Microfabric and mineralogical studies on the stabilization of an expansive soil using cement by-pass...

Microfabric and mineralogical studies on thestabilization of an expansive soil using cement by-pass dust and some types of slags

Amer Ali Al-Rawas

Abstract: This paper describes the microfabric and mineralogical aspects of the expansive soil of Al-Khod (northernOman) treated with cement by-pass dust (CBPD), copper slag, slag-cement, and granulated blast furnace slag (GBFS).First, the engineering properties and chemical and mineralogical composition of the untreated soil were determined.The soil was then mixed with the additives at 3, 6, and 9% of the dry weight of the soil. The microfabric and mineral-ogical characteristics of the treated soil were determined. The high amounts of calcium ions and calcium oxide, whichproduces calcium ions, react with the clay particles through a cation exchange process resulting in the formation of ag-gregations and reduction of the swell potential of the soil. Mineralogical tests on the treated samples indicated a gen-eral reduction in all clay minerals peak intensities, particularly in the case of CBPD treated samples. The fabric of theuntreated soil is composed of dense clay matrices with no appearance of aggregations or ped formations with increas-ing amounts of pore spaces. However, aggregations and few connectors were formed due to the addition of the stabiliz-ers. Aggregations and bindings were formed for all of the soils treated with GBFS and for those with 9% additions ofCBPD and slag-cement. The mineralogical and microfabric results were correlated with the swell percent and swellpressure of the treated samples. The formation of aggregations and reduction in clay minerals peak intensities resultedin the reduction of the swell pressure and swell percent values.

Key words: microfabric, mineralogy, stabilization, expansive soils, SEM, XRD.

Résumé : Cet article décrit les aspects de microfabrique et de minéralogie du sol gonflant de Al-Khod (Oman du nord)traité avec de la poussière de ciment de tamis (CBPD), du laitier de cuivre, du laitier de ciment, et du laitier de haut-fourneau granulé (CBFS). On a d’abord déterminé les propriétés mécanique et chimiques et la composition minéralo-gique de sol non traité. Le sol a alors été mélangé avec les additifs à 3, 6, et 9 % du poids sec du sol. Les caractéristi-ques de la microfabrique et de la minéralogie du sol traité ont été déterminées. Les grandes quantités d’ions de calciumet d’oxyde de calcium qui produisent des ions de calcium réagissent avec les particules d’argile par le processusd’échange de cations résultant en la formation d’agrégats et la réduction du potentiel de gonflement du sol. Les essaisminéralogiques sur les échantillons traités ont indiqué une réduction généralisée des intensités de tous les pics minéra-logiques, particulièrement dans le cas des échantillons traités au CBPD. La fabrique du sol non traité est composée dematrices d’argile dense sans apparence d’agrégats ou de formations de peds avec l’accroissement des espaces intersti-tiels. Cependant, des agrégats et quelques liens se sont formés par suite de l’addition d’agents stabilisants. Avec les ad-ditions de GBFS et de 9 % de CBPD et du laitier de ciment, des agrégats et des liens se sont formés. Les résultatsminéralogiques et de microfabrique ont été corrélés avec le pourcentage de gonflement et la pression de gonflementdes échantillons traités. La formation des agrégats et la réduction des intensités des pics des minéraux ont résulté en laréduction de la pression de gonflement et des valeurs de pourcentages de gonflement.

Mots clés : microfabrique, minéralogie, stabilisation, sols gonflants, SEM, XRD.

[Traduit par la Rédaction] Al-Rawas 1167

Introduction

The properties of expansive soils can be significantlychanged when treated with soil stabilizers such as lime, ce-ment, cement by-pass dust, and other additives containing

calcium ions. When such a stabilizer is added to expansivesoils, calcium ions increase in the interlayer of clays result-ing in an attraction between the clay particles and the forma-tion of aggregations or flocs. The cation exchange processcontinues until all charges on the interlayer and edges aresatisfied. This addition is primarily responsible for enhanc-ing the soil workability, but does not result in an increase instrength (Sherwood 1993; Bell 1996).

Further additions of stabilizers lead to pozzolanic reac-tions that occur between the calcium ions of the stabilizerand the silica and alumina of the clay minerals resulting inthe formation of cementitious products such as calcium sili-cate hydrates (CSH), calcium aluminate hydrates (CAH),

Can. Geotech. J. 39: 1150–1167 (2002) DOI: 10.1139/T02-046 © 2002 NRC Canada

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Received 19 October 2000. Accepted 25 April 2002.Published on the NRC Research Press Web site athttp://cgj.nrc.ca on 16 September 2002.

