Unfired Clay Masonry Bricks Incorporating Slate Waste

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    Radical improvements are needed in various construction,

    manufacturing and quarrying practices in order to minimise

    their detrimental effects on the natural environment  (Cole, 1998,

    1999; ODPM, 2001; Oti et al ., 2009a) are needed. Since

    environmental impacts have immediate implications for 

    planning, design and operation of civil engineering

    infrastructure, conducting appropriate research on construction

    materials will potentially help in the evaluation of the success

    of any development. With ever-changing legislative

    requirements and technological advances, a research partnership

    aimed at generating possible applications of slate waste may be

    a way forward. Using slate waste in unfired clay masonry brick 

    production has great potential. Converting these specific waste

    streams into a usable resource is important in terms of resource

    preservation and would contribute to improving the local

    environment in North Wales from a visual impact and amenity 

    point of view. The embodied energy in recovering and reusing

    slate waste for the production of unfired bricks is less than that

    in clay quarrying.

    The key industrial problem this paper aims to address is the

    current lack of significant engagement regarding the utilisationof wastes and by-products from various industrial processes

    (currently creating a huge environmental burden in landfill sites

    in the UK) in the building industry. The use of slate waste in

    unfired clay masonry products is rare in the UK. The use of 

    activated slag (GGBS) with clay in building components (outside

    its use in normal concrete applications) is new. This paper 

    proposes that any viable building product(s) emerging from this

    research are therefore quite innovative. It is hoped that this

    paper will also transfer knowledge on the workability of unfired

    clay masonry bricks incorporating slate waste as compared with

    mainstream construction (fired) bricks, as well as outlining

     various economical and environmental benefits.

    2. METHODOLOGY 

    2.1. Materials

    The materials used in the research were slate waste (SW), Lower 

    Oxford Clay (LOC), two different types of lime (L1 and L2),

    ground granulated blast-furnace slag (GGBS) and Portland

    cement (PC).

    2.1.1. Slate waste.  The SW used for this study originated from

    quarrying operations in Gwynedd, North Wales. The waste

    consists of an assemblage of discrete particles of various shapes

    and sizes. In order to group these various particles into separate

    ranges of sizes and to determine relative proportions by dry massof each size range, a particle size analysis of the slate (see  Table 1)

    was conducted in accordance with BS 1377-2: 1990 (BSI, 1990a).

    This analysis provided the research team a basis upon which the

    engineering properties of the slate waste could be broadly assessed

    and an indication of the feasibility of incorporating slate waste in

    the stabilisation process. More detailed information about the

    morphology of individual particles, as well as the composition of 

    the slate waste, was obtained using a Carl Zeiss SMT 1430

    scanning electron microscope (SEM), equipped with an Inca-suite

     version 4.01 Oxford instrument, linked to an energy dispersive x-

    ray (EDX) machine capable of analysing electrons in the range of 

    10–100 atomic weight. From the EDX spectra obtained from theSEM (Figure 1), the key elements in the slate waste aggregate were

    determined (Table 2). The key compounds are silica (quartz, SiO2)

    and alumina (Al2O3), constituting 92.9% of the morphology of 

    slate waste. Other minor compounds include albite (NaAlSi3O8),

    magnesia (MgO), feldspar (CaAlSi3O8) and wollastonite (CaSiO3);

    titanium (Ti) and manganese (Mn) crystals were also detected.

    2.1.2. Lower Oxford Clay.  The LOC used in this study wassupplied by Hanson Brick Company Ltd, from the Stewartby brick 

    plant in Bedfordshire, UK. Its mineralogical composition is shown

    in Table 3 and its chemical and physical properties are shown in

    Table 4. This material is currently used by Hanson to make fired

    ‘London’ bricks. It is a challenging choice of clay material because

    it is generally hard to stabilise (especially with lime) because of its

    high organic and sulphate contents. However, it has advantages

    for this research because it is currently being used for fired brick 

    manufacture, therefore making comparison of fired and unfired

    products easier.

    2.1.3. Lime.  Two different types of lime (L1 and L2) were used inthis study. L1 is a quicklime (CaO) and L2 is a hydraulic lime; both

    were supplied by Ty ˆ -Mawr Lime Ltd, Llangasty, Brecon. The

    chemical and physical properties of both limes are also shown in

    Table 4. In the current work, after several trials, a maximum value

    of 1.5 wt% lime was chosen for the activation of GGBS. The

    selection of these binders was made for the following reasons.

