Expansive soil stabilization

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International Journal of Geotechnical Engineering
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The combined effect of wood ash and lime on the
engineering properties of expansive soils
Chukwuebuka Emeh & Ogbonnaya Igwe
To cite this article: Chukwuebuka Emeh & Ogbonnaya Igwe (2016): The combined effect of
wood ash and lime on the engineering properties of expansive soils, International Journal of
Geotechnical Engineering
To link to this article: http://dx.doi.org/10.1080/19386362.2015.1125412
Published online: 06 Jan 2016.
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*Corresponding author, email ogbonnaya.igwe@unn.edu.ng
DOI 10.1080/19386362.2015.1125412 International Journal of Geotechnical Engineering 2016 VOL. XX NO. X
The combined effect of wood ash and lime on
the engineering properties of expansive soils
Chukwuebuka Emeh* and Ogbonnaya Igwe
This work assessed the combined effect of wood ash a waste product from a bread bakery and lime
(calcium oxide) on the geotechnical properties of expansive soils collected from Awgu (southeastern
Nigeria). The mineralogical composition of the soil and chemical composition of the wood ash were
analyzed using X-ray diffraction and X-ray fluorescence method, respectively. The geotechnical properties
of the soil such as grain size distribution, consistency limits, free swell potential, compaction, and
unconfined compressive strength of the natural soil and that of the soil with varying proportion of wood
ash and lime was also examined. The results revealed that the natural soil which is classified as highly
plastic inorganic soil, on addition of wood ash and lime in the optimum proportion of 78%-18%-4% by
weight of soil-wood ash-lime admixture showed reduction in the plasticity index and linear shrinkage,
thus improving the workability of the natural soil. There was also reduction in the free swell potential
of the natural soil, improvement in the compaction properties of the natural soil, and increase in the
shear strength value of the natural soil which drastically improved more after 28 days of curing. It was
therefore concluded that high plastic inorganic soils can successfully be stabilized for use in pavement
construction with the combined effect of wood ash and lime, which will not only reduce the cost
of carrying out engineering projects, but also reduces the environmental problems associated with
indiscriminate disposal of wood ash.
Keywords: Wood ash, Lime, Expansive soil, Stabilization, Geotechnical properties
Introduction
Expansive soils, which mostly originate from argillaceous
sediments, are soils characterized by expansion in wet condi-
tions and shrinkage in dry conditions. Wetting and drying of
expansive soils result to its heaving and cracking respectively;
a behavior that results from its high clay mineral contents (over
65% of the total mineralogy). Most often, this heaving and
cracking result in failure of civil engineering structures sup-
ported by the expansive soil (Holtz, 1983; Taylor and Smith,
1986; Uduji et al., 1994; Wray and Mayer, 2004). Due to the
damages (failures) caused by expansive soils, there is always
need to improve their bearing capacity through mechanical
or chemical stabilization. Works by Al-Rawasa et al. (2005),
Buhler and Cerato (2007) reveal that chemical stabilization
using substances like lime and Portland cement, which are the
conventional stabilizers, is more effective and/or economical
than mechanical stabilization like vibro-flotation and heavy
weight compaction of expansive soils. For example,Anifowose
(1989) have shown that the engineering properties of soils sta-
bilized with lime (calcium oxide) are better than those that
are mechanically stabilized. This is because, while lime has
high amount of calcium oxide (CaO) which undergoes cation
exchange with the clay minerals that cause the expansion,
mechanical stabilization only reduces the void ratio of the soil
(Eskisar, 2015; Mitchell and Soga, 2005; Show et al., 2003;
Sivapullaiah, 2006). However, industrial stabilizing sub-
stances like lime, quick lime, and Portland cement are most
often expensive which warrants the researching into alternative
cheaper source of stabilizers.
Works by Okagbue and Onyeobi (1999), Baser (2009)
and Agrawal and Gupta (2011) reveal that the use of marble
dust as soil stabilizer reduced its (soil) plasticity, increased
the strength (unconfined compressive strength and California
bearing ratio), and reduced the maximum dry density (MDD).