A.A. Al-Rawas. Department of Civil Engineering, College ofEngineering, Sultan Qaboos University, P.O. Box 33, Al-Khod 123, Muscat, Sultanate of Oman.(e-mail: [email protected]).

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and calcium aluminium silicate hydrates (CASH). Thesepozzolanic products contribute to the flocculation process bybonding adjacent flocculated soil particles together, and ascuring occurs, they strengthen the soil. The nature of the ex-changeable cation can have a significant effect in soils con-taining smectite clay minerals (Bell and Coulthard 1990).The stabilization of clayey soils depends on the type of soil,type and amount of additive, water content, curing condi-tions, method of application, and type of construction.

Literature review

Several investigations were carried out on stabilized soilsusing scanning electron microscopes (SEMs) (Basma andTuncer 1991; Keshawarz and Dutta 1993; and Abduljauwad1995). The main purpose of these investigations was to ob-serve the changes in the microstructure of stabilized soils.

Basma and Tuncer (1991) carried out a study on the effectof lime on volume change and compressibility of expansiveclays. Lime was added to clay soils at 3, 6, and 9% of thedry weight of the soil. It was found that as the lime percent-age increased, the soil became more granular. Keshawarzand Dutta (1993), who investigated the effect of fly ash,lime, and cement on the fabric of soils, reported that in thecase of untreated soil, the particles appear as a blocky ar-rangement of loosely packed particles. However, the limetreated soil showed a discernible aggregation of soil parti-cles, while the cement treated soil showed an abundance oftobermorite crystals. Spherical fly ash particles were ob-served in the soil treated with fly ash. Abduljauwad (1995)investigated the effect of lime and potassium nitrate on theplasticity and swell potential of expansive clays. The stabi-lizers were added to the clay soils at different percentages(3, 5, and 8% of the dry weight of the soil). The resultsshowed that the addition of lime and potassium nitratecaused flocculation of the clay particles and an increase intheir apparent grain size.

Previous work on clay mineralogy (Eades and Grim 1960;Willoughby et al. 1968; Lambe and Whitman 1959; Mitchell1976; Abduljauwad 1995; Bell 1996) was undertaken usingX-ray diffraction techniques to investigate the mineralogicalchanges and identify the reaction products formed whenlime is added to clay soils. It was found that new reflectionswere visible at d-spacings of 3.87, 3.67, 3.035, 1.619, and1.582 Å (1 Å = 0.1 nm), which appeared to be indicative ofcalcite, feldspars, kaolinite, and chlorite, respectively.Abduljauwad (1995) showed that the major peaks of the clayminerals (i.e., smectite, palygorskite, and illite) of untreatedsoils were significantly altered upon treatment. For example,in the case of potassium nitrate treatment, the smectite claymineral peaks almost disappeared.

The author conducted a literature survey that revealed thelack of detailed comparative studies on the microfabric andmineralogy of expansive soils treated with various stabiliz-ers. Most of the studies carried out by Collins and McGown(1974, 1983) and Collins (1984) were on the microstructureof untreated natural soils. Microstructure studies on un-treated Omani expansive soils were published by Al-Rawasand McGown (1999). Furthermore, Al-Rawas et al. (2002)carried out a study on the stabilization of expansive soils us-

ing cement by-pass dust (CBPD), copper slag, slag-cement,and granulated blast furnace slag (GBFS). The study focusedon the engineering and chemical characteristics of thetreated soil. However, no work was published on the micro-fabric and mineralogy of stabilized Omani expansive soils.Therefore, there was a need to undertake these detailed in-vestigations to fully characterize the behavior of the treatedsoil.

Objectives

The main objectives of this study were (1) to investigatethe microfabric and mineralogical characteristics of Al-Khod(northern Oman) soil treated with CBPD, copper slag, slag-cement, and GBFS, and (2) to correlate the microfabric andmineralogical results with the swell potential (swell percentand swell pressure) measured for the same soil used by Al-Rawas et al. (2002).

