed and screened. A fraction 32-40 µm was investigated with polarized light in an optical
microscope and the number of glassy, optical isotropic grains was determined.
For conductivity measurements, 25 g of slag was placed in 250 g of deionized water in an E
flask with cover. The electrical conductivity and pH were analyzed with an electrode after 1, 2,
5
3, 7, 14, 28, and 42 days, and then every week up to a total of 129 days. Analyses of the
leachates were performed by ICP-AES and ICP–MS.
3.2 Leaching tests
A long-term leaching test, based on the pre-standard NVN 7347 [7] for granular materials, was
performed in order to determine if the leaching mechanism is dissolution or diffusion leaching.
The material to be tested was placed in a 4-6-cm thick layer in a wide-necked flask,
saturated with deionized water and covered with a 2-cm layer of glass beads. After 24 hours,
the layer of glass pearls was covered with at least 5 cm of deionized water and the flask was
sealed to prevent prolonged contact with air. The free volume of water was changed after 0.25,
1, 2, 4, 8, 16, 32 and 64 days with the aim of obtaining free diffusion of ions from the material.
The collected water was mixed, filtered and analyzed for pH, electrical conductivity and ion
content. The test was done in duplicate (A and B) with 429 g and 343 g of air-cooled and
granulated BF slag, respectively, in test A and with 245 g and 141 g of each material in test B.
The determination of leaching mechanism was done with use of the analyzed amounts of the
inert elements sodium (Na) and potassium (K). By plotting the logarithm of the cumulative
release versus the logarithm of time, the slope (k) of the relation can be determined and used
for deriving the leaching mechanism. However, using the cumulative measured release implies
that deviations in a given period are included in the subsequent periods and complicate the
interpretation. The cumulative release can thus be calculated using only the release in the given
period and assuming that leaching is controlled by diffusion. The calculated cumulative release
for all N periods is then:
1,
ii
iiit tt
tMM
where Mt,i is the cumulative release of the component through the period i (mg/m2), Mi is the
release during period i (mg/m2), ti is the contact time after period i (seconds), and ti-1 is the
contact time after the period (i-1).
6
When the values for the measured and the calculated cumulative release are the same the
leaching is actually controlled by diffusion.
Verification of the findings was done according to NVN 7347 [7] from the slope (k) using
the values for three time intervals: 1-8, 2-8 and 3-8, omitting the first time fractions, as the
release may be affected by the layer of glass beads. If the slope in one of the intervals is 0.5 ±
0.1, the release is controlled by diffusion and the effective diffusion coefficient can be
calculated. Dissolution leaching results in a slope of >0.8 and a wash-off from the surface in a
slope <0.4.
A short-term leaching test was carried out using the standardized compliance test EN 12457-
3 [8]. The test is used to determine whether the material complies with leaching behaviour
identified by basic characterization tests of materials according to the European directive
regarding landfills, 2000/33/EC. The test is performed in two steps and mainly indicates the
amount of elements that are leached out as wash-off and as surface leaching (dissolution). 175
g of the material, <4 mm, was shaken with deionized water, first at L/S = 2 for six hours, and
then at L/S = 8 (new water) for 18 hours, resulting in a final L/S = 10. Conductivity and pH
were measured in the filtered leachates. The test was done in duplicate with material of the
same laboratory sample.
The availability test, NT ENVIR 003 [9], is designed for determination of the maximum
leachable part of a material under certain conditions in a geological time frame. 8 g of slag of a
particle size 95% <125 µm was leached in 800 g of water (L/S = 100) in two steps, first at pH =
7 for three hours, and then at pH = 4 for 18 hours. The two leachates were mixed and analyzed.
Duplicate tests of the same laboratory sample were performed.
4. Results
4.1. Physical properties, conductivity and pH
The results from the characterization of the materials are summarized in Tables 1 and 2. The
development of the electrical conductivity of the water in which the materials were immersed is
shown in Fig. 1. The pH in the water increased until it was stabilized after ~50 days at 10.8±0.1
for the air-cooled BF slag and 11.2±0.1 for the granulated BF slag.
7
Table 1. Chemical composition of the major elements in air-cooled (AC) and granulated (G)
BF slag.
FeO +Material Fe2O3 SiO2 Al2O3 CaO MgO K2O Na2O S
% % % % % % % %
AC BF slag 0.13 33.7 12.5 30.4 16.9 0.50 0.46 1.3G BF slag 0.32 33.2 13.4 29.9 16.2 0.54 0.54 1.2
Table 2. Fraction <1.18 mm, compact density, the specific surface area and glass content of air-
cooled (AC) and granulated (G) BF slag.
< 1.18 mm Comp.density Spec.surface area Glass contentMaterial
% g/cm3 m2/g %
AC BF slag 39.3 3.0 0.91 12G BF slag 6.0 2.7 2.63 96
Fig.1. Electrical conductivity measured for 129 days.
0
200
400
600
800
1000
1200
1400
0 20 40 60 80 100 120 140Time (days)
Con
duct
ivity
(µS/
cm)
Granulated BF slagAircooled BF slag
8
As can be seen in Table 2, the amount of fines (<1.18 mm) in the crystalline material was
higher than in the granulated material in spite of the successive crushing and sieving. However,
the specific surface area was larger in the granulated material, due to the high porosity.
4.2. Long-term leaching test
The values of the measured and the calculated release/m2 of the duplicate tests are not
presented as mean values, as the test parameters were not equal and there is probably an
imprecision in the measured diffusion area (cross-sectional area of the beaker). The measured
area (a) was 0.63 dm3 and 0.67 dm3 in test A and 0.50 dm3 for both slag types in test B. The
measured cumulative releases of both tests, A and B, are shown in Table 3.
Table 3. Measured cumulative release (mg/m2) of the major elements from the air-cooled (AC)
and the granulated (G) BF slag in duplicate, A and B, of the long-term test.
Test A Test B
Element AC BF G BF AC BF G BF
mg/m2 mg/m2 mg/m2 mg/m2
Ca 31148 3741 27574 3140
Fe 4.6 0.9 3.9 5
K 15501 1820 12130 1581
Mg 234 372 207 366
Na 6557 3031 5345 2443
S 288268 6330 224574 5203
Si 657 1589 592 1166
Al 59 33 145 68
.9
The results for the duplicate tests showed similar development regarding the release, and the
values of Test A are summarized in release plots for Na and K from the air-cooled BF slag and
the granulated BF slag, Fig. 2 and Fig. 3. The content of K was below the detection limit of the
instrument in the three first leachates with the granulated BF slag, Fig. 3.
9
Fig. 2. Release plot for Na and K from air-cooled BF slag. Cumulative release (x); calculated
cumulative release ( ).
Na from Aircooled BF slag
100
1000
10000
100000
1 10 100 1000 10000time (hour)
cum
ulat
ive
rele
ase
(mg/
m2)
K from Aircooled BF slag
100
1000
10000
100000
1 10 100 1000 10000time (hours)
cum
ulat
ive
rele
ase
(mg/
m2)
10
Fig. 3. Release plot for Na and K from granulated BF slag. For symbols, see Fig 2.
Na from Granulated BF slag
10
100
1000
10000
1 10 100 1000 10000time (hour)
cum
ulat
ive
rele
ase
(mg/
m2)
K from Granulated BF slag
10
100
1000
10000
1 10 100 1000 10000
time (hour)
cum
ulat
ive
rele
ase
(mg/
m2)
Conductivity and pH values of the leachates are given in Table 4. The slope (k) of the
relation: time and cumulative release of Na and K for the time intervals 1-8, 2-8 and 3-8 for
both tests, A and B, is shown in Table 5.
11
Table 4. pH and conductivity ( S/cm) in the analyzed leachates of air-cooled (AC) and
granulated (G) BF slag in Test A.