Their works revealed that the maximum strength of the stabi-
lized soil was attained at about 8% marble dust and 92% soil
admixtures but can only be successfully used as base of lightly
trafficked and sub-base of heavily trafficked flexible pave-
ments. In using limestone ash waste to stabilize soil, Okagbue
and Yakubu (2000) discovered that the an addition of about
6% of limestone ash waste to 94% of soil improved the soil
by reducing the plasticity index and increasing the strength
(California bearing ratio and shear strength) and also that
double quantity of the limestone ash waste may be required
to achieve the same level of soil stabilization as would be
achieved by use of conventional lime. Brooks (2009) also
reported reduction in swelling ability and strength gain of
© W. S. Maney & Son Ltd 2016
Received 13 October 2015; accepted 23 November 2015
Department of Geology, University of Nigeria, Nsukka, Nigeria
Downloadedby[ogbonnayaigwe]at05:2430January2016

Emeh and Igwe The combined effect of wood ash and lime on the engineering properties of expansive soils
International Journal of Geotechnical Engineering 2016 VOL. XX NO. X2
expansive soil stabilized with rice husk ash, while Okagbue
(2007) reported an improvement in the gradation, reduction
in the plasticity and MDD of an expansive soil stabilized
with wood ash (wood combustion by-product). Authors like
Ene and Okagbue (2009) used pyroclastic dust, while Cokca
(2001), Kumar and Sharma (2004), Ji-ru and Xing (2002) and
Wong (2015) used fly ash (byproduct of coal power plant) to
also improve the engineering properties of expansive soil and
each got result quiet similar to the cases earlier stated.
Some researchers also assessed the effect of two stabiliz-
ers (conventional and unconventional stabilizer) and discov-
ered that such combinations are better stabilizers than only
one material. For example, Rao et al. (2012) used rice husk
ash and lime to stabilize marine clay and discovered that on
the addition of 25% rice husk ash, the plasticity index (PI),
optimum moisture content (OMC), and differential free swell
(DFS) decreased by 30, 18.5 and 72.8%, respectively while
the MDD) and California bearing ratio (CBR) increased by
17 and 282%, respectively. Their work further revealed that
on addition of the two 25% rice husk ash and 9% lime, the PI,
OMC, and DFS decreased by 56.4, 42.6, and 77.2%, respec-
tively while the MDD and CBR increased by 12 and 449%,
respectively. Other authors like Ismaiel (2006) and Malhotra
and Naval (2013) combined lime and fly ash, while Amu et al.
(2005) used cement and fly ash also got higher improvement
in the geotechnical property of the soil than using only one
stabilizer. However, no author has yet combined a conventional
stabilizer and wood ash irrespective that Kersten et al. (1998)
and Babayemi and Dauda (2009) have shown that enormous
wood ash is regularly generated and improperly disposed into
the environment from bakeries, restaurants, and homes of some
countries like Nigeria, and the environmental and health impli-
cations associated with indiscriminate dumping of wood ash
to the environment have been highlighted by Pitman (2006),
Risto et al. (2005), and Pasquini (2006).
This work assesses the effect of combined wood ash and
lime (CaO) on the engineering properties of soils and their best
admixture ratio in stabilizing expansive soil.
Study methodology
Field observations and sampling
The observations that led to this study were performed atAwgu
town of southeastern Nigeria where it was observed that most of
the civil engineering structures like roads and residential build-
ings develop cracks shortly after their construction, and in some
cases lead to heaving or total failure of the structure. Reddish
brown soil underlying the area that showed highest structural
damage was collected at 30 cm depth, air-dried for two weeks
to attain complete drying, and preserved for analyses.
The lime (calcium oxide) used was obtained from an indus-
trially grade chemical store while the wood ash (the residue
powder left after the combustion of wood) was obtained from
the furnace of a wood-fired oven of a bread bakery. Following
Okagbue (2007), the wood ash was left undisturbed for 1 h
to cool to ambient temperature after it was removed from the
bakery furnace, passed through BS sieve of 63 μm to obtain
the size needed for ash clay reaction, and preserved in an air-
tight bag to eliminate its possible reaction with the atmospheric
carbon dioxide.
