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doi:10.1016/j.bu

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Building and Environment 42 (2007) 810–819

www.elsevier.com/locate/buildenv

Recycling of stone slurry in industrial activities:Application to concrete mixtures

Nuno Almeida�, Fernando Branco, Jose Roberto Santos

Instituto Superior Tecnico, Dep. Eng. Civil e Arq., Av. Rovisco Pais,1049-001 Lisboa, Portugal

Received 2 June 2005; received in revised form 9 September 2005; accepted 27 September 2005

Abstract

To solve the problem of the waste generated by the natural stone industry, several technical solutions consider the incorporation of this

type of waste in other industrial activities as a by-product. This paper presents an overview of current solutions and the results of a

research project where natural stone slurry is used to replace fine aggregates in concrete mixtures. The concrete mechanical properties are

presented and the technical viability of this new construction material is illustrated.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Stone slurry; Microfiller; Concrete; Aggregates

1Silicates present less than 2mm, and are characterized by low stiffness,

are disintegratable, present a tendency to expand in the presence of water

1. Introduction

A high consumption level of basic raw materials by theconstruction industry generates serious depletion onmineral resources and the associated environmental da-mage. Concrete industry is particularly important as it isnot only responsible for consuming natural resources andenergy but also for its capacity of absorbing otherindustries waste and by-products. To reduce these effectsthe common practice is to substitute the more expensivecomponents (e.g. cement), thus neglecting affordablecomponents, regardless of the environmental impactsinvolved in their extraction and transformation process.

As natural sources of natural fine aggregates arebecoming exhausted, it turns out urgent to developconcrete technology capable of incorporating also ‘‘artifi-cial’’ fine aggregates, industrial by-products or waste toreduce the use of those natural resources.

As natural fine aggregates (sand) are used directly fromthe exploration sites, some technical specifications impose

e front matter r 2005 Elsevier Ltd. All rights reserved.

ildenv.2005.09.018

ing author. Tel.: +351 218 418 340; fax: +351 218 418 339.

esses: nalmeida@civil.ist.utl.pt (N. Almeida),

st.utl.pt (F. Branco), roberto@civil.ist.utl.pt (J.R. Santos).

that the clay1 content on these materials must be limited inorder to be used in concrete mixtures.The usual procedure to guarantee the fulfilment of these

specifications is to limit the quantity of material passing thesieve of 74 mm [2]. Therefore, the rejection of very finematerials based solely on dimensional criteria has beencommon practice. However, in the light of state-of-the-artconcrete technology, this practice might be a mistake [3].Concerning the natural stone extraction and processing

industries, there are huge amounts of waste generation,which is continuously accumulated in open-air dumpsitesand constitute an unsolved environmental problem. Forexample the total amount of stone slurry accumulated eachyear, in Portugal, is estimated as 600.000 ton.2

This paper presents an overview of solutions to absorbthe stone slurry and demonstrates the technical viability forproducing white cement concrete with carbonated stone

[1] and are susceptible of being adsorbed by the cement particles (thus

interfering with the hydration reactions) [2]. For these reasons, clay is not

accepted to be used in concrete mixtures.2Estimates concerning 1998 [4]. However, there are statistics of the year

2000 [5] attesting that this quantity might be significantly higher (around

1.000.000 ton).

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slurry generated from marble and limestone processingactivities.

2. Research significance

There are records about the utilization of natural stonesince the Palaeolithic Era (500.000 B.C.) [6]. In fact, in thehistory of mankind, regardless of the different civilizations,the use of natural stone for noble purposes is widelydetected. As in other activities, the industrialization ofnatural stone processing activities was inevitable, culminat-ing its growth from the 1950s.

Actually, worldwide natural stone industry offers anoutput of 68 million tons of the processed product. In 2000,the EU alone accounted for 38.8% of the total amount,assuming the leadership of the sector, followed by Asia,which owed its comfortable share of 35% to the contribu-tion of China and India [5].

