Construction and Building Materials 18(2004) 377–381

0950-0618/04/$ - see front matter� 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2004.03.011

Effectiveness of Class C fly ash as an expansive soil stabilizer

Zalihe Nalbantoglu*˘

Department of Civil Engineering, Eastern Mediterranean University, Gazimagusa, Mersin 10, Turkey

Received 6 January 2003; received in revised form 11 March 2004; accepted 11 March 2004

Abstract

Fly ash produced in the combustion of subbituminous coals exhibits self-cementing characteristics and can be used in a widerange of stabilization applications. Fly ash treatment can effectively reduce the swell potential of highly plastic clays and preventthe swell beneath the smaller foundation pressures. The geology and climatic condition in Cyprus produce a wide distribution ofexpansive soils. These soils present problems in construction and possess a variety of undesirable characteristics such as lowstrength, high plasticity, difficult compaction and high swell potential. Laboratory test results on these soils indicate that fly ashis effective in ameliorating the texture and plasticity of the fly ash treated soils by reducing the amount of clay size particles,plasticity index and the swell potential. Cation exchange is one of the important reactions responsible for the improvement in thesoil characteristics. In the study, cation exchange capacity(CEC) values have been used to indicate the changes in the mineralogyof the fly ash treated soils and explain the reduction in the plasticity and water absorption potential.� 2004 Elsevier Ltd. All rights reserved.

Keywords: Expansive soils; Fly ash; Stabilization

1. Introduction

In the field of geotechnical engineering, it has longbeen known that swelling of expansive soils caused bymoisture change result in significant distresses and hencein severe damage to overlying structures. Expansivesoils are known as shrink–swell or swelling soils.Different clays have different susceptibility to swelling.The greatest problems occur in soils with a high mont-morillonite content. Such soils expand when they arewetted and shrink when dried. This movement exertspressure to crack sidewalks, basement floors, driveways,pipelines and foundations. The damages due to expan-sive soils are sometimes minor maintenance but oftenthey are much worse, causing major structural distress.According to Nelson and Millerw1x, 10% of the 250 000new houses built on expansive soils each year in theUnited States experience significant damage, somebeyond repairw2x.There are a number of additives, which may be

utilized for ground modification. The most commonlyused additives for soil modification are ordinary Portland

*Corresponding author.E-mail address: zalihe.nalbantoglu@emu.edu.tr(Z. Nalbantoglu).˘

cement, lime, fly ash and lime-fly ash. In Degirmenlikand Tuzla villages in North Cyprus, some serious prob-lems caused by large upward and downward movementsof the underlying soils have been observed on manybuildings and road pavements. Especially light structureshave been badly cracked. In these areas, the soils aremainly a clayey formation and the predominant mineralin the clay is montmorillonite. This clay mineral has avery high adsorptive capacity for water and hencepresents engineering problems that can be adequatelysolved by the use of fly ash soil admixture.The mechanism of swelling is complex and is influ-

enced by a number of factors: the type and amount ofclay minerals present in the soil, the specific surfacearea of the clay, structure of the soil and the valency ofthe exchangeable cationw3x. When lime and fly ash areadded into a soil, this results in a rapid hydration processand a simultaneous cation exchange that flocculates thesoil into larger lumps. The cementation of these lumpsby pozzolanic reaction produces the new pozzolanicreaction products, calcium silicate hydrates(CSH) andcalcium aluminate hydrates(CAH) that are responsiblefor the long term strength increase in soils.

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Table 1Physical, chemical and mineralogical properties of Degirmenlik andTuzla soils

Properties Degirmenlik Tuzlasoil soil

Liquid limit (%) 67.8 48.4Plastic limit (%) 22.2 25.0Plasticity index(%) 45.6 23.4% Clay fraction(-2 mm) 33.0 40.0Activity 1.38 0.59CEC (meqy100 g) 18.8 17.5Calcite(%) 23.0 17.0Quartz(%) 20.0 9.0Chlorite (%) 5.0 3.0Illite (%) 3.0 1.0Plagioclase(%) 4.0 24.0Dolomite (%) 7.0 2.0Kaolinite (%) 21.0 32.0Montmorillonite (%) 17.0 12.0