Analyses procedure
The wood ash was subjected to X-ray fluorescence analysis to
determine its chemical composition and to specific gravity test
following BS 1377 (1975) standard to determine its specific
gravity. The pH of the wood ash was determined following
ASTM C25-93a (1993) standard while the chemical composi-
tion and physical properties of the lime have already been given
on the container of the lime by the producing industry (specialty
mineral incorporated, 2009). The soil sample was subjected
to X-ray diffraction (XRD) analysis using Shimadzu X-ray
diffractometer (XRD-6000) to determine its dominant miner-
alogical composition. It was further subjected to sieve analy-
sis, Atterberg limits, specific gravity, free swell index (FSI),
linear shrinkage (LS), compaction, and unconfined compres-
sive strength (UCS) tests. The sieve analysis/Atterberg limits,
UCS, and FSI test were carried out according toASTM D2487
(2011), ASTM D2166/D2166M-13 (2013) and IS: 2720-XL
(1985) standards, respectively, while the specific gravity and
compaction test were performed following BS 1377 (1975)
standard.
About 940 g of the soil and 60 g of the wood ash (corre-
sponding to 94% soil and 6% wood ash) were thoroughly mixed
with a hand trowel and the wood ash–soil admixture divided
into five portions. The five portions were subjected toAtterberg
limits, FSI, LS, compaction, and UCS tests, respectively, in
order to determine the effect of wood ash on the geotechnical
properties of the soil sample. The mixing, dividing, and geo-
technical tests were repeated for three more times using 88%
soil and 12% wood ash; 82% soil and 18% wood ash; 76%
soil and 24% wood ash. For each of the geotechnical tests, the
soil–wood ash admixture that gave the best (optimum) geo-
technical property was selected and mixed with lime (calcium
oxide) in the ratio of 49:1 (i.e. 2% lime and 98% soil-wood ash
admixture). The wood ash-soil-lime admixture was also divided
into five portions and subjected to Atterberg limits, FSI, LS,
compaction, and UCS tests in order to ascertain if the addi-
tion of lime will improve or depreciate the tested geotechnical
properties of the soil. The mixing, dividing, and geotechnical
testing were repeated three more times using 4% lime and 96%
soil–wood ash admixture; 6% lime and 94% soil–wood ash
admixture; and 8% lime and 92% soil–wood ash admixture.
In each case, the geotechnical tests were done following the
earlier stated standards.
The wood ash–soil and wood ash-soil-lime admixtures
were each further compacted at the OMC and specimen was
molded using the split mold of dimension 38 mm in diameter
and 76 mm in height. The molded samples were each carefully
extruded and divided into four portions. Each of the 4 portions
was cured moist (storing in polythene bags at 98% humidity
and 25 °C) for 7, 14, 21, and 28 days, respectively. The cured
samples were thereafter subjected to UCS test to determine
their possible strength gain/lose.
Results and discussion
The index properties and dominant mineralogy of the expansive
soil are shown in Table 1 and 2, respectively. The physical and
chemical properties of the wood ash are shown in Table 3, while
those of lime are shown in Table 4.

3International Journal of Geotechnical Engineering 2016 VOL. XX NO. X
From Table 1, the soil was classified as high plasticity inor-
ganic clay (CH) with high expansivity. Based on findings done
by Prakash and Sridhara (2004), the free swell ratio of the soil
showed that the soil contains both swelling and non-swelling
clays, which corresponds with its (soil) montmorillonite and
illite content shown in Table 2. Table 3 reveals that the wood
ash contains over 13 oxide compounds. Chemical composition
of wood ash varies greatly because there are many factors that
determine it such as the type and burn process (Campbell, 1990;
Etiégni and Campbell, 1991; Hakkila, 1989; Someshwar, 1996),
Table 1 Index properties and classification of the natural soil
Property Numerical value
Specific gravity (g/cm3
) 2.43
Liquid limit (%) 57.00
Plastic limit (%) 26.84
Linear shrinkage (%) 16.51
