Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=yjge20 Download by: [ogbonnaya igwe] Date: 30 January 2016, At: 05:24 International Journal of Geotechnical Engineering ISSN: 1938-6362 (Print) 1939-7879 (Online) Journal homepage: http://www.tandfonline.com/loi/yjge20 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. Submit your article to this journal Article views: 5 View related articles
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Full Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=yjge20
Download by: [ogbonnaya igwe] Date: 30 January 2016, At: 05:24
The combined effect of wood ash and lime on theengineering properties of expansive soils
Chukwuebuka Emeh & Ogbonnaya Igwe
To cite this article: Chukwuebuka Emeh & Ogbonnaya Igwe (2016): The combined effect ofwood ash and lime on the engineering properties of expansive soils, International Journal ofGeotechnical Engineering
To link to this article: http://dx.doi.org/10.1080/19386362.2015.1125412
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 soilsChukwuebuka 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
IntroductionExpansive 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
Emeh and Igwe The combined effect of wood ash and lime on the engineering properties of expansive soils
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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 methodologyField observations and samplingThe observations that led to this study were performed at Awgu 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 procedureThe 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 to ASTM 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 to Atterberg 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 discussionThe 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.
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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) 2.43Liquid limit (%) 57.00Plastic limit (%) 26.84Linear shrinkage (%) 16.51Plasticity index 30.17Sand (%) 49.00Silt (%) 36.00Clay (%) 15.00Soil classification (USCS) CHFree swell ratio 1.23Activity 2.00Swell potential 8.80
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 13–14Percent fines (%) 98Bulk density 1.12 g/cm3
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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 soilAtterberg limits, shrinkage limits, and free swell indexFigure 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
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Maximum dry density and optimum moisture content of the soilThe 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 OWSA causes 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 OWSA shall 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
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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
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natural soil, again this soil mixed with 18% wood ash taken as the OWSA was 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 OWSA and 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 curingFrom 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
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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
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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).
ConclusionTable 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)− + Al2O3(Clay Alumina) → CAH
Table 5 Summary of the experiments and the results
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 28.08 167.23 S + 12% W 60 49 11 11 1.47 21.5 26.32 170.54 S + 18% W 60 49 7.01 11 1.48 20 20.93 200.6 1050 1380 1490 15905 S + 24% W 61 49 7.5 12 1.47 21 26.32 195.1Sample S + 18 % W = OWSA6 OWSA + 2%
L65 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.
AcknowledgmentAuthors are grateful to Mr Ojo Johnson, and Mr Ganiyu of National Steel Raw Materials Exploration Agency, 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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