Release time (days)Material 0.25 1 2 4 8 15 32 58
AC BF pH 9.0 8.1 8.6 8.6 9.3 9.3 9.8 9.9S/cm 97.1 116.5 193.8 248 346 541 947 1077
G BF pH 8.6 8.1 7.7 9.1 9.0 9.2 9.1 9.1S/cm 44.2 22.8 19.4 27.1 58.4 99.1 175.1 182.8
Table 5. The slope (k) of the cumulative release and time relation for sodium, k(Na), and for
potassium, k(K), in duplicate (A and B) of the long-term test of air-cooled (AC) and granulated
(G) BF slag.
Test A Test BAC BF slag G BF slag AC BF slag G BF slag
Time interval k (Na) k (K) k (Na) k (K) k (Na) k (K) k (Na) k (K)
1-8 0.72 0.71 0.75 0.83 0.62 0.66 0.63 0.692-8 0.68 0.64 0.92 0.95 0.62 0.63 0.78 0.783-8 0.58 0.53 0.99 1.02 0.70 0.64 0.79 0.83
The test was completed in 58 days. The liquid/solid relation (L/S) for the accumulated
leachates was 9.4 for the air-cooled BF-slag and 12.4 for the granulated BF-slag in Test A and
9.8 and 14.5, respectively, in Test B. This is a deviation from NVN 7347, in which the flask
should be immersed in a large vessel with water, and the L/S ratio thus become higher.
12
5. Discussion
5.1. Long-term leaching and leaching mechanism
The values of the measured and the calculated release were not equal for Na or K in either of
the tests, which should be the case if leaching was controlled by diffusion. As can be seen in
Fig. 2 and Fig. 3, there was an obvious change in the release slope towards the end of the test
for both materials. The k-values (slope) for the three time intervals in Table 5 showed similar
development for both K and Na in the granulated BF slag in the duplicate tests and indicate that
the leaching is controlled by dissolution from the surface (k>0.8). The leaching from the air-
cooled BF slag, on the other hand, seemed to develop towards diffusion (k=0.5) for K in test A.
The k-values for test B were different, which is why the results are not sufficient for
determination of the leaching mechanism according to the method described in NVN 3747.
The cumulative amounts of dissolved K and Na were significantly higher from the air-
cooled BF slag compared to the granulated slag and, as can be seen in Fig. 1 a similar
behaviour, measured as conductivity, were found when the material was immersed in water for
129 days. This difference between the materials is probably due to the higher surface reactivity
of the freshly crushed air-cooled BF slag and the fact that the granulated material was already
washed during the water granulation.
Both Mg, Si, Al and Ca are major elements in the BF slag, and the air-cooled slag consists
largely of ternary compounds of theses elements, of which the most common is melilite, a
series of solid solutions from akermanite (2CaO ·MgO · 2SiO2) to gehlinite (2CaO ·Al2O3
·SiO2). These minerals react in aqueous solutions and transform into more stable hydrated
phases. It seems plausible that such phases, which reduce the dissolution towards the end of the
test, are formed on the surfaces of the air-cooled BF slag.
The change in the release towards the end of the test, from both the air-cooled and the
granulated BF slag, shows that the duration of the long-term test was not sufficient for
determining how the solubility, or stability, of the materials will develop under such conditions.
13
5.2. Comparison with the compliance test and the availability test
The release of major elements was higher from the air-cooled than from the granulated slag in
both the long-term test and the compliance test. Exceptions are magnesium (Mg) and silicon
(Si). The releases of these two elements, as well as the inert elements, Na and K, according to
the three leaching tests are shown in Fig. 4. The deviation from the mean value for the
duplicate of the compliance test was low for all the elements, <1% for the granulated slag and
<2% for the air-cooled slag.
Fig. 4. Cumulative release of Na. K. Mg and Si (mg/kg) from air-cooled and granulated BF
slag according to the long-term leaching test, the compliance test and the availability test
(values for the weathered air-cooled slag).
Release of Na
1
10
100
1000
10000
cum
ulat
ive
rele
ase
(mg/
kg) Air-cooled BF slag
Granulated BF slag
long-term test compliance test availability test
Release of K
1
10
100
1000
10000
cum
ulat
ive
rele
ase
(mg/
kg)
Air-cooled BF slagGranulated BF slag
long-term test compliance test availability test
14
Release of Mg
1
10
100
1000
10000
100000cu
mul
ativ
e re
leas
e (m
g/kg
)
Air-cooled BF slagGranulated BF slag
long-term test compliance test availability test
Release of Si
1
10
100
1000
cum
ulat
ive
rele
as (m
g/K
g) Air-cooled BF slagGranulated BF slag
long-term test compliance test
The long-term test and the compliance test resulted in similar amounts of released Na and K.
The L/S relation was in the same range in both tests2, but the time and the percolation of the
liquid were completely different. The compliance test ([8] is designed to determine what can be
dissolved, assuming the system reaches equilibrium or semi-equilibrium during the test. The
similar results from the two tests thus support the findings that the major leaching mechanism
for both materials in the long-term test probably is dissolution. It is not obvious how the result
would have differed with a higher L/S ratio according to the pre-standard NVN 7347 [7].
The varying release of Mg and Si in the three tests (Si not analyzed in the availability test),
shows that these elements are present in minerals that react with the leachant and leaching
parameters, such as L/S, percolation, etc, have a major influence on the result. Quenching the
2 L/S 10 for both slag types in the compliance test, L/S 9.4 (AC BF slag) and 12.4 (G BF slag) in the long-term test.
15
molten slag, thereby increasing the glass content, is a means of enhancing the cementitious
properties of the slag, as has been reported by several authors e.g., Daugherty et al. [10],
Murphy et al. [11] and Shij [12]. Immersion in water, for such a material, implies a greater
extent of phase transformations during the leaching test compared to the air-cooled material
and may explain the higher release of Mg and Si from the granulated BF slag in all three tests.
The availability test [9] is designed to optimize leaching conditions; e.g., high surface area,
high L/S ratio and intense percolation. A material with puzzolanic properties has ample
opportunity to undergo several phase transformations, resulting in both stabilizing and
dissolution of elements. The deviation from the mean values of dissolved elements in the
duplicate of the availability test of the fresh air-cooled, the weathered air-cooled and of the
granulated BF slag is shown in Fig. 5. The results of the inert elements Na and K and the major
elements, Ca, Al, Mg and S, are plotted. As can be seen, the deviation is high, particularly for
the granulated slag, and implies that the difference in release of K from the two slag types with
the availability test in Fig. 4 is not significant.
Fig. 5. Deviation from the mean value of dissolved element in the duplicate of the availability
test, expressed in percentage (%).
0
5
10
15
20
25
30
35
40
45
50
Granulated BF slag fresh AC BF slag weathered AC BF slag
mea
n va
lue
+/-
(%)
K Na Al Ca Mg S
The differences in the correlation of the duplicate tests for the three materials may be due to the
varying reactivity of the materials, and indicate in that case that the weathered slag has become
16
more reactive during storage, probably due to reactions with the atmospheric moisture and
CO2, etc.
The impact of the puzzolanic properties on the leaching result seem to be obvious when
comparing the results with the availability test of the air-cooled and the granulated BF slag in
this investigation, with results of two materials with no cement-forming minerals: a crystalline
rock material, gabbro-diorite, and an amorphous fuming slag with a concentration of elements
almost identical to the composition of the mineral fayalithe (Fe2SiO4) [13]. The solubility of Ca
and Mg was 45% and 42% in the granulated BF slag, and 23% and 15% in the air-cooled BF
slag. The release from the gabbro-diorite was 3.9% and 1.6%, and from the fuming slag 0.9 %
and 0.5%, of Ca and Mg, respectively.