Fig. 1 shows producing countries that surpassed 1million tons, representing 73% of the total productionduring 2000 [5].

According to the expected growth ratio for the sector, itis likely that the world interchange of natural stone willquadruplicate in the next 20 years [5].

Although available statistics concerning generated wasteare not always compliant with reality, and taking inaccount that the total amount of waste generated dependson technology availability, it is possible to estimate around40% of the finished stones, the amount of stone slurrygenerated along processing. Due to the huge amounts ofnatural stone slurry generated around the world, it is thenimportant to incorporate this kind of waste in otherindustrial activities as a by-product.

3. Technical solutions for the consumption of stone slurry

3.1. Natural stone industry waste

There is a huge range of natural stone varieties, whichcan be grouped according to its geological and miner-alogical constitutions. The waste generated from theextracting and processing activities are almost exclusively

0 2000 4000

France

Greece

Turkey

Portugal

Spain

ChinaItaly

India

Brasil

USA

South Africa

produc

Fig. 1. Main world producers of proce

dependent on the raw material and on some abradingagents or wear out debris of equipment required to processharder stones like granite. This waste can be constituted bycalcium carbonate or silica aluminates if the originalmaterial is marble and limestone or granite, respectively.There are two types of natural stone processing waste:

solid and semi-liquid (slurry). Solid waste consists of stonefragments with variable dimension which present fractures,inadequate dimension, low commercial value or any otherfactor that does not fit into its use at a technological oreconomical level. Natural stone slurry results from thephysical processes such as extraction, sawing and polishing.The equipments used on the latter activities require large

amounts of water, which plays an important role incooling, lubrication and cleaning of the resultant verysmall particles. This mixture of water and very smallparticles produces a semi-liquid substance that is generallyknown as ‘‘natural stone slurry’’, due to its appearance.This slurry is then subjected to different treatment

activities, according to the technology available in theprocessing plant (e.g. sedimentation tanks can be used torecover part of the water used in the processing activities,thus reducing environmental impacts and improvingeconomic performance).Due to its composition, this slurry presents a great

potential of being used as a by-product in mineralconsuming industries, thus reducing the environmentalimpact of the natural stone industry.

3.2. Construction industry

The social, environmental and economic impacts of theconstruction industry have been widely discussed duringthe last decades. The need to find solutions towards thereduction of consumption levels of basic raw materialsare recognized issues regarding sustainable construction.Examples are now illustrated.

3.2.1. Cement industry

Recent research studies [7] concluded that there istechnical viability to incorporate massive quantities ofnatural stone slurry as ‘‘raw material’’ in the production of

6000 8000 10000 12000

tion (Mtons)

ssed natural stone during 2000 [5].

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BF

B

A+B A+B

A+B

27 28 29 30 31 32 33 342θ angle

C

A - Alite B - Belite C - Tricalcium AluminateF - Ferrite

A

Fig. 2. Difractograms obtained from clinker produced with natural stone

slurry (above) and from the usual process (below) [7].

N. Almeida et al. / Building and Environment 42 (2007) 810–819812

clinker, without any previous complex treatments. Fig. 2compares the difractogram obtained of clinker producedwith natural stone slurry with the one produced by theusual process.

As an example, Portuguese cement industry is respon-sible for the consumption of 12 million tons of rawmaterials each year, about 10 million tons of which islimestone [7]. It is even possible to refer that the Portuguesenatural stone slurry produced annually consists solely in3.5% of the total limestone raw material needed by thenational cement industry.

As the nature of the main raw material used by thecement industry is similar to the one of the natural stoneslurry, the technical and theoretical viabilities weredemonstrated [7,8]. Nevertheless, the solution has not yetbeen generally adopted.