Table 2Physical and chemical properties of Soma fly ash

Properties Test values

Specific gravity 2.22Loss on ignition(%) 1.10Fineness cmyg2 3818.00Retained on� 325 sieve(%) 18.00Oxides(%) CaOs14.80

MgOs2.70Al O s25.502 3

Fe Os5.702 3

SiOs47.402

SOs2.803

CEC is the quantity of exchangable cations requiredto balance the charge deficiency on the surface of theclay particles. Clays with larger specific surface areausually have higher CEC, higher surface activity andconsequently higher water absorption potential. In thepresent study, unlike the previously published research,cation exchange capacity(CEC) values have been usedto explain the effect of the new pozzolanic reactionproducts on the particle size and the swell potential ofthe treated soils.

2. Materials

2.1. Soils investigated

Soils containing clays with predominantly expansivelattice type minerals such as montmorillonite have thehighest degree of tendency to swell. The semi-aridclimate in Cyprus, aggravate swelling problems in someareas. Two soils of different origins and physical prop-erties were selected from the deposits of marine claysof North Cyprus. One of the soil samples was obtainedfrom the site located in Degirmenlik and the other onewas obtained from the Tuzla village. The sites selectedfor the investigation were based on the reported struc-tural damage in the areas in which some serious crackshave been observed on many buildings and pavements.Degirmenlik soil has a very active clay formation. Thismiddle miocene formation gives a blue–grey color infresh exposures and on hydration produces the khakicolor w4x.The second soil sample taken from the Tuzla village

has an active clay formation derived from the recent-pliocene sediment. It gives a dark-brown colour onhydration and on drying it gives a light yellow colour.The physical, chemical and mineralogical properties

of Degirmenlik and Tuzla soils are given in Table 1.

The soils vary in composition with the amount of claycomponents and plasticity. The clay size fraction ofDegirmenlik soil is 33% with 63% silt and 4% sandwhereas the clay size fraction of Tuzla soil is 40% with57% silt and 3% sand. According to the Unified SoilClassification System, Degirmenlik soil is classified asCH (clay with high plasticity) and Tuzla soil as CL(clay with low plasticity). Mineral identification withX-ray powder diffraction technique for two soils showsthat both soils are calcareous soils, which contain highpercentage of calcite and dolomite. The soils also con-tain some percentage of montmorillonite minerals, whichgive them the expansion character.

2.2. Fly ash

In the study, an industrial waste Soma fly ash pro-duced in Soma thermal power station in Turkey wasused as a chemical additive to improve the engineeringproperties of the soils. Soma fly ash is a lignite coaland approximately 4 million tons of fly ash is producedper year in the area. The physical and chemical proper-ties of Soma fly ash used in this study are given inTable 2.According to ASTM C618, Soma fly ash can be

classified as Class C fly ash due to its chemicalcomposition. It is a high calcium fly ash with a limecontent of 16%. Class C Soma fly ash has self cementingcharacteristics which provide an inexpensive source ofhigh quality soil stabilizing agent.

3. Methods

An experimental program was performed on Degir-menlik and Tuzla soil specimens collected from a depthof 1.5 m below the ground surface. A series of testswere first performed on compacted soil specimens with-out admixture followed by additional tests, in which flyash was added in different percentages, in order toevaluate the changes in engineering properties of thesoils.

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Fig. 1. Effect of fly ash on plasticity index and linear shrinkage limitvalues of fly ash treated Degirmenlik and Tuzla soils.