    (a) Quicklime has been used successfully in low-temperature

    regions for clay stabilisation for road construction and for 

    columns for the improvement of slope stability. Quicklime

    removes water from the stabilised mix or surrounding soil,

    thereby contributing to rapid stability of the mix or slope

    (Greaves, 1996; Holmes and Wingate, 2003). In a stabilisedsoil system, quicklime can react with pozzolan and enhance

    autogenous healing.

    (b) Hydraulic lime has silicates that are predominately in the di-

    silicate form (belite), with only trace amounts of highly 

    reactive tri-silicate (alite). Hydraulic lime thus has a slower 

    setting time and gains strength over time.

    The use of two different limes gave an indication of their 

    performance profiles and provided the research team with

    flexibility, especially when making recommendations for the

    commercial production of unfired clay bricks.

    2.1.4. Ground granulated blast-furnace slag. GGBS was suppliedby Civil and Marine Ltd, Llanwern, Newport.  Table 4 shows that

    GGBS contains calcium oxide, reactive silica and alumina, which

    can be successfully utilised in pozzolanic reactions  (Oti et al .,

    Sieve size:mm

    Amountpassing: %

    Amountretained: %

    Classification

    5000 100.0 0.0 Gravel particles3350 99.5 0.5 Gravel particles2000 79.5 20.0 Gravel particles1180 50.5 29.0 Sand particles

    600 27.5 23.0 Sand particles425 21.0 6.5 Sand particles

    300 16.0 5.0 Sand particles212 12.0 4.0 Sand particles150 9.0 3.0 Sand particles

    63 1.5 7.5 Sand particles

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    2008a). GGBS was used as a key ingredient because of the

    proximity of slag works in the South Wales region; owing to the

    reduced cost of transporting GGBS, it is hoped that a market

    opportunity for brick manufacturers in this region might be

    created.

    The amorphous glass content in GGBS is considered to be the

    most significant variable and certainly the most critical to its

    hydraulicity. The rate of quenching, which influences the glass

    content, is thus the predominant factor affecting the strength

    of slag cements. Dhir  et al . (1996) showed a linear relationship

    Full Scale 29869 cts Cursor: –0.172 keV (1 cts) keV

    Ca

    Ca

    Ca Mn

    Mn

    Na

    Mg

    Ti

    TiTi

    Si

    Al

    K

    K

    K

    O

    Mn

    0 1 2 3 4 5 6 7 8 9 10 11

    Figure 1. EDX spectra of slate waste

    Elementalsymbol

    Compound Chemicalformula

    Weight: %

    O/Si Quartz SiO2   81.85Na Albite NaAlSi3O8   1.38Mg Magnesia MgO 1.29Al Alumina Al2O3   11.05K Feldspar CaAlSi3O8   3.04Ca Wollastonite CaSiO3   0.42Ti Titanium Ti 0.58Mn Manganese Mn 0.39

    Total 100.00

    Table 2. Composition of slate waste

    Chemical formula Composition:%

    Illite (K,H3O)Al2Si3,AlO10(OH)2   23Kaolinite Al2Si2O5(OH)4   10Chlorite (OH)4(SiAl)8(Mg.Fe)6O20   7Calcite CaCO3   10Quartz SiO2   29Gypsum CaSO4.2H2O 2Pyrite FeS2   4Feldspar CaAlSi3O8   8

    Organic materials – 7

    Table 3. The mineralogical composition of lower Oxford clay

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    between glass content and strength in a GGBS-based system,

    but no well-defined or single relationship has been reported. In

    the presence of alkaline activators, the more basic the GGBS

    blend, the greater the hydraulic activity  (Hewlett, 2003).  At

    constant basicity, the strength increases with alumina content,

    and a deficiency in calcium can be compensated for by a larger 

    amount of magnesia. Hydraulic activity increases with

    increasing calcium, alumina and magnesia content and

    decreases with increasing silica content (Frearson and Higgins,

    1992). Other researchers (Ganesh Babu and Sree Rama Kumar,2000) have reported that the alumina content of the slag

    influences its sulphate resistance. The reactive glass content

    and fineness of GGBS alone influences the cementitious/

    pozzolanic efficiency, or its reactivity.