3.2.2. Red ceramic bricks and tiles

An European Research Project [9] concluded that it ispossible to incorporate large amounts of natural stoneslurry by substituting conventional calcium carbonate usedin the production of red ceramic bricks, without compro-mising the behaviour of the obtained final product [10].

The presence of this slurry in a 2–3% ratio solved theexpansion problems usually associated with structuralceramic materials [9]. Furthermore, depending on the kindof basic raw material used, it is possible to use up to 25%of slurry [11].

Confirmation on the behaviour of this kind of recycledmaterial was also obtained from similar research done in

Brazil and India, where red ceramic tiles are produced with20% of stone slurry input [8].Although limestone slurry is preferred, it is also possible

to produce red ceramic materials with granite slurry, if theabrasive materials used in the processing are sorted [12].

3.2.3. Tiles

India also presents successful cases related to theproduction of tiles containing 90% of stone slurry bondedby 10% of resin [8].

3.2.4. Mortars and concrete

Technical possibilities of producing concretes andmortars containing stone slurry have been studied withpositive results in several countries [8]. Research works inPortugal [13,14] led to similar conclusions, demonstratingimprovements in several properties.

3.2.5. Other cement-based products

Cement-based products such as structural blocks [15],lightweight blocks [16], soil–cement bricks [17], pavementcoatings [18] and even acoustic panels developed at anexperimental level that contained granite slurry, limestoneaggregates, cement and cork industry waste [10] are alsoknown applications.

3.2.6. Pavement

Stone slurry was not considered as suitable for pavementuse [20], but some laboratory tests demonstrated that it ispossible to incorporate this by-product in asphalt mixturesas a commercial filler substitute [20]. The use of slurry inroad works is not consensual. In fact, there are researcheswhich attest that it is possible to use marble slurry inroadwork layers [8] which account for 25–35% of the totalpavement thickness [19].

3.2.7. Embankment

As occurred in the pavements, opinions about the use ofstone slurry in embankments are not unanimous. Despitethe existence of bibliography referring the possibilityof using stone slurry in embankments (taking advantageof the insulation capability of the slurry) [19] or mixed in a25% ratio with soil [21], there are also several studiesreferring to environmental impacts related with thepresence of this industrial waste in soil [13].

3.2.8. Agglomerate marble

Agglomerate marble is the designation for products thatbind pieces of natural marble together with speciallyformulated polyester resin. This process allows thereconstruction of large recycled ‘‘marble’’ blocks, similarto the ones extracted from quarries, both in quality andvisual aspects, which can be submitted to the sameprocessing activities as natural stone. A research concern-ing the reutilization of marble slurry as a substitute ofcalcium carbonate was developed for agglomerate marblefabrication. For a total amount of slurry that reached up to

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6% of the total compounds, it was technically possible toadopt this procedure [22].

3.2.9. Other construction materials

Research undertaken in former Yugoslavia showed thatstone slurry could be used for producing glues and paints,achieving the required properties [23].

3.3. Other industrial activities

Apart from the construction industry, there are otheroptions to promote sustainable solutions for the naturalstone industry and for the recycling or reutilization of itsby-products.

3.3.1. Paper industry

There are references revealing the possibility to recyclestone slurry in pulp production, substituting the vegetal-based raw material generally used in the paper industry [7].

Research [22] showed evidence of opportunities toincorporate limestone and marble slurries substituting upto 30% of the kaolin generally used as a mineral pigment.This study showed also the feasibility of recycling slurryin this industry with improved results concerningphysical properties, nevertheless the visual properties wereinferior [22].

3.3.2. Ceramics industry (faience)

Granite slurry can substitute 50% of the raw materialsusually used to produce floor ceramic tiles [24,25].Additionally, other researchers addressed the possibilityof producing faiences with stone slurry [7,26].

In a particular case, it was confirmed that it was possibleto substitute calcite by marble and limestone slurry (in a12% proportion of the total material) with no negativerepercussions on the different production phases, as well asfor the final product quality [22]. The main advantage ofthis procedure was to reduce the need for special sand usedas raw material, which is one of the most expensivematerials in the mixture.