Plasticity index 30.17
Sand (%) 49.00
Silt (%) 36.00
Clay (%) 15.00
Soil classification (USCS) CH
Free swell ratio 1.23
Activity 2.00
Swell potential 8.80
Table 2 Dominant mineralogy of the expansive soil
Mineral present Percentage abundance
Na-montmorillonite 6.21
Illites 33.01
Kaolinites 12.14
Sepiolite 18.69
Sanidine 8.97
Table 3 Chemical and physical properties of the wood ash
Compounds/property Concentration unit
P2
O5
3.40%
SO3
1.82%
K2
O 15.1%
CaO 71.58%
TiO2
0.46%
Cr2
O3
0.02%
V2
O5
0.091%
MnO 2.37%
Fe2
O3
2.30%
CuO 0.070%
ZnO 0.17%
Ag2
O 2.10%
BaO 0.40%
Re2
O7
0.2%
LOI 20.01%
pH 12–13
Specific gravity 2.81
Table 4 Chemical and physical properties of the lime (after specialty minerals Inc., 2009)
Compounds Concentration unit
CaO 96%
Mg 0.8%
Fe2O3
0.1%
LOI 0.1%
pH 13–14
Percent fines (%) 98
Bulk density 1.12 g/cm3

For the case of Atterberg limits, the addition of 6% wood ash
resulted in a 12% increase in liquid limit and 12.2% increase
in plastic limit, while an addition of 18% wood ash resulted
in a 3% increase in liquid limit and 22.2% increase in plas-
tic limit. This higher increase in plastic limit than in liquid
limit resulted to the lowest decrease (19%) in the PI on addi-
tion of 18% wood ash. The lowest decrease of 9.5% was
also recorded in the LS on addition of 18% wood ash. These
results agree with those of Bhuvaneshwari et al. (2005) and
Ismaiel (2006) and Okagbue (2007) who used fly ash and
wood ash to stabilize expansive soil. Terzaghi and Peck (1996)
and Nalbantoglu and Gucbilmez (2001) explained that the
reduction in plasticity of the soil was due to the decrease in
the thickness of the double layer of the clay particles as a
result of cation exchange reaction which causes increase in the
attraction force therefore leading to the flocculation of the par-
ticles. Similarly, the lowest decrease (2.15%) in FSI was also
recorded on addition of 18% wood ash. However, the trend of
the FSI was more fluctuating than others (see Fig. 1(a)). This
fluctuation is probably due to the variation in the mineralog-
ical composition of the natural soil as the reaction between
clay and lime depends on the cation exchange capacity of the
the tree components (Hakkila, 1989; Waring and Schlesinger,
1985), the species of tree (Ayininuola and Oyedemi, 2013;
Misra et al., 1993; Someshwar, 1996), and the burn tempera-
ture (Etiégni and Campbell, 1991; Misra et al., 1993). There
is high percentage amount of CaO in this wood ash and this
should make it a good additive for expansive soil stabiliza-
tion because it will not only increase the alkalinity of the soil
to promote solubility of silica and alumina (Okagbue, 2007),
but also provides enough calcium ion for the cation exchange
reaction. Table 3 also revealed the presence of some heavy
metals like Zn, Cr, and Cu, but their concentrations are within
the permissible limit of most environmental regulatory bodies
(Pitman, 2006).
Effect of the additives on the
geotechnical properties of the soil
Atterberg limits, shrinkage limits, and free swell
index
Figure 1(a) shows the variation of Atterberg limits, LS, and
FSI of the expansive soil with varying quantities of wood ash.
1a Variation in Atterberg limits, LS, and free swell index with varying percentages of wood ash
1b Variation in Atterberg limits, LS, and free swell index with 18% wood ash and varying percentages of lime

Maximum dry density and optimum moisture
content of the soil
The OMC and MDD with varying quantities of wood ash are
shown in Figs. 2(a) and 2(b) while the variation in the OMC and
MDD at their optimum wood ash (18%) with varying quantities
of lime are shown in Fig. 2(c) and 2(d). The compaction curves
of the soil, soil–wood ash, and soil-wood ash-lime admixtures
are shown in Fig. 2(e).
Figures 2(a) and 2(b) show that there is an initial sharp
decrease in the MDD from 1.49 to 1.46 mg/m3
and a corre-
sponding 4.5% increase in OMC upon addition of 6% wood
ash to the natural soil. There was then gradual increase to the
highest MDD (1.48 mg/m3
) and a corresponding decrease to
the lowest OMC (3.5%) on the addition of 18% wood ash.