    2.1.5. Portland cement.   PC

    manufactured in accordance

    with British standard BS EN

    197-1: 2000 (BSI, 2000) was

    supplied by Lafarge Cement

    UK. Table 4 shows its chemical

    and physical properties. Themineralogic compositions of 

    the major compounds in PC are

    shown in Table 5.

    2.2. Mix composition, sample preparation and testing

    Table 6 shows the mix compositions of cylinders made using

    lime/PC-activated GGBS-LOC-SW mixtures at 20% maximum

    stabiliser content. The stabilisers were blended at a ratio of 

    20%PC/lime:80%GGBS. These blending ratios were adopted in

    consideration of their superior potential performance in relation

    to strength, economy and environmental benefits relative to

    other blending ratios (Oti et al ., 2008a).

    Proctor compaction tests were carried out in accordance with BS1924-2: 1990 (BSI, 1990b) in order to establish maximum dry 

    density (MDD) and optimum moisture content (OMC) of the

    unstabilised LOC. The MDD and OMC values were found to be

    1.42 Mg/m3 and 29% respectively. The approximate range of 

    Composition: %

    LOC*   L1y   L2z   GGBSx   PC||

    CaO 6.15 89.20 66.60 41.99 63.00SiO2   46.73 3.25 4.77 35.35 20.00Al2O3   18.51 0.19 1.49 11.59 6.00MgO 1.13 0.45 0.56 8.04 4.21Fe2O3   6.21 0.16 0.71 0.35 3.00

    MnO 0.07 0.05 0.08 0.45 0.03–1.11S2–   –   

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    moisture content over which at least 90% MDD (1 .28 Mg/m3)

    could be achieved was 22–40%. In this work, the moisture

    content was 27%, 30% and 33% for all stabilised blends, with a

    target mean dry density of 1.40 Mg/m3

    . The samples weretherefore expected, within experimental error, to be of the same

    density and volume for all the material compositions in a given

    stabiliser system. This does, however, mean that within each

    system, specimens of different composition may deviate slightly 

    from their individual MDD and OMC values.

    Dry materials capable of producing three compacted cylindrical

    test specimens (50 mm diameter, 100 mm length) were

    thoroughly mixed in a variable-speed Kenwood Chef Major 

    KM250 mixer for 2 min before slowly adding a calculated

    amount of water. Intermittent hand mixing with a palette knife

    was performed for a further 2 min to achieve a homogeneousmix so that the full stabilisation potential was realised.

    Immediately after mixing, the materials were compressed into

    cylinders, to the prescribed dry density and moisture content.

     A steel mould fitted with a collar was used to accommodate all

    the material required for one sample; specimen compaction was

    carried out using a hydraulic jack (Figure 2). The cylinders were

    extruded using a steel plunger. They were then weighed,

    wrapped in cling film and labelled before placing them in sealed

    plastic containers at room temperature of about 20 28C. The

    samples were then moist cured for 3, 14, 28, 56 and 90 days. At

    the end of the curing period, three samples per mix composition

    were tested for compressive strength using a Hounsfield testingmachine at a compression loading rate of 1 mm/min. The

    moisture content at the ageing time of testing for each sample

    was also recorded.

    Durability – as applied to stabilised clay bricks incorporating

    slate waste – is the ability to resist the effects of varying degrees

    of exposure. It is generally accepted that the applicability of any 

    new clay-waste product will depend on whether the product is

    able to withstand passive, moderate and severe exposure in

    water. If a brick is designed for external application, the

    expansive behaviour of the material upon soaking in water is

     vital. Two cylindrical test specimens representing each of the various mix compositions were prepared for durability testing

    (i.e. resistance to linear expansion upon partial soaking in

    water). In order to effect partial soaking, the bottom 10 mm of 

    cling film wrapping the test specimens was carefully cut using a

    sharp razor and then removed. The exposed surfaces were then

    placed on a platform in a plastic tank.

    The specimens were allowed to moist cure in the tank by 

    ensuring that water was always present below the platform

    upon which the test specimens rested. To minimise evaporation

    and drying out of the test specimens, the plastic container was

    covered with a lid, which was fitted with a digital gauge to

    monitor linear expansion (Figure 3). Moist curing was allowed

    to take place for the initial 7 days after specimen preparation.