Materials

Site selection and samplingThe site selected for obtaining the samples is located at

Al-Khod (about 40 km west of Muscat and approximately10 km west of Seeb International Airport) in northern Oman.The site was selected because it contains expansive soils thatwere encountered in a trial pit at a depth of about 1 m andoverlain by a layer of sand and gravel. The disturbed expan-sive soil was excavated, placed in plastic bags, and trans-ported to Sultan Qaboos University for testing.

Properties of the untreated soil

Chemical propertiesThe chemical and geotechnical properties of the untreated

soil are given in Table 1. The chemical analyses includedpH, cation exchange capacity (CEC), and exchangeable cat-ions. The chemical composition tests were performed in ac-cordance with Omanian Standard 25 (Ministry of Commerceand Industry, Oman 1979), which is equivalent to ASTMC114-99 (ASTM 1999). The exchangeable cations were ex-tracted with ammonium acetate, with subsequent determina-tion of the cations by standard methods (Chapman 1965).The Na+ and K+ were measured by the flame photometrymethod, and Ca2+ and Mg2+ were measured by atomic ab-sorption spectrophotometry. The CEC was measured by thesodium acetate method. The CEC, Na+, and Ca2+ values ofthe soil are 70 (mequiv./100 g), 41%, and 6%, respectively(Table 1). The reason for only determining Na+, Ca2+, Mg2+,and K+ is because of their importance in the swelling phe-nomenon. From a chemical point of view, this indicates thatthe soil has the potential to swell due to the high percentageof Na+ (Grim 1968).

Geotechnical propertiesAll basic geotechnical tests were performed in accordance

with British Standard 1377 (British Standards Institution1990). Based on the Casagrande plasticity chart, the soil wasclassified as inorganic silt of high plasticity (MH). The mostcommon swelling potential tests involve the use of a one-dimensional consolidation apparatus or oedometer, which

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was used in this study. There are no universal standard test-ing methods for determining the swell percent and swellpressure. Nelson and Miller (1992) summarized the varioustesting methods used by previous researchers. The methodsused in this study were developed by the author (Al-Rawas1993; Basma et al. 1998; Al-Rawas et al. 2002).

To prepare remolded samples, the soil was first cut intosmall pieces and air-dried for 24 h. It was then pulverizedrepeatedly using a plastic hammer. Because of its cohesivenature, the soil was then fully soaked in water for another24 h. After soaking, the soil disintegrated into its individualcomponents and additional lumps were broken by hand. Thesoil was then placed in an oven at 105°C for 24 h to ensurecomplete dryness. The dry soil was further pulverized to< 2 mm. At this stage, the soil was ready for remolding.

An amount of dry soil required for the natural dry unitweight was weighed and mixed with stabilizers at 3, 6, and9% by dry weight of the soil. The required amount of waterwas also weighed. The soil-additive mixture was thoroughlymixed and then placed into the mold and lightly compactedusing a wooden hammer (25 blows) to exactly fit the cuttingring. All remolded specimens were left in a desiccator for24 h before testing. The swell percent and swell pressuretests were conducted on samples compacted at their naturalmoisture content in the consolidation ring.

The swell percent of each test specimen was measured us-ing the loaded-swell method (Al-Rawas 1993). In this test,the specimen was loaded to a seating pressure of 25 kPa(equivalent to the exiting overburden pressure) and thenflooded with water and allowed to swell. Readings weretaken until full swell was attained. The increase in verticalheight of a sample, expressed as a percentage, due to the in-crease in moisture content was designated as the “swell per-cent.” The swell pressure of each test specimen wasmeasured using the constant volume method (Al-Rawas1993). The specimen placement conditions were the same asin the swell percent test. The specimen was then floodedwith water while the specimen volume was kept constant.Loads were applied using sand until deformation ceased. Atthis stage, the swell pressure was calculated as the load re-quired to prevent swelling divided by the area of the speci-men. For detailed information on the swelling tests, thereader is referred to Basma et al. (1998). The soil showed aswell pressure of 250 kPa and a swell percent of 9.4%.

Additives used in the studyIn this study, CBPD (a by-product), copper slag (a by-

product), slag-cement, and GBFS were utilized. CBPD, slag-cement, and GBFS were supplied by the Oman CementCompany. GBFS is imported by the Oman Cement Com-pany from Italy for the production of slag-cement. Copperslag was produced as a waste product during the extractionof copper in Sohar (northern Oman).