The fuming slag is almost completely glassy but contains no puzzolanic minerals and
remains very stable during the availability test, while both types of the BF slag react with the
water and are dissolved to a high degree. The stabilizing reactions of the BF slag seem to have
little impact in this test, and equilibrium is most likely not reached in that respect. This very
important property of the slag may hamper evaluation of leaching results if the reactions have
only partially taken place. Similar discussion should be conducted regarding results from other
leaching tests of reactive materials, such as BF slag and other slags from iron-and steel making.
This effect is most likely not as enhanced in all leaching tests.
6. Conclusions
Both the crystalline and the amorphous BF slags showed mainly dissolution-controlled
leaching in the three different tests in this investigation. The duration of the long-term test was
not sufficient for determining if, and when, diffusion controls the release from either of the two
types of BF slag.
Quenching of a slag for increased glass content and reactivity implies a greater extent of
phase transformation in contact with water and results in both dissolution and stabilizing of
elements. The stabilizing reactions have probably only partially taken place in the tests used in
this investigation.
17
The reactions of the cement-forming minerals in slags hamper the evaluations of leaching
results and should therefore be considered. The impact of the reactivity seems to be enhanced
in the availability test in which the leaching conditions are optimal.
Minerals with puzzolanic properties in a slag will probably react with moisture, CO2, etc.
during storage and to some degree undergo phase transformations, which may result in leaching
results that differ from those of freshly produced slags. Amorphous slag without cement-
forming minerals, on the other hand, shows low solubility and a leaching behaviour much
different from those slags investigated here.
Acknowledgements
This work was financed by MiMeR, the Minerals and Metals Recycling Research Centre, and
Vinnova. The authors wish to thank the members of MiMeR for the opportunity to present the
data and results.
18
References
[1] White WB. Dissolution mechanism of nuclear waste glasses: A critical review. In:
Advances in Ceramics, 20 Nuclear Waste Management II, Editor: Clark DE, White WB,
Machiels AJ. the American ceramic Society, 1986, p. 431-442.
[2] Yan J, Neretnieks I. Is the glass dissolution rate always a limiting factor in the leaching
processes of combustion residues? The Science of the Total Environment, 1986; 172: 95-
118.
[3] Chandler AJ, Eighmy TT, Hartlén J, Hjelmar O, Kosson DS, Sawell SE, van der Sloot HA,
Vehlow J. Municipal Solid Waste Incineration Residues. Elsevier, Amsterdam, 1997.
[4] van der Sloot HA., Heasman L, Quevauviller Ph. Harmonization of Leaching / Extraction
tests. Studies in Environmental Science 70. Elsevier, Amsterdam, 1997.
[5] Crank J. The mathematics of diffusion. Clarendon press, Oxford, 1975.
[6] Kosson DS, Kosson TT, van der Sloot HA. Evaluation of Solidification / Stabilization
Treatment Processes for Municipal Waste Combustion Residues. NTIS PB93-229 870/AS,
1993.
[7] NVN 7347, pre-standard. Determination of the maximum leachable quantity and the
emission of inorganic contaminants from granular construction materials and waste
materials - the compacted granular leach test. Draft revised November 1996. Nederlands
normalisatie instituut, Delft, The Netherlands. 1994.
[8] EN 12457-3. Characterization of waste – Leaching – Compliance leaching of granular
waste materials and sludges - Part 3: two stage batch test at a liquid to solid ratio of 2 and 8
19
l/kg for materials with particle size below 4 mm (without or with size reduction). CEN,
2002.
[9] NT ENVIR 003. Solid wastes, Granular inorganic material: Availability test. 1995:11.
ISSN 1238-4445, NORDTEST, Espoo, Finland, 1995.
[10] Daugherty KE, Saad B, Weirich C. Eberendu A. The Glass Content of Slag and Hydraulic
Activity. Silicates Industriels, 1983. 4-5, p 107-110.
[11] Murphy JN, Meadowcroft TR, Barr PV. Enhancement of the cementitious properties of
steel-making slag. Canadian Metallurgical Quarterly, Vol 36, No 5, 1997, p 315-331.
[12] Shij C. Steel slag – its Production, Processing, Characteristics, and Cementitious
Properties. Journal of Materials in Civil Engineering. ASCE. 2004, p 230-236.
[13] Tossavainen M, Forssberg E. Studies of the leaching behaviour of rock material and slag
used in road construction: A mineralogical interpretation. Steel Research, 71/2000, No
11, 2000, p 442-448.
20
Paper V
CHARACTERISTICS OF MODIFIED STEEL SLAGS FOR USE IN
CONSTRUCTION
M. Tossavainen1a, Q Yangb, F. Engströmb, N Menadb
aDivision of Mineral Processing bDivision of Process Metallurgy
Luleå University of Technology
SE-971 87 Luleå
Sweden
Submitted to Waste Management, May 2005
1 Corresponding author; [email protected], tel.;+46 920 491841; fax; +46 920 97364
2
Abstract
Four types of steel slag have been modified by semi-rapid cooling in crucibles and rapid
cooling by water granulation. The aim was to determine if rapid cooling results in a glassy slag
with improved properties regarding leaching and volume stability.
Optical microscopy, XRD, SEM, an expansion test and a compliance test for leaching have
been used for investigation of a ladle slag, a BOF slag and two different EAF slags.
The disintegrated ladle slag is made volume-stable by water granulation. The volume
stability of the granulated BOF slag is most likely improved, as MgO and Ca3SiO5, the major
silicate phase, were observed distributed in a glass-containing matrix.
The results of the leaching test, prEN 12457-2, show that the glass-containing matrix does
not prevent leaching of minor elements. The solubility of chromium, molybdenum and
vanadium varies in the three modifications, probably due to their presence in different minerals
and their different distributions.
The long-term leaching behaviour of the slag must be investigated for an assessment of the
stability in construction.
Rapid cooling reduces the number of phases in the slag and consequently the control of the
slag properties is improved.
A better understanding of the mineral formation during cooling is important for control of
the leaching of specific elements.
Key words: steel slag, modifications, water granulation, matrix, stability, leaching, volume
stability
3
1. Introduction
Large quantities of material are used in the construction and maintenance of roads each year. In
Sweden, the production of rock material in 2003 was 70 million tonnes, 50% of which was
used for road making and 10% in the manufacture of concrete (SGU, 2004). In Sweden, two
interim targets regarding the environmental quality objective “A Good Built Environment” are
that (by 2010) reused materials will represent at least 15% of the aggregate used and landfilled
waste will be reduced by at least 50% by 2005 compared to 1994 (http://miljomal.nu, 2004).
Due to its high strength and durability, steel slag is a construction material that is in many
cases superior to rock material, and using slag in construction also helps to reduce the amount
of landfilled waste. In 2002, only 25% of the Swedish steel-slag production (896 Kt) was sold
as external products (private communication).
In addition to the lack of rules and guidelines regarding testing, assessing and using slag in
Sweden, the technical and environmental obstacles for use of slag in construction include low
volume stability and leaching of elements. Other impediments are a long tradition and
knowledge of using rock material and the fact that in Sweden there are still quite good
resources of high-quality rock material. The fear that slag, generally, is environmentally
hazardous is also something that has to be considered.
By modification of the cooling conditions, both solubility and volume stability can be
affected positively. Rapid cooling by water granulation can result in an amorphous slag,
enclosing metals, and thereby having lower solubility of the metals compared to rock material
used for road making (Tossavainen and Forssberg, 2000). The formation of a glassy material
depends on both the chemical composition and the cooling conditions. According to Daugherty
et al. (1983), glass was easier to produce, as the acidity of the slag increased for a series of
synthetic slag compositions that was quenched and annealed. Ionescu et al. (1998, 2001) have
shown how water quenching of steel slag results in products with a high content of glassy
material. Silicate melts have high viscosity due to long molecule chains, and rearrangement
into crystals only takes place slowly. If the cooling is rapid, the slag passes from a liquid state
to a solid without development of a crystalline structure (Lea, 1983). Glasses, such as
granulated slags, are to be regarded as greatly under-cooled liquids having a very high
viscosity.