    Thereafter, the specimens were partially immersed in water to a

    depth of 10 mm above their base by carefully increasing the

    water level in the tank with a siphon, thus ensuring minimal

    disturbance to the specimens. This process of curing after 

    raising the water level is referred to as soaking.

    Both the moist curing and soaking environments were sealed

    systems. This was to reduce the availability of carbon dioxide

    that would otherwise cause carbonation of the lime in the

    hydrating system (which may reduce the amount of lime

    available for pozzolonic reaction). In the soaking process,

    Mix code Stabiliser Moisture Mass: g Totalcontent: % mass: g

    L1 L2 PC GGBS LOC SW Water

    L1-GGBS-LOC-SW 4%L :16%GGBS 27 10.50 0.00 0.00 42.00 236.25 26.25 85.00 400L2-GGBS-LOC-SW 4%L :16%GGBS 27 0.00 10.50 0.00 42.00 236.25 26.25 85.00 400PC-GGBS-LOC-SW 4%PC : 16%GGBS 27 0.00 0.00 10.50 42.00 236.25 26.25 85.00 400

    L1-GGBS-LOC-SW 4%L :16%GGBS 30 10.25 0.00 0.00 41.00 231.30 25.70 92.00 400

    L2-GGBS-LOC-SW 4%L :16%GGBS 30 0.00 10.25 0.00 41.00 231.30 25.70 92.00 400PC-GGBS-LOC-SW 4%PC : 16%GGBS 30 0.00 0.00 10.25 41.00 231.30 25.70 92.00 400

    L1-GGBS-LOC-SW 4%L :16%GGBS 33 10.00 0.00 0.00 40.10 225.54 25.06 99.30 400L2-GGBS-LOC-SW 4%L :16%GGBS 33 0.00 10.00 0.00 40.10 225.54 25.06 99.30 400PC-GGBS-LOC-SW 4%PC : 16%GGBS 33 0.00 0.00 10.00 40.10 225.54 25.06 99.30 400

    Table 6. Mix composition of the blended LOC mixtures (material for one cylindrical test specimen)

    Prefabricatedsteel mould

    Steel plunger

    Cylindertest sample

    Figure 2. Specimen preparation equipment

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    deionised water was used to avoid specimen contamination

    from metallic or other ion species in the water.

    Linear expansion measurements were recorded during moist

    curing and subsequent soaking. This was carried out

    automatically and the readings were recorded digitally every 12

    hours, until no further significant expansion was observed.

    Monitoring of the linear expansion measurements was

    completed on a daily basis until no further significant

    expansion was observed.

    3. RESULTS

    3.1. Strength development

    Figure 4 shows the compressive strength of the lime/PC-

    activated GGBS-LOC-SW specimens at varying compactionmoisture contents (27, 30 and 33%) at curing ages of 3, 14, 28,

    56 and 90 days. The control mixtures (PC-GGBS-LOC-SW) tend

    to show lower strength at all curing ages and at all compaction

    moisture contents. It should be noted that the 90-day strengths

    of the mixtures incorporating the lime-activated GGBS

    mixture are higher than those of the PC-activated mixtures.

    Overall, in all mixtures, the strength values of the test specimen

    with L1-GGBS-LOC-SW were higher at all curing ages.

    Figure 5 shows the rate of increase in strength with age for all

    the activated mixtures relative to the 28-day strength.

    Interestingly, it was observed that at a later curing age (56–90days), the blends containing a lime-activated GGBS mixture

    exhibited progressively high rate of increase in strength value

    compared with the PC-activated GGBS mixtures.

    3.2. Moisture content

    Figure 6 illustrates the moisture content of various test

    specimens at the time of testing, after moist curing the

    specimens for 3, 14, 28, 56 and 90 days. Specimen L1-GGBS-

    LOC-SW had a lower moisture content at 7-days of moist curing

    before testing while L2-GGBS-LOC-SW and PC-GGBS-LOC-SW 

    had higher moisture contents. Similar trends were observed up

    to the end of the 90-day moist curing period.

    3.3. Expansion behaviour 

    It can be seen from  Figure 7 that there is variation in the linear 

    expansion behaviour with compaction moisture content. The

    linear expansion behaviour of all the stabilised cylinder samples

    increases with increasing compaction moisture content. At the

    end of the 7-day moist curing period, the percentage linear expansion of the stabilised cylinders, L1-GGBS-LOC-SW, L2-

    GGBS-LOC-SW and PC-GGBS-LOC-SW are as follows: for 27%

    compaction moisture content, 1.99, 1.79 and 1.48%

    respectively; for 30% compaction moisture content 2.19, 1.87

    and 1.63% respectively; while for 33% compaction moisture

    content, 2.32, 2.00, and 1.70% respectively.