Al-Rawas et al. (2002) carried out a full chemical analysisof all the additives used in this study (Table 2). Copper slaghas the highest value of exchangeable Na+ (38%) and thelowest percentage of Ca2+ (4%). Slag-cement and GBFSshowed relatively close values of CEC, Na+, and Ca2+,which are significantly different from those of copper slag.CBPD showed extremely high values of CEC (140mequiv./100g) and Ca2+ (92%) and the lowest value of Na+

(0.1%). It is generally known that the swelling potential of asoil is greatly influenced by its chemical composition. Thehigher the CEC and Na+, the higher the swelling of the soiland vice-versa. On the other hand, the higher the Ca2+, thelower the swelling potential of the soil. Based on the chemi-cal results alone, it is expected that copper slag will increasethe swell potential of the soil due to its high amount of Na+,while the other additives will reduce the swell potential ofthe soil by varying degrees.

Al-Rawas et al. (2002) performed swell potential tests onsamples treated with the additives used in this study (Ta-ble 3). The results showed that the GBFS was the most ef-fective additive, since it reduced the swell pressure from 250to 162 kPa at the addition rate of 3%. On the other hand, theaddition of copper slag caused an increase in the swell pres-sure by about 100 kPa. CBPD and slag-cement treated sam-ples produced similar swell pressure values with theexception of the 3% slag-cement treated sample, which ex-hibited a swell pressure of 311 kPa. All additives reducedthe swell percent by varying degrees, except for copper slag,which caused a slight increase in the swell percent. Themaximum reduction in swell percent was achieved by mix-ing the soil with 9% CBPD.

Methods of testing

Instrumentation — scanning electron microscope(SEM)

The present study is based entirely on the use of a JEOLJSM-840A SEM utilizing the secondary electron mode.Magnification ranges from × 20 to × 5000 were used for the

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CharacteristicsValues anddescriptions

Colour YellowishDepth (m) 1.2Natural water content (%) 8.9Field dry unit weight (kN/m3) 17Specific gravity 2.8Percent passing no. 200 seive 60Clay content (less than 2 µm) (%) 20Liquid limit (%) 50Plastic limit (%) 30Plasticity index 20Clay activity 1.03Swell pressure (kPa) 250Swell percent 9.4Unified Soil Classification System (USCS) MHCompaction properties (approximate)

Optimum water content (%) 21Maximum dry weight (kN/m3) 17.5

Exchangeable cations (%)Sodium (Na) 41Calcium (Ca) 6Magnesium (Mg) 1Potassium (K) 1

Cation exchange capacity (CEC) (mequiv./100g) 70pH 9.2

Table 1. Physical and chemical properties of the untreated soil.



examination of the soil fabric. This SEM allows the viewingof relatively large specimens up to 10 × 10 × 5 mm in size.

Sample preparation for SEMThe samples were initially prepared following the same

procedures performed for the swell percent and swell pres-sure tests as described in the section entitled “Properties ofthe untreated soil” except that the samples were not testedfor swelling.

Al-Rawas and McGown (1999) made an extensive reviewof the drying techniques (Sides and Barden 1971; Collins1978; Zein 1985) and concluded that there is no single tech-nique that can be claimed to be the best for specimen drying.Air drying was therefore used in this study. Two specimensfrom each sample were tested, one representing the verticalplane and the other representing the horizontal plane. Fourspecimens were cut for each sample, two specimens to rep-resent the vertical plane and the other two the horizontalplane, as suggested by Collins (1978). Prismatic specimensapproximately 1 cm3 were carefully cut using a sharp knife.The specimens were dried slowly at a relatively constanttemperature of approximately 25°C over a period of abouttwo weeks.

The dried specimen was then fractured by cutting a V-shaped groove around the middle of the specimen and apply-ing a combined bending and pulling action as suggested bySmart (1967) and Smart and Tovey (1981, 1982). The baseof the specimen was then trimmed flat and cemented to theSEM aluminum stub. After mounting, the specimen was sub-jected to between 50 and 100 cellotape applications and re-movals to remove debris and particles damaged duringfracturing; this process was referred to as peeling by Bardenand Sides (1971). The peeled specimen surface was vacuumcoated with gold to make it electrically conductive to pre-vent charge build up on the specimen. Following this, thespecimens were kept in a sealed plastic box and stored in adesiccator to avoid contamination from the surroundings for24 h before testing. Direct handling of the specimens waskept to a minimum at all stages to avoid contamination.