By enhancing the amount of amorphous material in a slag the potential hydraulic properties are
increased and the material can also be used in cement and concrete products of higher quality
4
and value compared to conventional road-making materials (Murphy et al. 1997, Ionescu et al.,
1998, 2001, Shij, 2004). For a disintegrated slag, use in concrete is particularly interesting, as
grinding costs can be reduced. Gravel is used in concrete and according to the environmental
quality objectives (http://miljomal.nu, 2004), by 2010, the extraction of natural gravel in
Sweden shall not exceed 12 million tonnes per year, as compared to 20.3 million tonnes that
was produced in 2003 (SGU, 2004).
Besides glass formation, controlling cooling conditions can be a means of affecting mineral
transformation and consequently the solubility of elements like chromium. Chemical
compounds containing hexavalent chromium (Cr6+) are generally considered far more toxic
than those containing the trivalent form (Cr3+). According to Lee and Nassarella (1998), Cr6+ is
usually formed at lower temperatures and a rapid cooling reduces the formation by limiting the
kinetics of the formation.
Regarding volume stability, according to Juckes (2003), there are many mechanisms that
can cause a breakdown of steelmaking slag: an expansive phase inversion of dicalciumsilicate,
C2S, oxidation of FeO to magnetite, hydration of sulphide phases, hydration of silicate phases,
carbonation of calcium- and magnesium hydroxides and hydration of free lime and periclase,
MgO. For aggregates of steelmaking slag, the major cause for expansion is most likely
unassilimilated free lime. Volume stability is one of the most important requirements for slag
used in road making or in concrete.
This paper presents a study regarding four different types of steelmaking slags; a ladle slag,
a basic oxygen furnace slag and two types of electric arc furnace slags, modified by different
cooling conditions. The aim was to determine if rapid cooling by water granulation would
result in a glassy slag with improved properties regarding volume stability and leaching. The
disintegrating and expansion properties of the ladle slag and the basic oxygen furnace slag in
particular are examined. For electric arc furnace slag, the leaching of metals such as chromium,
molybdenum and barium is a concern. The qualities of the matrix of the modified slag are
studied and the possibility of enclosing metals in a glassy matrix, and thereby reducing the
leaching, is discussed.
5
2. Materials
2.1. Investigated steel making slag
20-30 kg representative samples of four different types of steel slag were supplied by
steelmaking companies in Sweden:
A. Ladle slag ladle slag
B. Basic oxygen furnace slag BOF slag
C. Electric arc furnace slag, type 1 EAF slag 1
D. Electric arc furnace slag, type 2 EAF slag 2
The materials, except the disintegrated ladle slag, were crushed with a jaw crusher, Retsch
BB3, to < 30-40 mm before splitting into 1-1.5 kg sub-samples.
3. Methods for characterisation
3.1. Physical properties and chemical and mineralogical composition
The total composition of each material was analyzed by Ovako Steel AB (Sweden) with
inductively coupled plasma emission spectroscopy, ICP, and x-ray diffraction spectroscopy,
XRD. Titration has been used for analysis of Fe and FeO and infrared adsorption spectroscopy,
IR, for carbon and sulphur.
The specific surface area was determined according to the BET method with a
Micromeretics Flowsorb 2300 and density was measured with a Micromeretics Multivolume
Pycnometer 1305 on material prepared for leaching, <4mm.
The expansion was analyzed by SSAB Merox AB (Sweden) according to the John Emery
expansion test corresponding to the test “Determination of the expansion of steel slag”, EN
1744-1 (CEN, 1998).
An isothermal calorimetric measurement was performed in order to investigate how the
cement hydration was affected when the disintegrated ladle slag was mixed into a cement paste
(Moosberg-Bustnes, 2003). The heat development was determined for pastes with 20% of the
6
cement replaced with the original or granulated ladle slag. Pure cement was used as a reference
material.
All the slag samples were crushed to a particle-size of <4 mm and leached according to the
one-stage batch test prEN 12457-2 (CEN, 2002a) except two samples, ladle slag and granulated
EAF slag 1, that were leached according to the two-stage batch test prEN 12457-3 (CEN,
2002b). The leaching tests were done in duplicates and the results are presented as a mean
value. The leachates were analyzed by the accredited laboratory Analytica AB (Sweden).
The mineralogy of the slag phases was studied on polished thin samples using optical
microscopy2 and scanning electron microscopy (SEM), Philips XL 30, with energy dispersive
analysis (EDX). X-ray diffraction analysis (XRD) was performed on pulverized material with a
Siemens D5000 x-ray diffractometer with an x-ray generator producing copper radiation (Cu
K ).
4. Modification trials
The slag was modified in two ways for comparison with the original slag:
1. re-melting and water-granulation (rapid cooling)
2. re-melting and cooling in the crucible (semi-rapid cooling)
The ladle slag was only modified by re-melting and water-granulation.
4.1 Crucible systems
Due to different demands on the refractory material for the slags, two different crucible systems
for re-melting were developed. A graphite crucible system was used for the ladle slag with low
content of Fe-oxides and an MgO crucible system for the two different types of EAF slag and
the BOF slag with high values of CaO/SiO2 and a high content of Fe oxides.
The graphite crucible system is shown in Figure 1. The outer crucible was made of refractory
castable (MgO 80%). With a refractory cover the system could be closed to minimize air
intrusion and oxidization of the inner, graphite crucible.
2 Ekstöm Mineral AB, Sweden
7
Figure 1. Graphite crucible system (A) and equipment for water granulation (B). (in Appendix)
The MgO crucible system consisted of an outer crucible made of castable with 94% Al2O3,
enclosing a graphite crucible and an inner MgO crucible.
4.2 Modifications
The ladle slag re-melted in the graphic crucible system became liquid within one hour. For
granulation, the liquid slag ( 1550°C) was poured into the granulation head, as shown in
Figure 1. Water jets formed in the granulation head hit the pouring slag and the generated slag
granules were collected at the bottom of the water tank. The duration for the tapping and
granulation was less than one minute.
For re-melting the EAF- and BOF slags in the MgO crucible system, a thermocouple was
placed above the slag and the heating rate was controlled to 4-6 C/minute. The time for melting
the slag varied from six to eight hours.
For the semi-rapid cooling, the re-melted slag was left to cool in the MgO crucible. The
cooling time from a temperature of 1500°C to room temperature was estimated at five hours.
5. Results
5.1 Physical properties
The re-melted slag that was left to cool in the crucibles (semi-rapid cooling) resulted in large
pieces that were crushed to <4 mm for leaching tests. The water-granulated material of the
BOF slag, the EAF slag1 and the EAF slag 2 consisted of granular particles, 2 - 4 mm. The
disintegrated ladle slag reacted in the water granulation (rapid cooling) to a non-dusting, brittle
and porous product. In Table 1, the compact density, the BET surface and the results from the
expansion test of the original and one water granulated slag sample are shown. The BET
surface was reduced substantially in the granular particles, mainly due to a reduction of the
amount of fines.
8
Table 1. The compact density (g/cm3), the BET-surface (m2/g) and the expansion (%) in the
slag samples.
Compact density BET-surface ExpansionSample Original Granulated Original Granulated Original Granulated
g/cm3) g/cm3) m2/g m2/g % %
Ladle slag 3.03 2.76 0.75 0.81 n.a. 0BOF - slag 3.53 3.65 2.35 0.21 8.5 n.a.EAF - slag 1 3.25 3.34 2.23 0.05 0.7 n.a.EAF - slag 2 3.59 3.77 1.23 0.59 0.8 n.a.*not analysed
The BOF slag expanded by 8.5%, while the volume increase for the two different types of EAF
slag was less than one percent, Table 1. Due to material shortage, the volume stability was not
tested on the modifications of these slags. It was not possible to examine the original ladle slag,
as the disintegrated material passed the bottom screen of the test equipment. The water-
granulated ladle slag, on the other hand, was tested and did not expand.