The initial sharp decrease was also observed and explained by
Okagbue and Yakubu (2000) to have been caused by floccu-
lation and agglomeration of the clay particles but their reason
for the subsequent gradual drop did not agree with the result
obtained in the present work. An explanation for the gradual
drop in the MDD may be that the lime content in the 6% wood
ash added was enough only for the initial flocculation and
agglomeration reaction and thus an increase in the quantity of
wood ash resulted in a slower reaction rate. Generally, as the
amount of wood ash increases, the OMC and MDD fluctuate
in which case none of the wood ash-soil OMC decreased up
to that of the natural soil and none of the wood ash-soil MDD
increased up to that of the natural soil.
Figures 2(c) and 2(d) reveal a general progressive increase
in OMC and decrease in MDD as lime is added to the OWSA.
The OMC increased by 12.5% while the MDD decreased by
0.15 mg/m3
upon addition of 8% lime to the OWSA. Okagbue
(2007) explained that the decrease in the MDD is due to floc-
culation and agglomeration of the clay particles (caused by
cation exchange reaction) resulting in increase in void vol-
ume consequential reduction in the weight–volume ratio. The
increase in the OMC is because of the hydration of quick lime
(reaction of quick lime and water to form calcium hydroxide).
An exothermic reaction that normally leads to the drying of
soil and thus requires more water for the subsequent reaction,
which is disassociation of the calcium hydroxide into Ca2+
minerals present and the concentration of lime (Bell, 1996).
Another explanation is that the wood ash does not quickly
produce enough calcium ions (Ca2+
) that can favorably go
into cation exchange reaction since it (wood ash) contains
other high valence ions (like Fe3+
, Cr3+
, and Ti4+
) that may
mask the effect of Ca2+
.
Since the lowest PI, LS, and FSI were obtained on the
addition of 18% wood ash to 82% soil; this was taken as the
optimum wood ash–soil admixture (OWSA) and was added
varying percentages of lime. Figure 1(b) shows the variation
in the Atterberg limits, LS, and FSI at their OWSA with var-
ying percentage of lime. The figure shows that the addition
of 4% lime increases the liquid and plastic limits by 5 and
2%, respectively but an addition of 8% lime decreases the
liquid and plastic limits by 11 and 21%, respectively. The
result is that the PI showed a 6 and 10% increase on addition
of 4 and 8% lime, respectively. Similar progressive increase
shown by PI is also shown by the LS (see Fig. 1(b)). The
addition of 4 and 8% lime to the OWSA showed a 1.5 and
3.5% increase in the LS. Ismaiel (2006) also gave similar
report on stabilization of expansive soils with the combined
effect of fly ash and lime. It implies that the addition of lime
to the OWSA does not significantly improve the PI and LS
of the soil.
Figure 1(b) however reveals that an addition of 4 and 6%
lime to the OWSAcauses a further 18.66 and 18.44% decrease
in the FSI (i.e. relative to that of OWSA), respectively. It is
expected that the addition of 5% lime to the OWSAshall result
to the lowest decrease (19.13%) in the FSI. These results agree
with those of Buhler and Cerato (2007), Malhotra and Naval
(2013) in using fly ash and lime to stabilize soil and also that
of Rao et al. (2012) in using rice husk ash and lime to stabilize
soil. The reduction in the swell potential of the natural soil was
achieved by the initial reaction of lime which releases calcium
ion (Ca2+
) that migrates to the surface of the clay particles dis-
placing water and other ions thereby reducing the swell ten-
dency. A process regarded as flocculation and agglomeration
and it generally occurs in a matter of hours, though can sub-
stantially improve with time of curing and pozzolanic reaction
(Dempsey and Thompson, 1968; National Lime Association,
2004).
2a Variation in OMC with varying percentages of wood ash

Interestingly, the wood ash-lime-soil admixture mois-
ture–density curves (see Fig. 2(e)) showed a more flattened
and OH-
ions (National Lime Association, 2004; Okagbue and
Yakubu, 2000).