Microfabric characterizationCollins and McGown (1983) developed a microfabric

characterization scheme, which was used in this study to de-

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CharacteristicsAdditives Cementby-pass dust (CBPD) Copper slag Slag-cement

Granulated blastfurnace slag (GBFS)

pH — 9.1 10.3 8.8CEC (mequiv./100g) 140 76 55 67Exchangeable cations (%)a

Sodium (Na) 0.1 38 15 10Calcium (Ca) 92 4 27 22Magnesium (Mg) 4.9 2 3 1Potassium (K) 0.3 2 2 1

Compounds (%)SiO2 15.84 35.70 31.50 33.97Al2O3 3.57 1.43 8.50 14.51Fe2O3 2.76 47.35 1.90 1.43CaO 63.76 9.40 47.60 41.24MgO 1.93 4.37 5.10 7.56SO3 1.65 0.28 2.90 2.22K2O 2.99 0.22 0.33 0.33Na2O 0.33 0.15 0.22 0.20TiO2 0.48 0.98 0.39 0.54Mn2O3 0.07 0.04 0.33 0.31Cl 1.09 0.06 0.03 0.01LOI 5.38 — 1.30 1.71

Note: LOI, loss on ignition.aOnly Na+, Ca2+, Mg2+, and K+ were determined.

Table 2. Some chemical characteristics of the additives used (Al-Rawas et al. 2000).

Additives (%) Swell percent Swell pressure (kPa)

Untreated soil 9.4 250Cement by-pass dust

3 5.1 2596 6.9 2619 4.1 202

Copper slag3 10.2 3536 10.6 3549 11.9 356

Slag-cement3 6.0 3116 5.0 2629 5.0 200

Granulated blastfurnace slag3 7.4 1626 7.6 1899 8.0 190

Table 3. Swell potential results of the treated soil (Al-Rawas etal. 2000).



scribe the microfabric features of the specimens tested. The“elementary particle arrangements” involve interaction be-tween a small number of like constituent particles of clay,granular, or organic matter, and a number of subforms andsubdivisions are thus identified. The “particle assemblages”are units of particle organization and have definable physicalboundaries. They consist of one or more forms of elemen-tary particle arrangement or smaller particle assemblages.Basic and higher order assemblages are identified with thelatter representing higher levels of particle assemblages or-ganization. Subforms and subdivisions are defined in eachcase. The “pore spaces” occur within, between, and acrossthe various particle arrangements and are classified based onCollins and McGown (1983) scheme.

Instrumentation — X-ray diffraction (XRD)The X-ray diffractometer used in this study was a Philips

PW1700 automated powder diffractometer. The generatorsettings were 40 kV and 40 mA and CuKα radiation, λ =1.5418 Å was used. A scanning rate of 2° 2θ per minutefrom 2 to 35° 2θ was used for all samples. The X-raydiffractometer was controlled by a µPDP11 computer thatgave both a trace of the X-ray patterns of the minerals and acomputer output listing the d-spacings, angle of diffraction,intensities, etc.

Sample preparation and treatment for XRDSample preparation consisted of separating the clay frac-

tion, preparing oriented samples, and treating the orientedsamples. The soil was initially air dried for several days,then it was crushed until it passed a no. 200 sieve (75 mm).About 15 g of the sieved material was placed in a beakercontaining 600–800 mL of distilled water, and a few dropsof a dispersing agent (sodium hexametaphosphate) wereadded. Then the suspension was stirred thoroughly and leftfor at least 6–8 h. The top 4 cm of the suspension wasthen transferred into 50 mL centrifuge tubes. Clay frac-tions (< 2 µm) were obtained by centrifuge fractionation(5000 rpm for 30 min).

The procedures used for the preparation of oriented sam-ples and their chemical and heat treatments were as sug-gested by Whittig and Allardice (1986) and Brown andBrindley (1980). For the preparation of oriented samples onglass slides, 100 mg of the clay fraction was mixed with1 mL of distilled water and stirred thoroughly. Approxi-mately 50 mg of the clay in the suspension was then care-fully transferred onto a 2.6 × 2.3 cm glass slide. The clayparticles were allowed to settle freely on the slide. Similarly,the remaining 50 mg was also transferred onto another slide.The samples mounted on the slides were allowed to dry be-fore examination.