3.3.3. Agriculture soil corrective

One of the applications discovered for the stone slurry,already tested at an experimental level, was the correctionand improved performance of soil dedicated to agricultureexploitation, by increasing the pH level of those soils [27].

Nevertheless, some authors [7] claim that the dryingprocess of the slurry induces the formation of solid blocksthat are harmful for agricultural purposes and, in order forthe process to be valid, it is necessary to submit the slurryto previous treatments that are economically unacceptable,when compared with the traditional solutions.

Furthermore, the potential of air contamination arisingfrom soil revolving during agriculture activities is a realitythat can evoke concerns related to human health [7].

Experimental realizations using basalt processing by-products for agricultural applications showed that the

hygroscopic properties prevented evaporation, thus retain-ing further humidity in the soil [9]. This solution wasstudied in Catania (Italy) and it is predictable that they canbe also used in other dry areas in EU (Southeast of Spain,Canary Islands and Greece) or other Mediterraneancountries [9].

3.3.4. Acid water treatment

Taking advantage of the capability of the slurry toincrease pH level, Soares [7] refers to the possibility oftreating acid water by means of using the stone slurry in theprocess.

3.3.5. Dumpsites sealing

In a Portuguese region where dimension stone industryassumes a relevant economic and social role, an innovativeproject was developed towards the periodic sealing ofdomestic solid waste cells, which formed a dumpsite, usingthe large amounts of stone slurry generated by localprocessing plants [28]. The project authors concluded thatthe ideal humidity to be contained in the slurry forcompacting operations should be 19% [28].Firstly, the construction of the cells should be preceded

by a watertight compacted layer with 20 cm, with thefunction of avoiding infiltration of leached substances [28].Then, according to the study, a rock fill must be done,which is under the deposit of domestic waste accumulateduntil the complete formation of a cell (with the proper carein order to avoid accumulation of biogas). After comple-tion, the cell is sealed with another layer of compactedslurry [28].The final result of the project was positive and the slurry

accomplished the functional demands, promoting imper-meability, thus minimizing infiltration of water inside thecells and reducing the existence of insects and otherundesired occurrences [28].

3.3.6. Other applications

Taking into account that very fine particles of limestoneand marble slurry present chemical and mineral naturesimilar to the ground carbonated stones (calcium carbo-nate) used in different industrial ends, there is a wholerange of potential applications for the slurry.Markets such as the USA consume large quantities of

these kinds of products, thus existing companies exclusivelydedicated to transportation of limestone slurry [29]. Thisreality shows the competitiveness of these materials.An independent research undertaken in Spain focused

on the fact that it was possible to insert marble andlimestone slurry in the market as calcium carbonate to beused in sectors such as cement, paints, plastics andpolymers, paper, ceramics and glass [11].Furthermore, it is possible to speculate that ground

marble (or marble slurry) can be used in the manufacturingof industrial applications such as varnishes, rubber, latexapplications, vinyl compounds, PVC and foams or even in

ARTICLE IN PRESSN. Almeida et al. / Building and Environment 42 (2007) 810–819814

the food industry (composed food for animals, flours,pastry, cereals, gums, etc.) [30].

According to the latter studies, other authors refer to thepossibility to apply this waste in plastics [19], rubber,paints, siderurgy, sugar, pharmaceutics, textiles or inarticles such as soaps or candles [7].

In conclusion, the options are not exhausted once itis realized that the possibility to use this waste as aline marker in sports stadiums can be a businessopportunity [31].

4. Experimental programme

4.1. General procedures

An experimental programme was undertaken at theTechnical University of Lisbon—IST (Portugal) to evalu-ate the main mechanical properties of concrete mixturesincorporating semi-liquid natural stone waste (slurry),namely in concrete mixtures containing white cement.