     At the end of the 7-day moist curing and 43-days partial

    soaking in deionised water, the percentage linear expansion of 

    the stabilised cylinders, L1-GGBS-LOC-SW, L2-GGBS-LOC-SW 

    and PC-GGBS-LOC-SW increased as follows: for 27%

    compaction moisture content, 2.47, 2

    .17 and 1

    .92%

    respectively; for 30% compaction moisture content 2.60, 2.32

    and 2.15% respectively; while for 33% compaction moisture

    content, 2.67, 2.43 and 2.39% respectively.

    Digital displacementtransducer

    Capped opening forsiphoning water

    5 mm thick Perspex capon the end of the specimen

    Level of deionised waterduring the 7-day moist curing

    Water level during soaking(after moist curing)

    5 mm thickplastic platform

    Porous disc

    Multi-channeldigital data logger

    Plasticcover

    Plastictank

    Figure 3. Experimental set-up

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    Linear expansion during soaking of the blended specimens

    suggested relatively more rapid expansion upon 43-days partial

    soaking. The stabilised cylinders containing lime and GGBS

    showed higher expansion rates. The blend made with quicklime

    (L1-GGBS-LOC-SW) tended to show a relatively higher 

    expansive behaviour, with the highest recorded expansion rate

    of about 2.67% at 33% compaction moisture content. Thelowest linear expansion (at the end of the 43-day days partial

    soaking) of 1.92% was observed in specimens made using the

    PC-GGBS-LOC-SW blend. Interestingly, this is the stabilised

    blend that showed the least strength development.

    Reassuringly, the total overall expansion rate of the samples

    (1.92–2.67%) is within the acceptable limit for the durability of 

    stabilised clay masonry units. The total maximum 50-day linear 

    expansion behaviour for the stabilised cylinder specimens at

    27–33% compaction moisture content is summarised in Figure 8.

    4. DISCUSSIONThe increase in moisture content from 27–33% produced

    noticeable reductions in strength values (see Figure 4) for all the

    stabiliser blends at the optimal designed blending ratio shown in

    Table 6. An increase in moisture content of the stabilised

    specimens may have resulted in a decrease in magnitude of 

    particle forces within the system. The explanation for this

     variation is complex, due to the various pozzolanic and other reactions involved in the hydration process. For example, when a

    lime–GGBS mixture reacts with clay soil incorporating slate

    waste, an exothermic reaction that results in the liberation of heat

    will occur. This action can also accelerate the pozzolanic reactions

    between both the GGBS and the residual lime in the stabilised

    matrix and the activated GGBS and the clay soil, leading to a

    higher combined pozzolanic reaction rate, which promotes the

    accumulation of strength-enhancing calcium–silicate–hydrate

    (CSH) gel among other hydration products. The different stabiliser 

    blends studied contain varying amounts of residual lime, thereby 

    resulting in differences in the pH of the systems and hence

    differences in reacting ion species. Variations in the strengths of the stabilised systems arise due to differences in moisture and

    pore gel structure. Increasing calcium and sodium ion

    concentration in a blended system incorporating lime and GGBS

    0·4

    1·9

    3·4

    4·9

    0·4

    1·9

    3·4

    4·9

    0·4

    1·9

    3·4

    4·9

    0 7 14 21 28 35 42 49 56 63 70 77 84 91

    0 7 14 21 28 35 42 49 56 63 70 77 84 91

    0 7 14 21 28 35 42 49 56 63 70 77 84 91

    (a)

       C

      o  m  p  r  e  s  s   i  v  e  s   t  r  e  n  g   t   h  :   N   /  m  m

       2

    (b)

       C  o  m  p  r  e  s  s   i  v  e  s   t  r  e  n  g   t   h  :   N   /  m  m

       2

    Curing age: days

    (c)

       C  o  m  p  r  e  s  s   i  v  e  s   t  r  e  n  g   t   h  :   N   /  m  m

       2

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    Figure 4. Strength development of the lime/PC-GGBS-LOC-SWstabilised mixtures at compaction moisture content of (a) 27%,(b) 30% and (c) 33%