Initially, the samples mounted on the slides were exam-ined in three forms; as an oriented clay sample (untreated),as an oriented clay sample treated with ethylene glycol, andas an oriented clay sample heated to 550°C for 2 h. One ofthe prepared clay slides was used for both the untreated andheat treated samples, while the other was used for the sam-ple treated with ethylene glycol. Two drops of ethylene gly-col were directly applied on the clay film, which was left for24 h before testing. This technique was used because it wassimple and effective. Ethylene glycol and heat treatments

were used to provide additional information essential for theidentification of clay minerals.

For the preparation of the samples treated with various ad-ditives, 100 mg of the clay fraction extracted earlier wasmixed with each additive (after grinding to a powder form)at 3, 6, and 9% by dry weight of the clay. Two glass slidescontaining approximately equal amounts of additive treatedsamples were prepared as described earlier. No ethylene gly-col or heat treatment was used herein.

Results

Description of the micrographs

Untreated samplesFigure 1 shows SEM micrographs of the untreated sam-

ples that were taken at different magnifications. The generalfabric of the sample is composed mainly of clay matrix withan appreciable number of granular clothed grains embeddedwithin the system (Fig. 1a). At an intermediate magnifica-tion, the fabric of the sample is composed of very dense claymatrix with sand grains within the matrix (Fig. 1b). Porespaces of various types and sizes are clearly seen. A denseclay matrix associated with discernible pore spaces domi-nates the fabric of the sample when observed with high mag-nification (Fig. 1c). At all levels of magnifications, noaggregations or connectors were observed.

Samples treated with 3% additivesFigure 2 exhibits the soil fabric with low magnification

(×160) for samples treated with 3% of the additives used.The fabric of the CBPD stabilized sample (Fig. 2a) revealeda continuous mass of clay matrix with few granular grains,while pore spaces are not visible. Figure 2b represents thefabric of the sample treated with copper slag, consisting ofmasses of clay, granular matrices, and aggregations. Thegranular grains are clothed and embedded within aggregates.Intra-assemblage pore spaces are visible. Figure 2c revealsthat the slag-cement treated sample is composed mainly ofextensive clay matrices, whereas the fabric of the sampletreated with GBFS exhibits a clear aggregation that is gener-ally sub-rounded in shape and approximately 100–250 µm indiameter (Fig. 2d).

With medium magnification (×500), the CBPD stabilizedsample had a continuous clay matrix with rare appearancesof interassemblage pore spaces (Fig. 3a). An aggregation ofsub-rounded shape (≈ 30–50 µm in diameter) was observed.A clay matrix associated with intra-assemblage pore spacesdominates the fabric of the copper slag treated sample(Fig. 3b). Figure 3c reveals that the sample stabilized withslag-cement consists mainly of extensive masses of clay ma-trices with few partly discernible clay aggregations that wererelatively small in size (≈ 20–25 µm in diameter). The GBFSstabilized sample is composed of an irregular large aggrega-tion and clay matrix with interassemblage pore spaces(Fig. 3d).

When the CBPD stabilized sample was examined athigher magnification (Fig. 4a), very dense clay matriceswith few interassemblage pore spaces were observed. Thecopper slag treated sample revealed discernable clay matriceswith few granular grains (Fig. 4b). Aggregations interacting

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Fig. 1. SEM micrographs for untreated samples (a) magnification × 160; (b) magnification × 500; (c) magnification × 2000.



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Fig. 4. SEM micrographs (magnification × 1000) for samples treated with 3% of (a) cement by-pass dust (CBPD); (b) copper slag;and (c) slag-cement.



with the clay particle matrix were clearly seen for the GBFSstabilized sample (Fig. 4c). Most of these aggregations wererounded to sub-rounded in shape, with considerable varia-tion in size (≈ 30–75 µm in diameter).

Samples treated with 6% additivesThe micrographs taken at low magnification (×160) for

the samples treated with 6% of various stabilizers show ageneral fabric that consists of dense small aggregations withgranular grains. With magnification at ×500, CBPD(Fig. 5a), slag-cement (Fig. 5c), and GBFS (Fig. 5d) stabi-lized samples showed discernible clay aggregations associ-ated with pore spaces of different types and sizes. Theaggregations were small in size (≈ 20 µm in diameter). Thedevelopment of connectors can be clearly seen in the GBFSstabilized sample (Fig. 5d). A clustered clay system domi-nates the fabric of the copper slag treated sample (Fig. 5b).