The stone slurry generated in industrial plants wasreused as a component for concrete mixtures without being

Table 1

Properties of coarse and fine aggregates

Property FA1 FA2 CA1

Water absorption (%) 0.4 1 1.9

NP954 NP954 NP581

Dry specific density (kg/m3) 2594 2526 2576

NP954 NP954 NP581

Saturated surface dry specific

density (kg/m3)

2604 2551 2628

NP954 NP954 NP581

Bulk density (kg/m3) 1498 1537 1389

NP955 NP955 NP955

Microfines content (%) 0.5 1.7 3

NP86 NP86 NP86

Maximum size (mm) 0.595 2.38 9.51

NP1379 NP1379 NP1379

Fineness modulus 1.4 3.2 5.7

NP1379 NP1379 NP1379

0

10

20

30

40

50

60

70

80

90

100

sieve siz

perc

enta

ge p

assi

ng (

%)

0.1490.0740.0065 0.297 0.59 1.19

Fig. 3. Grading curv

submitted to any kind of previous treatment. However, itwas realized that this procedure was only possible throughthe use of high contents of superplasticizer and that it wasrecommended to previously disaggregate the existinggrumes of slurry (recycling).The light colour of the limestone and marble slurry that

was used and its very fine dimension enabled its use onspecial concrete mixtures containing white cement.The special concrete mixtures were designed with low

water/cement ratios, reduced maximum dimension of theaggregate and higher content of cement, combined with theuse of adequate superplasticizers.Taking these into account, eight concrete mixtures with

stone slurry (CMSS) were produced substituting 0%(reference mixture), 5%, 10%, 15%, 20%, 34%, 67%and 100% of the volume of fine aggregate (sand). Allconcrete mixtures were set with a slump of 230710mmand a spread of 550710mm, obtained by adjusting thewater/cement ratio.

4.2. Materials

4.2.1. Aggregates

The coarse aggregate used in concrete mixtures wasobtained from crushed limestone and the fine aggregateswere extracted from sedimentary deposits (sand). Table 1shows the properties of coarse aggregates (CA) and fineaggregates (FA1 and FA2), as well as the respectivespecifications used for testing. Fig. 3 presents the gradingcurves (determined according to NP1379).

4.2.2. White cement

Concrete mixtures were produced with white cementtype CEM II/B-L 32,5R (br), characterized by themechanical properties presented in Table 2 (determinedaccording to NP EN 196-1), the physical propertiespresented in Table 3 (as specified by the cement producer)and the chemical properties presented in Table 4 (cementproducer results).

e (mm)

CA1

FA2

FA1

4.762.38 6.35 12.79.52 19.1 25.4

e of aggregates.

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Table 2

White cement mechanical properties

Age (days) Bending (MPa) Compression strength (MPa)

2 3.4 18.9

7 5.8 32.2

28 6.9 41.1

Table 3

White cement physical properties

Property Specification Test result

Specific density 2.96 g/cm3

Sieve residue

90 mm 0.1%

45 mm 1.3%

32 mm 6.7%

Specific surface area (Blane) NP EN 196-6 5340 cm2/g

Average dimension of particles 9.4mmWater of normal consistency NP EN 196-3 29.0%

Setting time

Initial NP EN 196-3 95min

Final 170min

Expansion (Le Chatelier) NP EN 196-3 1.0mm

Brightness Y 85.5%

Whiteness index 64.8%

Table 4

White cement chemical properties

Property Existence (%)

Lost on ignition LOI 12.20

Insoluble residue IR 0.30

SiO2 17.50

Al2O3 2.15

Fe2O3 0.20

CaO 64.20

MgO 0.80

SO3 2.50

K2O 0.24

Na2O 0.11

Table 5

Chemical characterization of stone slurry

Designation Existence (%)