5.2 Chemical composition and leaching results
Chemical compositions of the four original slags are shown in Table 2. The content of iron
oxides is high in both the BOF slag and the EAF slag 2, and the amount of Al2O3 and MgO is
high in the ladle slag.
Table 2. Chemical composition of the original slag
% ppm
Samples Fe2O3 FeO Fe met. Al2O3 CaO MgO MnO SiO2 Cr Mo Zn Ni Cu K Na P Ti V
Ladle slag 1.1 0.5 0.4 22.9 42.5 12.6 0.23 14.2 2700 280 370 70 20 80 <20 <50 840 280
BOF slag 10.9 10.7 2.3 1.9 45.0 9.6 3.1 11.1 506 39 37 25 8 220 <10 2270 8270 14800
EAF-slag 1 1.0 3.3 0.1 3.7 45.5 5.2 2.0 32.2 32700 500 130 3180 140 590 150 <50 7910 310
EAF-slag 2 20.3 5.6 0.6 6.7 38.8 3.9 5.0 14.1 26800 70 260 90 160 <20 <20 2000 2400 1700
The solubility of five major elements in the matrix and of three minor elements expressed as
percentage of the element dissolved is shown in Table 3. The values reported by the laboratory
9
are in many cases low and the accuracy of the measurements is low. The duplicates agreed
fairly well in most cases.
Table 3. The percentage of the element dissolved with prEN 12457-2 or prEN 12457-3Slag sample Ca Mg Fe Si Al Cr Mo V
% % % % % % % %Ladle slagrapid coolinga 0.35 nd 0.02 0.03 0.22 0.008 0.04 0.14
BOF slagoriginal 2.21 nd 0.00 0.01 0.03 0.005 0.54 0.00semi rapid coolingb 1.31 nd 0.00 0.03 0.17 nd 0.19 0.01rapid cooling 0.63 nd nd 0.12 0.02 nd 0.17 0.05
EAF slag 1original 0.35 nd 0.00 0.02 0.70 0.002 0.78 0.09semi rapid cooling 0.21 0.01 nd 0.10 0.02 0.002 0.02 0.37rapid cooling 0.15 0.01 nd 0.09 0.02 0.002 0.02 0.09
EAF slag 2original 0.56 nd 0.00 0.01 1.80 0.022 1.14 0.02semi rapid cooling 0.87 nd 0.00 0.00 1.10 0.000 0.03 0.00rapid cooling 0.24 0.00 0.00 0.08 0.13 0.012 0.61 0.14nd = not detected a = re-melting and water granulation b = re-melting and cooling in the crucible
5.3 XRD
All four slags are basic (Mb3 >1) which, according to Daugherty et al. (1983), results in a
mainly crystalline slag. The value of Mb is 1.5, 3.9, 1.4 and 2.1 for ladle slag, BOF slag EAF
slag 1 and EAF slag 2, respectively. The results of the XRD investigation with graphs of the
original and the modified slags, Figure 2, show that all samples, except the granulated ladle
slag, consist largely of crystalline material. However, the peaks are quite broad and
overlapping due to amorphous material. The presence of glassy material and the very complex
composition makes the identification of phases difficult. The search and match program for
XRD analysis gives a high probability of several phases, but the fit to the peaks are not good.
The best fit phases are presented in Figure 2, and some phases listed are not likely.
Figure 2. XRD patterns of the investigated slags. A) Ladle slag, B) BOF slag,
C) EAF slag 1, D) EAF slag 2. (in Appendix)
3 Mb =(CaO+MgO)/(SiO2 + Al2O3)
10
5.4 Optical microscopy
Photomicrographs of the original slag samples are shown in Figure 3. For all four slags,
fragments with a matrix of glass and silicates (dark) enclosing opaque phases (light) are shown.
Figure 3. Optical microscopy of the original slags. A) Ladle slag, B) BOF slag, C) EAF slag 1,
D) EAF slag 2. (in Appendix)
The original ladle slag, (A) in Figure 3, consists of three types of transparent phases: fragments
of rock material, minerals and glass. The opaque phases are all iron-containing: magnetite,
hematite, an iron-rich particle and iron hydroxides were the most frequent. A few particles with
spinel composition AB2O44 were distinguished. The spinel phase is observed in the glassy
fragment as euhedral light crystals.
The original BOF slag, (B) in Figure 3, consists almost entirely of dark transparent glass-
containing fragments. The glassy matrix is iron-rich and encloses both silicates and opaque
phases. Silicate fragments were observed as well. Six different opaque phases were identified,
of which two Fe-Mg-Mn-containing and one iron-rich phase are the dominating. The two Fe-
Mg-Mn phases can be seen as the light particles in the glassy fragment in Figure 3.
Four different transparent phases, of which three were glassy, were identified in the original
EAF slag 1. The fragments contain several opaque phases. The most frequent opaque phase is a
spinel composition. It is observed in the silicate fragment in Figure 3 (C) as thin lists in
between the silicate crystals.
Four transparent phases, with varying content of silicate phases, were identified in the EAF
slag 2. They all include opaque phases. Several iron-containing phases, as well as traces of
magnetite, hematite, iron-hydroxides, and sulphides were distinguished. In Figure 3 (D) two
iron-rich phases can be seen as a corona in the matrix that consist of glass, silicate particles
(dark) and two opaque phases (light).
4 A = Mg, Fe2+, Zn, Mn2+ B = Al, Fe3+, Cr
11
The numbers of differentiated transparent and opaque phases in the original, the semi-
rapidly cooled and the granulated slag samples are listed in Table 4. Traces of minerals and
rock material are not included in the numbers. The rapidly cooled slags consist of few mineral
phases.
Table 4. The number of transparent and opaque phases differentiated with optical microscopy
in the slag samples.
Ladle slag BOF-slag EAF-slag 1 EAF-slag 2Sample transparent opaque transparent opaque transparent opaque transparent opaque
Original 3 6 2 6 4 6 4 5Semi rapid cooling 2 4 1 4 2 5Rapid cooling 1 1 2 3 2 2 1 2
6 Discussion
6.1 A mineralogical interpretation of the volume stability and the solubility
The investigations with XRD and optical microscopy are completed with SEM studies in order
to find a correlation of results to be used in an interpretation of the impact of different cooling
methods on the matrix of the slags and the effect on volume stability and leaching of minor
elements.
6.1.1 Ladle slag
The ladle slag is difficult to handle and store due to the disintegrating properties. The XRD
graphs, Figure 2, show that the ladle slag is the only one that becomes almost completely
amorphous by granulation. The major mineral in the original slag is mayenite, Ca12Al14O33,
followed in order by free MgO, periclase. -Ca2SiO4 and -Ca2SiO4 were identified. The -
form can undergo a phase transformation to -form during cooling at 400-500°C and the
volume increase (>10%) causes a pulverization of the slag (Monaco and Lu, 1996). The
expansive -Ca2SiO4 and the free MgO are plausible explanations for the disintegrated
material. The leaching solution of the original slag was not possible to filter, which might be
due to cement-forming properties of the slag.
12
Except glass, the granulated ladle slag contains one crystalline phase: unassimilated periclase,
MgO. With SEM and mapping of selected elements the two phases were identified as a matrix
consisting mainly of calcium, silicon and aluminum enclosing small fragments of MgO, Figure
4.