Soil–fly ash mixtures were prepared with two differ-ent percentages of fly ash; 15 and 25% fly ash by dryweight of soil. In selecting the levels of the treatment,previous studies on similar soils have been taken intoaccountw5,6x.The soil, oven-dried for 4 days at 508C was pulver-

ized toy40 sieve size. It was mixed with a calculatedamount of stabilizer and then water was added. Thesoil–stabilizer–water was thoroughly mixed by amechanical mixer with a speed of 140 rev.ymin for aperiod of approximately 5 min. After mixing, pre-curingof soil–stabilizer–water mixture was allowed for 24 h,after which the various tests were performed on speci-mens. All specimens were compacted before testing byusing a standard Proctor compaction effort at optimumwater content.In the study, for the determination of the clay size

fraction, approximately 1 kg of the soil in a compactedmold was thoroughly sieve-washed and the soil passingNo. 200 (-75 mm) sieve was oven-dried at 508C for4 days for the hydrometer analysis. The remaining soilin the compacted mold was used for the Atterberg limittests. The linear shrinkage limit values of the naturaland treated specimens were also determined accordingto BS 1377 standard. The optimum water content andmaximum dry unit weight of the natural and treatedspecimens were determined by using the standard Proc-tor compaction test.Swell test specimens with 20 mm height and 76 mm

diameter were prepared at optimum water content andmaximum dry density. The prepared specimens weresealed in waxed paper and then dipped in hot paraffinto ensure complete air-tight and allowed to cure at 228C and 70% relative humidity for the periods of 0, 7,30 and 100 days.After molding, natural and treated specimens were

placed in a standard oedometer device with a seatingpressure of 6.9 kPa. The samples were then submergedin water and the measurement of the expansion of thespecimens continued until equilibrium was reached.CEC values of the specimens were determined in accor-dance with AFNOR 80181 standardw7x. A certainconcentration of methylene blue solution is added indefinite volumes to a suspension of soil and the totalamount of methylene blue solution adsorbed is used forthe calculation of CEC values.

4. Discussion

Fig. 1 shows the effect of fly ash treatment on theplasticity index and linear shrinkage limit values of thenatural and treated Degirmenlik and Tuzla soils. Thefigure indicates that as percent fly ash increases theplasticity index and the linear shrinkage limit values ofthe treated soils decrease. The plasticity index of limeand fly ash treated soils decreases mainly due to an

increase in plastic limitw8x. Liquid limit may increaseor decrease depending on the type of soil. In the presentstudy, the treated Degirmenlik soils show a decrease inthe liquid limit values whereas an increase in the liquidlimit of Tuzla soil was obtained. Although this incrementin liquid limit was obtained for Tuzla soil, the incrementin plastic limit is high enough to offset that increase, sothat the overall plasticity index values of Tuzla soildecrease continuously.From the figure, it can be seen that the effect of fly

ash treatment on the plasticity index of Tuzla soil isvery small. This is due to the low plasticity of Tuzlasoil. Fly ash greatly reduces the plasticity index of highplasticity soils but has little influence on the plasticityindex of low plasticity fine soils. This is attributed tosmaller particle size, higher specific surface area andless crystallinity that make the clay minerals moresusceptible to lime attackw9,10x. The results of Atterberglimit test indicate that the natural Degirmenlik soil hasa much higher plasticity index value than the naturalTuzla soil. The low plasticity of Tuzla soil makes thesoil less susceptible to lime attack than the naturalDegirmenlik soil.Fig. 2 shows the results of swell potential values of

both Degirmenlik and Tuzla soils. Test results indicatethat the untreated Degirmenlik soil gives a swell poten-tial of 19.6% whereas Tuzla soil gives 6.5%. The figureindicates that fly ash is very effective in reducing theswell potential of Degirmenlik soil. With 15% fly ashtreatment, the swell potential decreases to 5% and with25% fly ash treatment, the swell potential of this soildecreases to 3.7%. Further reduction in the swell poten-tial of Degirmenlik soil is obtained with an increase in

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Fig. 2. Variation of swell potential with percent fly ash and curingtime for Degirmenlik and Tuzla soils.

Fig. 3. Effect of fly ash and curing time on the swell pressure valuesof Degirmenlik and Tuzla soils.