Lost on ignition LOI 43.40

Insoluble residue IR 0.90

SiO2 0.91

Al2O3 3.72

Fe2O3 0.40

CaO 54.29

MgO 0.30

SO3 0.09

Cl� 0.03

N. Almeida et al. / Building and Environment 42 (2007) 810–819 815

4.2.3. Stone slurry

The stone slurry was collected in an open-air dumpsite.The plant responsible for generating the collected speci-mens processed only marble and limestone. The watercontent of the different samples, at site, ranged from 1% to2%. When collected, the dried stone slurry was composedof dust and grumes. The grumes resulted from thefragmentation of compacted slurry obtained in the waterrecover operations performed at the processing plant. Inorder to test the stone slurry, the collected samples werereduced to dust.

Chemical tests reached the results presented in Table 5.The high content of CaO confirmed that the original stoneswere marble and limestone. It was also verified that theslurry did not contain any organic matter, thus confirmingthat it could be used in concrete mixtures.

The tested slurry had a specific density of 2.72 g/cm3.Furthermore, by using a Easy Particle Sizer M6.10

equipment, it was possible to compare both grading curvesof cement and slurry particles (Fig. 4). The specific surfacearea of the slurry particles was 7128 cm2/g and its averagesize was 5.0 mm (smaller than cement particles).The dimension of slurry particles was therefore compa-

tible with the filling and densing of the transition zone(measuring between 10 and 50 mm [1]) and of the capillarypores (which range from 50 nm to 10 mm of diameter [32]),thus being able to act as a microfiller. According to parallelspecific testing, it was concluded that the used slurry hadno hydraulic or pozzolanic activity.The average dimension of the slurry particles was

inferior to 74 mm (which would exclude its use as anaggregate for concrete production, according to theconventional concrete technology approach), their chemi-cal nature was exclusively dependent on the originalmaterial (without clay or other deleterious materials) andthe test results showed that the slurry was fit to be used inconcrete mixtures.

4.2.4. Concrete admixtures

A new generation polymer based on modified phospho-nates, with the commercial designation of Chrysofluid

Optima 100, compatible with potable water and acting asan high activity water reducer was used. The maximumdosage recommended by the producer (5 kg per 100 kg ofcement) was considered. This plasticizer conforms to CEmarking and NF 085 certification, whose technicalspecifications are those applied in the non-harmonizedpart of the NF EN 934-2. Table 6 presents more propertiesof the used chemical admixtures.

4.3. Concrete mixtures proportions

Proportioning was designed by Faury’s method, and theresults are presented in Table 7 (the water/cement ratioincludes the liquid phase of the admixture).

4.4. Preparation of specimens

All eight concrete mixtures were prepared accordingto a similar methodology. The studied properties were

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0

10

20

30

40

50

60

70

80

90

100

0.1 1.0 10.0 100.0 1000.0

particle size (µm)

infe

rior

size

per

cent

age

(%)

White cement

Stone slurry

Fig. 4. Comparison of cement and slurry particle size.

Table 6

Properties of the used superplasticizer

Nature Liquid

Density 1.0670.01 g/cm3

Colour White/yellow, slightly milky

pH 470.5

Freezing point About �3 1C

Cl� ions content Nil to BS5075

Na2O equivalent 0.3%

Dry extract (halogen) 3071.5%

Dry extract (EN 480-8) 3171.5%

N. Almeida et al. / Building and Environment 42 (2007) 810–819816

compressive strength at 7 and 28 days of age, splittingtensile strength and modulus of elasticity (the latter two at35 days of age). Compressive strength testing was under-taken upon 10 cm cubic specimens. Regarding splittingtensile strength and modulus of elasticity, cylinders with30 cm of height and 15 cm of diameter were cast. Allspecimens were demoulded 48 h after casting. All speci-mens were cured in a moist room until 14 days of age, andthen transferred to regular conditions till testing.

5. Results and analysis

5.1. Test results

Table 8 presents the results obtained for the mechanicalproperties and Fig. 5 plots their relative variation, whencompared with CMSS0 (mixture without stone slurry).