Figure 4. SEM picture of the water granulated ladle slag. Dark fragments of (1) MgO in a
matrix (2) with high content of calcium, silicon and aluminum. (in Appendix)
The periclase particles, MgO, are distributed in the matrix and do not expand, see Table 1. The
transformation of the original ladle slag into an almost completely amorphous phase is
probably a result of the high content of aluminum and the rapid cooling. According to Talling
and Kirvenko (1997), essential amounts of aluminium in an acidic slag result in a glassy
material, even at slow cooling. The ladle slag is not acidic, but the slag was cooled rapidly.
The result of the calorimetric measurements of cement pastes with 20% ladle slag
demonstrates that the untreated ladle slag retards the heat development curve severely, while
the granulated slag is clearly reactive and the nucleation and growth rates are almost the same
as for pure cement (Moosberg-Bustnes, 2003). This result is in agreement with the findings of
Meadowcroft et al. (1996) and Ionescu et al. (2001) that the reactivity of a slag is increased
with an increase in the glassy content.
The amorphous matrix is to some degree soluble, see Table 3. The total content and the
solubility of metals as chromium and molybdenum in the slag are very low.
6.1.2 BOF slag
The original BOF slag expanded by 8.5% in the expansion test, most likely due to free MgO as
dolomite is added to the furnace. The original BOF slag has high BET surface because of a
high content of fines and pores compared to the granulated material.
The original BOF slag consisted almost entirely of dark coloured glass fragments including
opaque iron-containing and silicate phases, see Figure 3. Similar transparent phases were
observed in the two modifications of the BOF slag. The low content of the glass-forming
elements silicon and aluminum implies a limitation of glass formation. According to the XRD
results, Figure 2, the major phase in the original BOF slag is larnite, -Ca2SiO4. With SEM,
mapping and line scan of selected elements, a high co-existence of silicon and calcium that fits
13
the findings of larnite as the major phase was shown in many fragments. Parts with high co-
existence of iron, manganese and magnesium were distinguished, as well as periclase, MgO,
see Figure 5.
Figure 5. SEM investigation of the original BOF slag. (1) calcium silicate, (2)MgO,
(3) fragment rich in iron, manganese and magnesium. (in Appendix)
According to the XRD analysis, the main mineral in the granulated BOF slag is Ca3SiO5.
Ca3SiO5 is formed as a primary phase at high temperatures but is liable to transform on cooling
to Ca2SiO4 and lime (Goldring and Juckes, 1997). Quick cooling (Luxán et al., 2000, Monaco
and Lu, 1996) as well as presence of impurity ions (Ionescu et al., 1998) prevent formation of
Ca2SiO4. In our tests, the transformation has probably taken place in the original and the semi-
rapidly cooled slags. The expansion in the original phase is likely due to the content of free
MgO and -Ca2SiO4.
With SEM, mapping and line scan, a calcium silicate phase was observed as big crystals as
well as fiber-shaped particles in the granulated BOF slag, Figure 6. The findings correspond
with the studies with optical microscopy. The euhedral prismatic microphenochrysts, that
according to Goldring and Juckes (1997) are typical for Ca3SiO5, were clearly distinguished.
Ca3SiO5 is cement-forming mineral and will hydrate and expand in contact with water.
MgO was observed with SEM as small spherical particles distributed in the glassy matrix
that contains high content of calcium and iron. The MgO has probably been exposed to water
during granulation and, distributed in the glassy matrix, will not expand.
Figure 6. SEM picture of the granulated BOF slag. (1) silicate, (2) MgO, (3) matrix with high
content of iron. (in Appendix)
The leaching of calcium and iron are reduced in the granulated BOF slag, see Table 3. Iron is
present in the glassy matrix, as discussed above, and the leaching is very low in all three slag
samples. Calcium, on the other hand, is also present in the major silicate phase, Ca3SiO5. The
solubility of silicon is increased in the granulated slag compared to the original. This might be
due to soluble Ca3SiO5 and indicates that it can be prone to hydration and expansion. The
leaching result shows that the dissolution of the minor elements is not prevented by enclosure
14
in an inert amorphous phase, see Table 3. Vanadium is most soluble in the granulated BOF
slag.
6.1.3 EAF-slag 1
The XRD graphs of the EAF slag 1, Figure 2, show that the original and the two modifications
contain a large proportion of crystalline phases. Merwinite, Ca3Mg (SiO4)2 was identified as
the main mineral in both the original slag and the two modifications. Ca2SiO4 was only
found in the original slag and might explain the high BET surface. The original slag has low
expansion value, Table 1, and no other expanding phases have been identified with XRD in the
two modifications. The EAF slag 1 has high content of silicon and the granulated slag most
likely contains more glass than the original slag in the matrix that can enclose potential
expansive phases.
The numbers of transparent and opaque phases identified with optical microscopy are
reduced from four and six, respectively, in the original EAF slag 1 to two of each kind in the
granulated slag, Table 4, which implies a more homogenous material. The number of
transparent phases is reduced already in the semi-rapidly cooled slag. With SEM, mapping and
line scan of selected elements, two matrix-forming phases and a spinel phase were
differentiated in the semi-rapidly cooled EAF slag 1, see Figure 7. Two of the phases correlate
with the XRD identification of merwinite and magnesiochromite (spinel phase). The other
matrix-forming phase contains aluminium and SEM results show co-existence with primarily
silicon, calcium and oxygen.
Figure 7. Semi-rapidly cooled EAF slag 1. Spinel phase (1), Al-Ca-Si-O phase (2), silicate
phase (3). (in Appendix)
The content of calcium and silicon is high in the EAF slag 1. The solubility of these two major
elements as well as aluminium, iron and magnesium is shown in Table 3. The leachability is
very low and varies in the three modifications. The solubility of aluminium is reduced
substantially in the semi-rapidly cooled and the granulated slag, which indicates that one of the
matrix forming phases is stable. There does not seem to be any obvious correlation between the
solubility of the major and the minor elements. The varying dissolution of the metals
chromium, molybdenum and vanadium is more likely a result of the presence in different
15
minerals and in different distributions in the material than an enclosure in glass. The solubility
of chromium is very low, 0.002% of the total chromium content, in all three samples.
Vanadium, on the other hand, is most leachable in the semi-rapidly cooled slag.
6.1.4 EAF slag 2
The XRD analysis, Figure 2, shows that the slag is very complex and some suggested phases
have varying content of substituted ions. The identified main mineral in the original slag and
the two modifications is the -phase of Ca2SiO4. The original slag did not expand and, as
discussed above for the EAF slag 1, the water-granulated slag will probably remain stable as
well.
With optical microscopy of the original EAF slag 2 several differently coloured transparent
fragments containing varying content of glass, silicates and opaque phases were distinguished,
Figure 3. The number of differentiated transparent phases is reduced from four to two in the
semi-rapidly cooled slag and to one in the granulated slag, Table 4. Five opaque phases could
be identified in both the original and in the semi-rapidly cooled slag, while only two opaque
phases were observed in the granular slag. The formation of transparent phases is slow due to
the high viscosity of the glass-forming elements and the material is more sensitive to a
reduction in cooling time. The content of iron is high in the EAF slag 2 and it is a substantial
part of the matrix. Fragments consisting of glass and small-sized transformed phases containing
iron, as well as metal droplets of the transformed phases, were observed in the granulated slag.
Calcium, iron and silicon are the major elements in the matrix of the EAF slag 2. As can be
seen in Table 3, calcium, aluminium and iron have the lowest leachability in the granulated
slag, while silicon as well as the minor elements chromium, molybdenum and vanadium are
most insoluble in the semi-rapidly cooled slag. A similar behaviour of the major elements takes
place for the BOF slag and the EAF slag 1, as well, but the effect on the leaching of the minor
elements is different.
16
7 Summary and remarks
This investigation, with four types of slag and three different cooling conditions, has resulted in
eleven slag samples with different solubility of both major elements and minor elements. The
formation of glass, major phases, phases containing minor elements and their distribution
depend on the chemistry and the cooling conditions. Each slag, and even each batch, results in
a slag material with properties that are different from the others.