At higher magnification, interconnected aggregations withinterassemblage and intra-assemblage pore spaces were ob-served for all of the treated samples (Fig. 6). The slag-cement treated sample showed visible grains and clay parti-cles in addition to the aggregations (Fig. 6c). The aggrega-tions appearing at this level were mostly irregular in shapeand large in size (≈ 20–60 µm in diameter).

Samples treated with 9% additivesThe micrographs of the samples stabilized at 9% exhibited

a continuous mass of dense clay matrices and aggregationsunder low SEM magnification. With medium magnification(×500), the CBPD treated sample reveals discernible verylarge aggregations (≈ 100–130 µm in diameter), mostly ir-regular in shape, interconnected with each other withinterassemblage pore spaces (Fig. 7a). An extensive andcontinuous mass of clay matrix dominates the fabric of thesample treated with copper slag (Fig. 7b). Figure 7c repre-sents the fabric of the slag-cement treated sample composedmainly of connectors, some aggregations, and granulargrains with some interassemblage pore spaces. A blocky ar-rangement of partly discernible irregular aggregations wasobserved in the GBFS treated sample (Fig. 7d).

Figure 8a shows that the CBPD stabilized sample is com-posed mainly of aggregations, approximately 30–50 µm indiameter, with large pore spaces of interassemblage type,when examined under high magnification. At the same level,the fabric of the sample stabilized with copper slag exhibiteda clay matrix with intra-elemental pore spaces (Fig. 8b).Connectors showing clearly the interaction between the clayparticles and the granular grains were observed in the slag-cement treated sample (Fig. 8c). The GBFS stabilized sam-ple revealed similar fabric to that observed with mediummagnification.

XRD results

Identification of mineralsA representative X-ray pattern of a sample of Al-Khod

clay is shown in Fig. 9, detailing some stages of ethyleneglycol and heat treatment. The clay mineral groups wereidentified in the clay of Al-Khod by their characteristicsbasal reflections: smectite (14.59 Å expands to 17.01 Å afterglycolation and collapses to 9.95 Å upon heat treatment for2 h at 550°C), palygorskite (10.98 Å and unaffected by

glycolation, and its density decreases at 350°C and disap-pears at 550°C), illite (10.04 Å and unaffected byglycolation or heat treatment for 2 h at 550°C), kaolinite(7.21 Å and unaffected by glycolation and disappears afterheating at 550°C), and serpentine (7.21 Å and unaffected byglycolation or heat treatment for 2 h at 550°C). The figurealso shows quartz (3.34 Å) and calcite (3.03 Å). The datasupport earlier reported results (Al-Rawas et al. 1998).

Effect of additivesThe XRD patterns of the treated samples are shown in

Fig. 10. Generally speaking, there was a reduction in thepeak intensity of most of the treated samples, as can be seenby the reduction in peak heights and broadness of some ofthe peaks, particularly for the samples treated with 9% addi-tives. A significant increase in the broadness of the smectite,palygorskite, and illite peaks was observed, particularly withthe addition of CBPD and copper slag. Similar behaviorcaused by these two additives was observed for the kaolinitepeak. However, the mineral peaks of samples treated withslag-cement showed minor changes with respect to peakbroadness. None of the peaks of the untreated sample disap-peared due to treatment except for the peak corresponding togypsum, which disappeared when treated with GBFS. It wasnoted that with the addition of 9% slag-cement, the peaksshifted away to the right from their original positions whilemaintaining the same patterns. This could be due to instru-ment distortion during the X-ray scanning process or condi-tions of the sample.

The changes in peak intensities mentioned above weredue to the addition of the stabilizers. The untreated sample,which was composed mainly of clay fraction (approximately< 2µm), exhibited clear and sharp peaks that correspond toclay minerals present in the sample. However, with the addi-tion of the stabilizers, chemical reactions took place betweenthe additive and the clay resulting in the reduction of theclay minerals intensities.

Discussion

Untreated samplesThe untreated soil exhibited a swell percent of 9.4 and a

swell pressure of 250 kPa. The high swell potential valuesmeasured were mainly attributed to three major factors:chemical composition, fabric, and mineralogy of the soil.The chemical analysis revealed that Na+, the most active cat-ion, was the predominant cation (41%), while Ca2+, which isknown to be stable, constituted only 6%. From a microfabricpoint of view, the soil is composed of a dense clay matrixwith pore spaces of various types and sizes with no aggrega-tions or connectors. Mineralogically speaking, smectite, themost active clay mineral, along with palygorskite and illite,which are intermediate in swelling, was present in the soil,thereby making it expansive. Furthermore, the existence ofexpanding clay minerals with predominantly Na+ in a denseclay matrix associated with pore spaces with little sign ofaggregations or connectors explains the expansive nature ofthe soil and the high swell potential values measured.