5.2. Compressive strength

When 5% of the initial sand content was replaced bystone slurry (CMSS5), 10.3% higher compressive strengthafter 7 days, and 7.1% higher compressive strength after 28days were detected, when compared with CMSS0. Thisincrease can be related to the higher concentration ofhydrated cement compounds within the available space for

them to occupy [9]. Furthermore, by acting as microfiller,the stone slurry promoted an accelerated formation ofhydrated compounds, thus resulting in a significantimprovement of compressive strength at earlier ages(7 days).In fact, the amount of slurry present in CMSS5 enabled

the very fine particles of it to act as nucleation points[33,34]. This is related to an effect of physical nature thatensures effective packing and larger dispersion of cementparticles, thus fomenting better hydration conditions.Moreover, the slurry particles completed the matrixinterstices (transition zone and capillary pores) andreduced space for free water [35,36]. The combination ofthese phenomena resulted in a better bonding among theconcrete components.CMSS10, CMSS15 and CMSS20 presented a reduction

of compressive strength ranging from 3.6% to 10.6% at 7days of age, and from 6.7% to 8.9% at 28 days of age(when compared to CMSS0). Lower performance ofCMSS10 could seem improbable taking into account itswater/cement ratio. However, for this extremely low water/cement ratio, the available space for accommodatinghydrated products was insufficient, thus inhibiting chemi-cal reactions.Regarding higher contents of stone slurry (substitution

of more than 20% of sand), the decrease of compressivestrength values was significant. The incorporation of suchamounts of very fine material did not permit the microfillereffect to prevail, which, in addition to a rather inappropri-ate grading, caused lower results.When substituting all the sand for stone slurry

(CMSS100), test results showed 50.3MPa at 28 days and30.1MPa at 7 days. While these results were acceptable bycomparison with conventional concrete, the relativereduction amounted to 40.9% for 28 days and 50.1% for7 days. Therefore, it is possible to conclude that fullsubstitution of fine aggregate for stone slurry is not reliablewhen compressive strength is a critical aspect to take inconsideration.

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Table 7

CMSS mix proportions

Concrete mixture Sand substitution (%) CA1 (kg/m3) FA2 (kg/m3) FA1 (kg/m3) Cement (kg/m3) Slurry (kg/m3) Water/cement ratio

CMSS0 0 1071 473 253 401 — 0.36

CMSS5 5 1080 477 219 405 38 0.33

CMSS10 10 1085 479 182 406 77 0.32

CMSS15 15 1071 473 144 401 114 0.36

CMSS20 20 1054 465 103 395 152 0.39

CMSS34 34 1037 458 — 388 256 0.44

CMSS67 67 1017 225 — 381 489 0.48

CMSS100 100 1013 — — 379 726 0.50

Table 8

CMSS mechanical properties

Mixture Compressive strength 7

days (MPa)

Compressive strength 28

days (MPa)

Spitting tensile strength

(MPa)

Modulus of elasticity (GPa)

CMSS0 60.3 85.1 4.2 40.5

CMSS5 66.5 91.1 4.8 43

CMSS10 55.3 79.4 4.2 41.4

CMSS15 58.1 79.5 4.3 38.8

CMSS20 53.9 77.5 4.0 36.9

CMSS34 41.1 60.8 3.3 33.5

CMSS67 36.4 58.2 3.2 30.7

CMSS100 30.1 50.3 3.0 26.7

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

0 10 20 30 40 50 60 70 80 90 100

substitution of fine aggregate for stone sludge (%)

perf

orm

ance

var

iatio

n Compressive strength (7 days)Compressive strength (28 days)Split tensile strengthModulus of elasticity

Fig. 5. Variation of performance of concrete mixtures with different contents of stone slurry.