    100

    110

    120

    130

    140

    100

    110

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    100

    110

    120

    130

    140

    0 7 14 21 28 35 42 49 56 63 70 77 84 91

    0 7 14 21 28 35 42 49 56 63 70 77 84 91

    0 7 14 21 28 35 42 49 56 63 70 77 84 91

       I  n  c  r  e  a  s  e   i  n  s   t  r  e  n  g   t   h  v  a   l  u  e  :   %

       I  n  c  r  e  a  s  e   i  n  s   t  r  e  n  g   t   h  v  a   l  u  e  :   %

       I  n

      c  r  e  a  s  e   i  n  s   t  r  e  n  g   t   h  v  a   l  u  e  :   %

    (a)

    (b)

    Curing age: days

    (c)

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    Figure 5. Rate of increase in strength (relative to 28-day strength)with age of activated GGBS-LOC-SW mixtures at compactionmoisture content of (a) 27%, (b) 30% and (c) 33%

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    may provide better dissolution of silicate and aluminates species,

    leading to increased inter-molecular bond strength.

     Another reason for the variation in strength of the lime- and

    PC-stabilised systems is that, during ionic exchange, the

    combination of negatively charged surfaces on the clay particles,positive cations and polar molecules in the system may result in

    the formation of an electric double layer. Since the addition of 

    lime produces rapid ion exchange, this may modify the electrical

    double layer, reducing the thickness of the water-absorbing layer 

    in the system and thus the degree of swelling.

    The results of the moisture content test (Figure 6) demonstrate

    that moisture has a profound influence on the long-term

    performance of stabilised soil incorporating slate waste material.

    The moisture content affects strength development and

    durability of the material. The variation in moisture content of 

    the unfired bricks made from L1-GGBS-LOC-SW, L2-GGBS-LOC-SW and PC-GGBS-LOC-SW blended mixtures is largely 

    due to the drying behaviour of each mixture. After 90 days of 

    moist curing, the hydration process is almost complete, yet

    10

    17

    24

    31

    10

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    24

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    10

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    0 7 14 21 28 35 42 49 56 63 70 77 84 91

    0 7 14 21 28 35 42 49 56 63 70 77 84 91

    0 7 14 21 28 35 42 49 56 63 70 77 84 91

    (a)

    (b)

    Curing age: days

    (c)

       M

      o   i  s   t  u  r  e  c  o  n   t  e  n   t  a   t   t   h  e   d  a  y  o   f

      s   t  r  e  n  g   t   h   t  e  s   t   i  n  g  :   %

       M  o   i  s   t  u  r  e  c  o  n   t  e  n   t  a   t   t   h  e   d  a  y  o   f

      s   t  r  e  n  g   t   h   t  e  s   t   i  n  g  :   %

       M  o   i  s   t  u  r  e  c  o  n   t  e  n   t  a   t   t   h  e   d  a  y  o   f

      s   t  r  e  n  g   t   h   t  e  s   t   i  n  g  :   %

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SW

    PC-GGBS-LOC-SW

    Figure 6. Moisture content of activated GGBS-LOC-SW mixturesat compaction moisture content of (a) 27%, (b) 30% and (c) 33%

    1·42

    1·77

    2·12

    2·47

    2·82

    1·42

    1·77

    2·12

    2·47

    2·82

    1·42

    1·77

    2·12

    2·47

    2·82

    1 7 13 19 25 31 37 43 49

    1 7 13 19 25 31 37 43 49

    1 7 13 19 25 31 37 43 49

    (a)

    (b)

    7-day curing + soaking period: days

    (c)

       L   i  n  e  a  r  e  x  p  a  n  s   i  o  n  :   %

       L   i  n  e  a  r  e  x  p  a  n  s   i  o  n  :   %

       L   i  n  e  a  r  e  x  p  a  n  s   i  o  n  :   %

    L1-GGBS-LOC-SW

    L2-GGBS-LOC-SWPC-GGBS-LOC-SW

    L1-GGBS-LOC-SWL2-GGBS-LOC-SWPC-GGBS-LOC-SW

    L1-GGBS-LOC-SWL2-GGBS-LOC-SWPC-GGBS-LOC-SW

    Figure 7. Linear expansion measurements during moist curing(7 days) and subsequent soaking of the stabilised cylinderspecimens made at compaction moisture content of (a) 27%,(b) 30% and (c) 33%