2b Variation in MDD with varying percentages of wood ash
2c Variation of OMC with 18% wood ash and varying percentages of lime
2d Variation in MDD with 18% wood ash and varying percentages of lime

natural soil, again this soil mixed with 18% wood ash taken as
the OWSAwas mixed with varying quantities of lime as shown
in Fig. 3(b). The result revealed that there was a significant
increase in the UCS. The UCS increased by 206.4 kpa upon
adding 2% lime to the OWSAand further increased by 181 kpa
upon the addition of 4% lime to the OWSA and decreased
upon the addition of more lime. Therefore, 4% lime and 96%
OWSA was taken as the optimum wood ash-soil-lime admix-
ture. However, Fig. 3(b) indicates that the highest UCS shall be
attained (about 400 kpa total increase) on the addition of about
4.5% lime to the optimum wood ash–soil. The OWSA (18%
wood ash content) and optimum wood ash-soil-lime admix-
ture (4% lime content) were selected and each cured for 7, 14,
21, and 28 days with the aim of determining the strength gain
of the admixtures with time, bearing in mind that pozzolanic
reaction is time dependent (Show et al., 2003), and this reaction
as shown below produces calcium silicate hydrate (CSH) and
calcium aluminate hydrate (CAH):
Ca2+
+ 2(OH)−
+ SiO2
(Clay Silica) → CSH
compaction curve than that of wood ash–soil admixture. This
was also observed by Sweeney et al. (1988) who explained that
the flattening is due to the short-term pre-compaction cemen-
tation reactions caused by the lime. This cementation mostly
concentrates between the inter-clay particles edges/faces offer-
ing greater resistance to compaction. Nicholson et al. (1994)
and Ismaiel (2006) further explained that the flattening of com-
paction curves makes it easier to achieve the required density
over a wider range of moisture contents thereby conserving
time, effort/energy, and hence reduction in the cost of operation.
Unconfined compressive strength and curing
From Fig. 3(a) it can be seen that, as in the case of OMC and
MDD, the UCS of the soil did not show significant increase
or decrease as wood ash is progressively added to it. The UCS
increased by only 7 kpa on the addition of 18% wood ash to the
soil. In order to determine if the increase in calcium oxide con-
tent of the wood ash will cause increase in strength value of the
2e Compaction curves of the natural soil and at varying proportions of additives
3a Variation in UCS with varying percentages of wood ash

3b Variation in UCS with 18% wood ash and varying percentages of lime
4a Effect of curing on the UCS
4b Stress–stain relationship of the natural soil, optimum wood ash admixture uncured and 28 days cured, and optimum lime–
wood ash admixture uncured and 28 days cured

seven days before significant strength gain could be observed.
However, pozzolanic reaction has been observed to last for
months even years as long as the pH of the soil remains above
10 (Biczysko, 1996; Ismaiel, 2006).
Conclusion
Table 5 shows the summary of the experiments and the results
obtained, and the following conclusions were drawn from this
work:
(1) The addition of wood ash into the studied expansive
soil reduced the PI and LS of the soil and thus generally
improved the workability of the soil. The mixing of 18%
wood ash and 82% soil (regarded as the OWSA) gave
the least reduction in PI (decreased by 19.00%) and LS
(decreased by 9.50%) of the soil. Addition of lime to
the OWSA did not show any significant improvement
in the PI and LS.
(2) The OWSA resulted in only 2.15% decrease in FSI of
the soil, while the addition of 4% lime to the OWSA
resulted in a further 18.66% decrease in the FSI.
(3) Addition of wood as to the soil has no significant effect
on its (soil) OMC and MDD but the addition of lime to
the OWSA resulted to a progressive increase in OMC
and progressive reduction in the MDD. Upon addition of
8% lime to the OWSA, the OMC increased by 12.50%
while the MDD decreased by 0.15mgm3
. Similarly,
the addition of wood ash to the soil has no immediate
significant effect on its UCS while the addition of 4%
lime to the OWSA resulted to 387.4 kpa increase in the
UCS of the soil. There is evidence that it will attain a
maximum increase (by 400 kpa) on the addition of 4.5%
lime to the OWSA.
(4) The strength of both wood ash–soil and wood ash-soil-
lime admixtures increases with curing duration. After
28 days curing at 98% humidity and a temperature of
25 °C, UCS of the wood ash–soil admixture increased
The calcium silicate gel formed initially coats and binds lumps
of clay together which, then in time, crystallizes to form an
interlocking structure which binds the soil particles together
thus, strength of the soils increases (Hadi et al., 2008; Terrel
et al., 1979).