N. Almeida et al. / Building and Environment 42 (2007) 810–819 817

5.3. Splitting tensile strength

The benefits obtained in compressive strength propertydue to the microfiller effect induced by stone slurryparticles was even further important regarding the splittingtensile strength tests (relative increase of 14.3% detectedfor CMSS5). These are coherent results, at the light of theexplanation advanced regarding the compressive strengthvariation of CMSS5.

As for the compressive strength, when the substitutionlevel of sand surpassed 20%, the tensile splitting strengthwas significantly reduced. Nevertheless, test results showthat tensile splitting strength is less sensitive to high contentof very fine particles than compressive strength. CMSS100

presented a result of 3MPa, correspondent to a quiteacceptable reduction of 28.6% relatively to CMSS0.

5.4. Modulus of elasticity

In accordance with the analysis made concerning theother mechanical properties, CMSS5 test results deter-mined that this was the concrete mixture with betterbehaviour in terms of modulus of elasticity (6.2% higherthan CMSS0) and that all mixtures containing less than20% of stone slurry obtained acceptable results. CMSS10also presented a slight improvement of 2.2% in behaviour.In the extreme case of slurry incorporation (CMSS100), theaverage of test results for the modulus of elasticity was

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26.7GPa (34.1% less than the reference concrete mixtureCMSS0).

It is known that cement paste modulus of elasticity isgenerally half the modulus of elasticity of aggregates [36].Therefore, when introducing stone slurry (very fineparticles, with slight inferior size than cement particles),the paste could be considered as increased, thus promotinga negative effect on the modulus of elasticity of thehardened concretes. This fact, in addition to the higherwater/cement ratio, could explain the lower modulusof elasticity attained for more than 15% substitution(inclusively).

Another reason that might explain the negative beha-viour detected for more than 15% of aggregate replace-ment for slurry, a part from the higher water/cement ratio,might be associated with a possible volumetric expansionoccurring among the different materials, withdrawingaggregates (which better contributes to a higher modulusof elasticity) from one another, thus losing some ability torestrain deformations (further dependent on the paste).

In light of this, better behaviour of CMSS5 and CMSS10can be explained by better grading and packing ofhardened concrete mixture, attained by reduced spaceamong the different particles.

6. Conclusions

Nowadays, worldwide natural stone industry offers anoutput of 68 million tons of processed product. As in otherindustrial activities, concerns related with the generationand destination of waste must be addressed. Among these,large quantities of semi-liquid wastes originated in naturalstone extraction and processing activities (stone slurry)need to be treated through sound solutions instead of beingaccumulated at open-air dumpsites.

This paper presented several technical solutions for theincorporation of this industrial by-product in constructionmaterials such as cement, red ceramic bricks and tiles,resin-based materials, mortars and concrete, agglomeratemarble, etc. or in other construction solutions rangingfrom pavement to embankments and road works. Further-more, the nature of this waste (depending mainly on itsmineralogical origin) allows other applications in a widerange of activities like the paper industry, the ceramicsindustry (faience), agriculture, water treatments anddumpsites sealing, among others.

A research developed to evaluate the mechanicalbehaviour of concrete mixtures containing stone slurrywas also undertaken. The results showed that the substitu-tion of 5% of the sand content by stone slurry inducedhigher compressive strength, higher splitting tensilestrength and higher modulus of elasticity. The feasibilityof incorporating up to 20% stone slurry in detriment of therespective amount of fine aggregate without prejudicingmechanical properties in a serious manner was alsodetermined.

In conclusion, natural stone slurry can be consumed byseveral industrial activities as a by-product and canspecifically be used as a fine aggregate and/or microfillerin concrete mixtures, inducing benefits on its mechanicalproperties.Although the technical solutions are abundant, facts like

the eventual need for expensive slurry treatments and/orthe geographical distance between the source and thedestination can compromise these solutions. Thus, it isimperative to continue the research and the diffusion ofknowledge, hoping some of these options will prevail.

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