As cooling time and thereby the influence of different factors increases, the number of
mineral phases formed increases. A rapid cooling e.g., by water granulation, results in a more
homogenous slag with few phases and the control of the properties is thereby enhanced.
One sample of an original slag at a plant will, in spite of standardized sampling, most likely
differ from another sample. Consequently, results from investigations of the original slag from
the same process will vary more or less.
The formation of glass in the investigated granulated slag samples has not been sufficient to
enclose the heavy metals and prevent them from leaching.
The solubility of elements such as chromium, molybdenum and vanadium for the
investigated slag types is in most cases very low in percentage and the differences between the
original and modified samples are low. Nevertheless, the difference of element dissolved can
imply that the value exceeds the limit value for inert landfill class according to the Official
Journal of the European Communities, (2003).
The steel slags in this investigation contain cement-forming minerals and glass that have a
tendency to react with aqueous solutions to form more stable hydrated phases. The reactions
have probably not taken place or the effect is low in a short-term leaching test with high L/S
(liquid/solid) content.
17
The tests used in this investigation are short-term compliance tests (24 h). The long-term
leaching behaviour needs to be investigated for an assessment of stability and use in
construction.
The volume stability of a steel slag is most likely enhanced by water granulation.
A better understanding of the mineral formation during cooling for each slag is important for
a better control of the properties.
Acknowledgements
This work was financed by MiMeR, Minerals and Metals Recycling Research Centre, and
Vinnova. The authors wish to thank the members of MiMeR for the opportunity to present the
data and for fruitful discussions about the tests and results.
18
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21
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yste
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Sys
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ion
10 20 30 40 50 60 70 80 902
Water granulation
Original
A 1 Ca12Al14O33 Mayenite
2 MgO Periclase
3 Ca2Al2SiO7 Gehlenite
4 -Ca2SiO4 Larnite
5 Ca2SiO4 Calcium silicate
2
2
2 2
2
2
22
1
1
1
11
1
11
1
35
1
5
1
41
155
4
4
51
3
1
5 51
15
10 20 30 40 50 60 70 80 90
2
Semi-rapid cooling
Original
Water granulation
B 1 -Ca2SiO4 Larnite2 Ca2Fe1.2MgO0.4Si0.4O5 Ca, Mg, Fe Silica3 Ca0.9Mn0.1O Calcium Manganese oxide4 Ca3SiO5 Calcium silicate5 Al3Ti0.78Mn0.25 Al Mn Titanium6 Mg0.9Mn0.1O Mg,Mn oxide
11
1
1
3
32
2
2
3
2
12 2 22
321
13
222
2
22
3
4
442
4
4
4222
65
4
6
2 44 4
6
4
10 20 30 40 50 60 70 80 90
Original
2
Water-granulation
Semi-rapid cooling
C
1
1
1
1156
7
6
56
11
1
1
56 6
57 6 6
1
11
111
11
11 1 7
6
7115
61
67
67
667
12
2
14
31
1
1
1
412111
4
1 Ca3Mg(SiO4)2 Mervinite
5 MgAlFeO4 Magnesium Aluminum iron oxide6 MgCr2O4 Magnesiochromite7 Al0.7Fe3Si0.3 Aluminum iron silicon
2 Ca2SiO4 Calcium silicate3 C Carbon4 Ca2Ti5SO12 Calcium titanium oxide
11
2
10 20 30 40 50 60 70 80 902
Water granulation
Semi-rapid cooling
Original
D 1 Ca2SiO4 Larnite2 FeO Wurstite3 Ca2Mg0.2AlFe0.6Si0.2O5Ca, Mg,Al,Fe silicate4 CaO Lime5 FeCr2O4 Chromite
6 MnFe2O4 Jacobsite7 MgFe2O4 Magnesite ferrite8 (Fe0.6Cr0.4)2O3 Iron, Chromium oxide9 Fe2O3 Hematite
10 Mn2AlO4 Mn, aluminum oxide
11
1
1
1765
1 1 1
675
13157
2114
1 26
571 1
31
1714
75
15
7
11
111
11
1
11111
1
1
8
8
8
5
5555
1010
10
4
1 11
11
1 11
11
1
9
9
98
8
4
Figure 2. XRD patterns of the investigated slags A. Ladle slag, B. BOF slag, C. EAF slag 1, D. EAF slag 2
23
x 160 x 80
x 80 x 160Figure 3. Optical microscopy of the original slags. A) Ladle slag, B) BOF slag, C) EAF slag 1, D) EAF slag 2
24
Figure 4. SEM picture of the water granulated ladle slag. Dark fragments of (1) MgO in a matrix
(2) with high content of calcium, silicon and aluminum.
1
2
25
Figure 5. SEM investigation of the original BOF slag. (1) calcium silicate, (2)MgO,
(3) fragment rich in iron, manganese and magnesium.
3
2
1
1
26
Figure 6. SEM picture of the granulated BOF slag. (1) silicate, (2) MgO, (3) matrix with high
content of iron.
1
2
3 1
27
21
3
Figure 7. Semi rapidly cooled EAF slag 1. Spinel phase (1), Al-Ca-Si-O phase (2), silicate phase
(3)
Paper VI
Mia Tossavainen, Fredrik Engström, Nourreddine Menad, Qixing Yang
STABILITY OF MODIFIED STEEL SLAGS
Department of Chemical Engineering and Geosciences, Luleå University of
Technology, SE-971 87 Luleå Sweden
Conference proceedings at the 4th European Slag Conference in Oulu. 20-21 June
2005
Abstract
Leaching of metals and low volume stability may constitute technical and
environmental obstacles for using slag in road building. By modifying the slag, these
properties can be improved and the use of the material in road construction can be
increased.
In order to investigate the effects on metal solubility and volume stability samples of
three different types of steel slag have been modified on a lab-scale basis, by semi-
rapid cooling in crucibles and rapid cooling by water granulation.
The three slag samples investigated, have different chemistries and the different
cooling conditions have resulted in slags with different properties.
The cooling rate has an influence on the formation and distribution of minerals and
phases and consequently the solubility. The solubility of chromium, molybdenum and
vanadium is very low in both the original and the modified slag samples.
Volume stability is most likely improved by granulation.
2
Introduction
An environmental target in Sweden is that, by 2010, reused material shall represent
at least 15% of the aggregates used (1). In 2002, only 25% (225 kt) of the steel slag
was sold as external products. Apart from the fact that leaching of metals and low
volume stability can be a hindrance for using slag in road building, experience
regarding testing, assessing and using slag for this purpose in Sweden is limited and
there are few guidelines.
By modifying the cooling conditions for slag, both the solubility and the volume
stability can be affected positively. Rapid cooling can result in an amorphous slag
having low solubility (2, 3). The formation of a glassy material is dependent on both
the chemical composition and the cooling conditions.
Besides glass formation, controlled cooling affects mineral transformation and
consequently the solubility of elements like chromium. The toxic hexavalent
chromium (Cr6+) is more soluble than the trivalent form (Cr3+), (4) and is usually
formed at lower temperatures (5). Molybdenum in steel slag is either metallic or in
oxidised form (Mo4+- Mo6+) and is soluble at neutral and basic pH (6). Depending on
the state of oxidation, vanadium is most likely found as oxides (V3+ - V5+) in the slag,
where V4+ is the most stable (7).
Rapid cooling is a way of preventing the formation of -Ca2SiO4, (C2S) (8). It also
results in lower content of free lime. These two components have a major impact on
the expansion of steel slag (9).