Fig. 4. Cation exchange capacity, CEC values of fly ash treatedDegirmenlik and Tuzla soils.

curing time. The swell potential of 25% fly ash treatedsoil decreases to zero at a curing time of 30 days.Test results of Tuzla soil indicate that fly ash treatment

is not very effective in reducing the swell potential ofthis soil. Fig. 2 indicates that 15% fly ash treatmentdecreases the swell potential of Tuzla soil whereas aninitial increase in the swell potential of 25% fly ashtreated soil is obtained. This initial increment isexplained by the high percentage of fly ash added intothis soil. Due to low susceptibility of Tuzla soil to limeattack, addition of high percentage of fine particlescauses the soil to behave like a fine material andconsequently result in an increase in the swell potential.However, with the increase in curing time, the swellpotential of this soil decreases to 2%.A soil with a high swell potential exhibits a high

swell pressure. The amount of swell is reduced by theoverburden pressure. The overburden pressure just suf-ficient to prevent swell upon inundation is termed theswell pressure. The higher the foundation pressure, thesmaller the swell due to the inundation of the activesupporting soilw11x. Fig. 3 shows the swell pressurevalues of fly ash treated Degirmenlik and Tuzla soils.The swell pressure values of the untreated Degirmenlikand Tuzla soils are 480 kPa and 320 kPa, respectively.Test results indicate the reduction in the swell pressurevalues of these soils. The swell pressure is obtained byconsolidating the preswollen specimens to its initialheight. From the figure, it can be seen that the swellpressure values of the treated Degirmenlik soil decreasewith an increase in percent fly ash treatment and furtherreduction in the swell pressure values was obtained withan increase in curing time.

Fig. 3 indicates that similar to the swell potentialvalues obtained for Tuzla soil, an initial increase in theswell pressure value of 25% fly ash treated Tuzla soilwas obtained. With an increase in curing time, a reduc-tion in the swell pressure value of this soil is alsoobtained.Cementation process due to pozzolanic reaction reduc-

es the swell potential of fly ash treated soils andsubsequent reduction in the swell pressure values ofboth soils is obtained. Cementation between particles isa major factor in limiting volume increase of clays onswelling w12x. Test results indicate that fly ash treatmentdecreases the clay size fraction of the soils and causesthe clay particles to amass by cementation. Fig. 4 shows

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Fig. 5. Effect of fly ash and curing time on the particle sizecharacteristics of Degirmenlik and Tuzla soils.

the effect of fly ash treatment on CEC values of bothDegirmenlik and Tuzla soils. CEC is the quantity ofexchangable cations held by that clay and is equal tothe amount of negative charge. Clays with larger specificsurface area usually have higher CEC, higher surfaceactivity and consequently higher water absorption poten-tial. The figure indicates that CEC values decrease withincreasing fly ash percentage. The decrease in CECvalues indicates the changes in the mineralogy of thetreated soils that are no longer representative of theuntreated soils. The decrease in CEC is explained dueto the formation of the new phasesw13,14x with coarserparticles that result in less water absorption potential.Fly ash treatment causes the soil to become moregranular, resulting in lower surface activity and henceless water absorption potential.Fig. 5 shows the increase in the particle sizes of the

fly ash treated Degirmenlik and Tuzla soils. Furtherincrease in sand size particles(2 mm–0.075 mm) withcuring time, substantiates the above findings that, flyash treatment and curing time bring the soil to a moregranular nature and decrease the water absorptionpotential.

5. Conclusion

The following conclusions can be drawn from theabove experimental investigation.

● The fly ash treatment is effective in improving theplasticity of Degirmenlik and Tuzla soils. The

crossing of the A-line from the clayey region to thesilty region occurred in both soils.

● The reduction in the swell pressure values of bothsoils indicates that the swelling of the soils isprevented under smaller pressure values. Thus, highswell potential values are not expected beneath thesmaller foundation pressures.

● The reduced CEC values indicate that fly ash treat-ment causes changes in the mineralogy of the treatedsoils and produces the new secondary reaction min-erals. The formation of these new pozzolanic reactionproducts causes the soils to become more granularand result in less water absorption potential.

Utilization of fly ash as a stabilization material forsoil appears to be one of many viable answers forhandling the fly ash waste problem. Since there is muchmore fly ash that is disposed of rather than utilized,making more productive use of fly ash would haveconsiderable environmental benefits, reducing land, airand water pollution.

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