Comparing Figs. 3(a) and 3(b) with 4(a), it can be seen that
the lower strength gained by the wood ash–soil admixture is due
to the calcium oxide in the wood ash is not readily available for
the pozzolanic reaction which is time dependent, noting that the
natural soil contains appreciable amount of Na-montmorillonite
(see Table 2) and excessive quantities of exchangeable sodium
affects the lime reactivity of soil (Mallela et al., 2004), therefore
at this point the wood ash has no pozzolanic value to the mix
but only as a filler (Abdullahi, 2006). This could be justified
by the increase in the strength value of the wood ash-soil-lime
admixture as compared with the one obtained with the optimum
wood ash admixture alone, that is from 200.6 kPa to 407 kPa
on addition of 2% lime which subsequently increases as more
lime is added, and also the surge up of the strength value after
7 days of curing from 200.6 kPa before curing to 1050 kPa and
to 1590 kPa after 28 days of curing, and at this point the wood
ash must have produced enough lime for Pozzolanic reaction.
This strength gain was also revealed in the stress–strain
curves of the natural soil, the optimum admixtures, and 28 days
cured optimum admixtures as shown in Fig. 4(b). The stress–
strain curves of the uncured samples showing plastic deforma-
tions compared to that of the cured samples that showed brittle
deformations. These behaviors are likely due to hardening of
the cured clay particles with time and agree with works of
Popescu et al. (1997) and Nasrizer et al. (2011). Curing of the
samples in this work did not only serve the purpose of deter-
mining the durability of the wood ash–-lime-stabilized soil, but
also revealed that the calcium oxide content in the wood ash
is not readily available or not adequate enough for pozzolanic
reaction within hours but has to last for a period of at least
Ca2+
+ 2(OH)−
+ Al2
O3
(Clay Alumina) → CAH
Table 5 Summary of the experiments and the results
Note: Where, W = wood ash, OWSA = optimum wood ash-soil admixture
S/No Admixture Consistency Limits Proctor com-
paction test
Free
Swell
Index
(%)
Unconfined compressive strength (kPa)
LL (%) PL (%) LS (%) PI (%) MDD
(mg/
m3
)
OMC
(%)
Curing period (days)
0 7 14 21 28
1 Soil sample
only (S)
57 26.84 16.51 30.17 1.49 19 23.08 193.7
2 S + 6% W 69 39 16 30 1.46 25.5 28.08 167.2
3 S + 12% W 60 49 11 11 1.47 21.5 26.32 170.5
4 S + 18% W 60 49 7.01 11 1.48 20 20.93 200.6 1050 1380 1490 1590
5 S + 24% W 61 49 7.5 12 1.47 21 26.32 195.1
Sample S + 18 % W = OWSA
6 OWSA + 2%
L
65 46 10.3 19 1.37 31 6.67 407
7 OWSA + 4%
L
65 47 9 18 1.38 30.5 2.27 588 1250 1690 2100 2500
8 OWSA + 6%
L
49 30 10.5 19 1.37 32 2.5 585
9 OWSA + 8%
L
49 28 11.04 21 1.33 32.5 6.38 507

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by 1389 kpa while that of wood ash-soil-lime admixture
increased by 1912 kpa. Curing of the samples in this
work did not only serve the purpose of determining the
durability of the wood ash–lime-stabilized soil, but also
revealed that the calcium oxide content in the wood
ash is not readily available or not adequate enough for
pozzolanic reaction within hours but has to last for a
period of at least seven days before significant strength
gain could be observed.
(5) The addition of industrial CaO to wood ash in the right
proportion improves the stabilizing ability of the wood
ash.
(6) Since wood ash is regarded as a waste material and it
is cheap, using it as a stabilizing material for expansive
soils will reduce the cost of carrying out engineering
constructions on expansive soils and also reduce the
environmental problems associated with indiscriminate
disposal of wood ash.
Acknowledgment
Authors are grateful to Mr Ojo Johnson, and Mr Ganiyu of
National Steel Raw Materials ExplorationAgency, Kaduna, for
providing geotechnical services. They are also grateful to the
management of Ife-best bakeries for providing the wood ash
used in this work and to Chinenye for her financial supports.
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