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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
120
130
140
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
17
24
31
10
17
24
31
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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