This paper presents a study regarding modification of steel slags with different
chemistries by different cooling methods. The aim was to investigate if rapid cooling
by water granulation results in a glassy slag with improved properties regarding
leaching and volume stability. The leaching of metals such as chromium and
molybdenum is of concern, particularly for the two types of EAF slag, and vanadium
for the BOF slag. The expansion properties of the BOF slag are discussed.
Materials
Three steel slags were selected in order to represent different types of slag: A BOF
slag and two types of electric arc furnace slag (EAF slag 1 and EAF slag 2). The
slags were selected as they have big differences in the content of both major and
minor elements. The sampled materials were analyzed with regard to chemical
composition, density, specific surface area and expansion value, which are described
by Tossavainen et al. (10). A selection of the chemical compositions is shown in
Table 1. The basicity, Mb1 is 3.9, 1.4 and 2.1 for the BOF slag, EAF slag 1 and the
EAF slag 2, respectively.
% ppmSample Fe-oxides Al2O3 CaO MgO SiO2 Cr Mo V
BOF slag 21.6 1.9 45.0 9.6 12.1 500 40 14800EAF slag 1 4.3 3.7 45.5 5.2 32.2 32700 500 310EAF slag 2 25.9 6.7 38.8 3.9 14.1 26800 70 1700
Table 1: Chemical composition of the original slag samples
Methods of Modification
The slag samples were modified on a lab-scale basis in two ways for comparison
with the original slag, which are well described by Yang et al. (11): 1) Re-melting and
water-granulation (rapid cooling) and 2) Re-melting and cooling in the crucible (semi-
rapid cooling).
31 Mb =(CaO+MgO)/(SiO2 + Al2O3)
Results and Discussion
Physical properties and solubility
The modifications were performed in order to study the effect on the matrix and the
distribution of the minor elements chromium, molybdenum and vanadium in the three
slag types.
The specific surface area (BET surface) of the water-granulated slag samples was
reduced significantly compared to the original samples, Table 2. The expansion test
(EN 1744-1) was not performed on the granulated slag samples due to shortage of
test material.
BET-surface ExpansionSample Original Granulated Original Granulated
(m2/g) (m2/g) (%) (%)
BOF - slag 2.35 0.21 8.5 n.a.EAF - slag 1 2.23 0.17 0.7 n.a.EAF - slag 2 1.23 0.59 0.8 n.a.*not analysed
Table 2: BET surface and expansion for the original and granulated slags
The slag samples were leached according to the compliance test, prEN 12457-2,
which is a short term leaching test (24 h). The results regarding chromium,
molybdenum and vanadium for the BOF slag and the two types of EAF slags are
presented in Table 3. The results are expressed as the percentage of the element
dissolved.
4
Cr Mo VSlag sample
% % %
BOF slagoriginal 0.01 0.54 0.002semi rapid coolinga nd 0.19 0.005rapid coolingb nd 0.17 0.05
EAF slag 1original 0.002 0.78 0.09semi rapid coolinga 0.002 0.02 0.37rapid cooling*b 0.002 0.02 0.09
EAF slag 2original 0.02 1.14 0.02semi rapid coolinga 0.00003 0.03 0.0009rapid coolingb 0.01 0.61 0.14nd = not detected a = re-melting and cooling in the crucible* tested with prEN 12457-3 b = re-melting and water granulation
Table 3: The leachability (%) according to prEN 12457-2.
Optical microscopy, scanning electron microscope (SEM) and x-ray diffraction (XRD)
were used for investigation of the mineralogy and interpretation of the results.
Volume stability
The original BOF slag sample expanded by 8.5 %, probably due to free MgO, as
dolomite is used in the furnace. Periclase, MgO, was observed with SEM. According
to the XRD analysis, the major phase in the original and the semi-rapidly cooled slag
is - Ca2SiO4 (C2S) and in the granulated slag it is Ca3SiO5 (C3S).
Ca3SiO5 is a cement-forming mineral that will hydrate and expand when in contact
with water. With optical microscopy a silicate phase formed to euhedral phenocrysts
was observed in the granulated slag, Figure 1. This phase was distributed in the
glassy matrix, which is rich in Fe, Mg and Mn, as well. According to Goldring and
Juckes (12) C3S occurs mostly as euhedral, prismatic microphenocrysts. 5
Figure 1: The silicate phase distributed in the matrix of Fe-Mg-Mn-phases (light
colour) and glass (dark) in the granulated BOF slag. Optical microscopy (x 40)
With SEM, mapping of selected elements, a silicate phase and small crystals of
periclase were identified distributed in a matrix of iron-rich phases and glass in the
granulated BOF slag. The enclosure in the glassy matrix will most likely reduce the
expansion of the slag.
Distribution of molybdenum, chromium and vanadium
Molybdenum
Small amounts of molybdenum are present in all three different types of steel slag
(BOF slag, EAF slag 1, EAF slag 2). Despite the low levels present, the percentage
of molybdenum leached is higher compared to the other studied elements. One
possible explanation is found in the E-pH diagram in Figure 2. In this figure, it can be
seen that when in contact with water, a high pH (10-12) and a slightly oxidising
6
atmosphere results in the formation of MoO42-. The three original slag samples have
higher solubility of molybdenum than the two modifications.
Figure 2: E-ph diagram of Mo – H2O system at 25°C
Due to the low content of molybdenum and the complex slag chemistry, no mineral or
phase containing molybdenum was identified in the slag samples.
Chromium
Chromium can form spinel minerals with magnesium, aluminium and manganese.
Such phases are identified with SEM in both types of EAF slag, but are particularly
distinctive in the EAF slag 1, Figure 3. According to this investigation the amount of
chromium dissolved from the EAF slag 1 is not influenced by the different cooling
rates, see Table 3.
The chemistry of chromium in the EAF slag 2 seems to be more complex. With SEM,
it was observed that other chromium containing minerals might be present, possibly
with calcium. As can be seen in Table 3, the solubility is very low in the semi-rapidly
cooled EAF slag 2.
7
Figure 3: Mapping picture of chromium in original EAF slag 1
Thermodynamic calculations show that the most stable chromium phases for the two
types of EAF slag are when chromium exists as a spinel together with magnesium or
iron. The major part of chromium is probably present in a spinel phase in both the
EAF slag 1 and the EAF slag 2.
Vanadium
The distribution of vanadium in the BOF slag varies in the differently cooled samples.
Figure 4 shows the distribution of selected elements in A) original and B) rapidly-
cooled BOF slag. In the original BOF slag, there is a co-existence of vanadium and
calcium, silicon and titanium. In the rapidly-cooled slag, vanadium is more evenly
distributed in the sample. The leaching of vanadium increases substantially in the
granulated BOF slag sample compared to the original sample. The different
distribution may be an explanation to the change in the solubility.
8
A B
Figure 4: Mapping picture of BOF slag: A) original, B)rapidly-cooling
Summary and Remarks
The cooling rate has an influence on the formation and distribution of minerals and
consequently the solubility of each element.
The three investigated slag types with different chemistries and the different cooling
conditions have resulted in slag samples with different incidence and distributions of
the studied elements.
The studied elements have very low solubility in both the original and the modified
slag samples.
Rapid cooling most likely enhances the volume stability of the slag.
Acknowledgements
This work was financed by MiMeR, Minerals and Metals Recycling Research Centre,
and Vinnova. The authors wish to thank the members of MiMeR for the opportunity to
present the data and the fruitful discussions about the tests and results.
9
10
References
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[4] K hn, Behmenburg, Capodilupo, Romera: Decreasing the scorification of
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between Slag and Magnesite-Chrome Refractory. Metallurgical and Materials
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[6] Jarrell, W.M., Page, A. L., Elseewi, A.A., Molybdenum in the environment.
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[9] Juckes, L. M.: The volume stability of modern steelmaking slag. Mineral
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[10] Tossavainen, M., Yang, Q., Engström, F., Nourreddine Menad: Characteristics
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in building applications. Securing the future 2005, International conference on
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