    Compaction moisture content: %   5   0  -   d  a  y  m  a  x   i  m  u  m

       l   i  n  e  a  r  e  x  p  a  n  s   i  o  n  :   %

    L1-GGBS-LOC-SWL2-GGBS-LOC-SWPC-GGBS-LOC-SW

    24 27 30 33 361·42

    2·17

    2·92

    Figure 8. Total (maximum) 50-day linear expansionmeasurements for moist curing (7 days) and subsequent soakingof the stabilised cylinder specimens (27, 30 and 33% compactionmoisture content)

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    differences in the drying behaviour still remain. Comparison of 

    the profiles in Figure 6 shows that the drying behaviour of 

    samples made with PC-GGBS-LOC-SW is slow compared with

    that of the L1-GGBS-LOC-SW and L2-GGBS-LOC-SW samples.

    This can be explained. For example, during hydration of the

    lime-activated GGBS-LOC-SW mixtures, more ettringite is likely 

    to be formed and this may imbibe some water during moist

    curing, resulting in a drying effect, or as a result of the

    exothermic reaction of slaking lime, which has a drying effect.

    Swelling and linear expansion of stabilised clay soil is common

    and is known to be associated with the formation of a colloidal

    product (gel intermixed with ettringite) that forms on the

    surface of the clay particles during curing. In a saturated

    condition, ettringite grows and develops from this colloidal

    product. It has a capability of imbibing large amounts of water 

    and this dramatically increases the swelling potential of the

    stabilised soil especially if lime is used as a stabiliser. (Other 

    hydration products may also fill the void space of the stabilised

    system, thus enhancing both strength and volume stability upon

    subsequent soaking.) However, the introduction of a cementing

    agent such as PC or GGBS modifies the chemical interaction of the clay–lime system, thereby altering the types of reactions that

    occur and potentially altering any disruptions that the reaction

    products may cause. It is therefore not surprising that the test

    specimens made using PC-GGBS-LOC-SW expanded much less

    than those stabilised using a lime–GGBS blended binder.

    Over the 50 days linear expansion observation period (see

    Figures 7 and 8),  all the samples either attained terminal linear 

    expansion or continued to expand at a negligible rate. Linear 

    expansion was immediate when the specimens were soaked in

    water after the 7-day moist curing period. This expansion was

    more stable for the rest of the 43 days of soaking. The PC-

    GGBS-LOC-SW samples showed significantly lower expansion

    than lime-GGBS-LOC-SW stabilised mixtures. The overall

    reduction in linear expansion is likely to be due to the

    formation of cementitious products. Cementitious gels cement

    the soil particles together and enable them to resist the

    considerable swelling pressures that can be generated when

    ettringite forms in the presence of water. Hydration of the PC-

    GGBS-LOC-SW stabiliser blend was much more rapid than the

    pozzolanic reaction of the lime-activated GGBS-LOC-SW 

    mixtures. This hydration reaction is known to consume lime.

    The fact that higher strengths were observed in the L1-GGBS-LOC-

    SW mixture and it is this mixture that expanded the most suggeststhat volume stability is a sensitive balance between void space and

    cementation. The presence of GGBS in the mixture may also play 

    the role of diluting the stabilised system, thus reducing the amount

    of expansive products in the pore space and also increasing the

    effective water to stabiliser ratio. This would enable a greater 

    degree of quicklime hydration. This minimises any possible

    disruption to the hardened product and the overall expansion may 

    be reduced. In addition, GGBS may also mitigate expansion by 

    providing a surface upon which lime can be adsorbed and

    subsequently interact by activating the hydration process with the

    enhanced pH environment (Oti et al ., 2008b, 2008c, 2009b). The

    overall expansive behaviour of all the blended mixturescorresponds to a reduced water absorption capability of the edge to

    face contact of the specimens in water, with a tendency to form a

    turbulent flocculated system with coagulation contact.

    In view of the positive developments of the initial research work,

    a full-scale steel mould to produce full-size bricks was fabricated.

    This enabled laboratory-scale production of full-size

    (215 102 65 mm) unfired clay building bricks incorporating

    slate waste, with immediate success (Figure 9). In order to ensure

    successful transition of the laboratory brick samples to actual

    brick production, the research team intend to collaborate with

    local brick manufacturing companies for the first industrial

    production of stabilised clay bricks incorporating slate waste. The

    outcome of this intended collaboration will be reported later. The

    same mix design could also be adapted for the production of 

    blocks and mortar for industrial-scale development of unfired clay 

    masonry material incorporating slate waste.

    Table 7 summarises an analysis of some major environmental

    concerns relating to new product development. The unfired clay 

    masonry bricks incorporating slate waste are compared with

    bricks used in mainstream construction. Environmental analyses

    are increasingly being used and include criteria such as

    transportation, carbon dioxide emissions, embodied energy,

    Figure 9. Laboratory-produced full-size unfired clay brick 

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    depletion of resources, use of waste material, landfill, occupants’

    health (regarding end-products), product reuse and overall

    perception in terms of care for the environment. Such analyses

    can lead to improvements in the life cycle of products and

    provide criteria for design decisions when choosing materials

    offering similar performances for a given application.

    For the analysis presented in Table 7, the environmental and

    sustainability scoring method of Breeam was used (BuildingResearch Establishment, 2008). Table 7 shows that the unfired

    clay bricks using slate waste demonstrate excellent results

    against most criteria, thus demonstrating high sustainability 

    characteristics that could be exploited for energy-efficient

    masonry wall construction. This new product fits in with current

    trends for flexible and ‘green’ product development in the UK.

    The new product material is breathable and provides better air 

    quality for building occupants.

    The environmental performance of the unfired clay bricks

    incorporating slate waste is excellent when compared with

    bricks in current mainstream construction. Total energy usage is

    estimated at around 657.1 MJ/t for the lime-activated GGBSsystem and 667.1 MJ/t for the PC-activated GGBS system;

    emissions for the unfired clay bricks has been calculated at

    40.952kgCO2/t and 42.952kgCO2/t respectively. For common

    fired bricks, energy usage (input) is 4186.8 MJ/t with equivalent

    output emissions of 202 kgCO2/t (Brick Development

     Association, 2008). This large difference in energy usage may be

    attributed to the high temperatures (900–12008C) used in kiln

    firing of conventional bricks to give the final product the

    strength and durability it requires to perform in service.

    Furthermore, firing clay-based material at such high

    temperatures generally results in the release of several gases

    other than carbon dioxide.

    Traditional sun-baked bricks tend to have the least energy usage

    (525.6 MJ/t), with emissions of 25.1kgCO2/t (Morton, 2008).

    However, the main deficiency of sun-baked bricks is their 

    susceptibility to water damage.

    For the UK government to achieve its current sustainability 

    goals, research focusing on waste utilisation should be

    encouraged. The prudent use of resources based on the

    prevailing ecological principles and the integration of key 

    aspects of  Rethinking Construction  (Egan, 1998) with

    environmental protection will be vital for a healthy built

    environment (Adetunji et al ., 2003; Bryant and Wilson, 1998;

    Joyner, 2005; Myers, 2003).

    5. CONCLUSIONS

    There is great potential for utilising slate waste and other 

    industrial by-products to facilitate a more sustainable

    construction environment. The strength characteristics of 

    unfired bricks incorporating slate waste were studied and found

    to be improved with the addition of lime and GGBS, which act

    as a bond on the soil particles. The following conclusions are

    drawn from the research.

    (a) Using LOC as the target stabilisation material, unfired bricksincorporating slate waste and lime-activated GGBS tend to

    have strength values superior to those of PC-activated systems.

    (b) An environmental comparison of the unfired clay bricks with

    fired bricks used in mainstream construction revealed that the

    unfired bricks show good environmental characteristics over 

    a range of important criteria. Most of the reported

    environmental emissions from conventional building brick 

    production are attributed to the energy used for firing kilns.

    (c ) The unfired clay technologyusing GGBS as the main stabilising

    agent for the production of building bricks incorporating slate

    waste will result in reduced energy costs compared with kiln-

    firing. Furthermore, it will reduce the environmental damage

    associated with the manufacture of traditional stabilisers andthus reduce greenhouse gas emissions.

    ACKNOWLEDGEMENTS

    The authors thank the Welsh Assembly government (WAG) for 

    funding the project by way of the collaborative industrial

    research programme and the knowledge exploitation funding

    initiatives. The authors also acknowledge the faculty of 

    advanced technology of the University of Glamorgan for 

    research facilities and staff resources.

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