© 2002 NRC Canada

Al-Rawas 1159



© 2002 NRC Canada


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© 2002 NRC Canada

Al-Rawas 1161

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© 2002 NRC Canada


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© 2002 NRC Canada

Al-Rawas 1163

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Cement by-pass dust (CBPD) treated samplesThe addition of 3 and 6% CBPD to the soil reduced the

swell percent from 9.4 to 5.1 and 6.9, respectively. However,no reduction in swell pressure was recorded. The addition of9% of CBPD has caused reduction in both the swell percentand swell pressure by 5.3 and 48 kPa, respectively (Table 3).The reason for not detecting a reduction in swell pressure at3 and 6% CBPD is probably due to the method of measuringthe pressure, which was through the use of dead loads ratherthan a load cell. In addition, since the reduction at 9% addi-tion was only 48 kPa, therefore, the expected reduction inpressure at 3 and 6% should be small.

Examination of the SEM micrographs of the treated sam-ples showed no evidence of alteration in the structure of thesamples treated with 3% CBPD. However, with additions of6 and 9% of CBPD, aggregations of particles were formed,particularly in the case of the 9% addition where aggrega-tions were large and discernible.

The XRD results showed that CBPD caused significant re-ductions in the XRD mineral peak intensities, particularlyfor smectite clay minerals. In addition, the X-ray patterns ofall of the CBPD treated samples were almost completelyoverlapped. This indicates that from a mineralogical point ofview there is no apparent effect due to the addition of differ-ent percentages of CBPD. Examination of the chemical anal-ysis of CBPD showed that it possesses extremely highamounts of exchangeable Ca2+ (92%) and CaO (63.76%),which produces calcium ions, that react with the clay parti-cles through a cation exchange process resulting in the for-mation of aggregations as can be seen from the SEM

micrographs (Figs. 5–8). It is believed that the reduction inthe minerals intensities, and the formation of aggregationshad contributed to the reduction in swell percent and swellpressure.

Copper slag treated samplesAll of the samples stabilized with copper slag showed a

marked increase in the swell pressure (about 100 kPa) and aslight increase in the swell percent (Table 3). The copperslag contains very high amounts of Na+ (38%) and lowamounts of Ca2+ (4%) and CaO (9.40%). The fabric of thesample is mainly composed of an extensive mass of clay ma-trices associated with pore spaces of various types and sizes.Few aggregations and connectors were observed at the addi-tion of 3 and 6%. It is surprising to see that most of the min-eral peaks of the samples treated with copper slag wereweaker than other treated samples while the swell percentand swell pressure of the treated samples increased signifi-cantly. From the chemical analysis it appears that Na+ haspromoted the activity of the treated soil, thereby increasingits swell percent and swell pressure. This has been comple-mented by the fact that the fabric was composed mainly ofdense clay matrices, which promote swelling.

Slag-cement treated samplesThe behavior of slag-cement treated samples is generally

similar to that of the samples treated with CBPD. It is inter-esting to observe an initial increase in the swell pressure by50 kPa with the addition of 3% slag-cement. However, a de-crease of about 60 kPa was achieved at the addition of 9%.

© 2002 NRC Canada


Fig. 9. XRD patterns of Al-Khod clay with different treatments.



© 2002 NRC Canada

Al-Rawas 1165

With regard to swell percent values, a consistent reductionwas measured with all of the additions. Chemically, theslag-cement contained appreciable amounts of exchangeable

Ca2+ (27%) and CaO (47.60%) and relatively low amountsof exchangeable Na+ (15%).

The microfabric features are characterized by the presence

Fig. 10. XRD patterns for an untreated sample and samples treated with additives.



of a clay matrix and granular grains with development ofsome aggregations at the addition of 6% slag-cement. Withthe addition of 9%, clear clay aggregations, connectors, andpossibly silt grains were observed. The XRD traces (Fig. 10)indicated slight reduction in the clay minerals peak intensi-ties (palygorskite and illite), due to the addition of slag-cement.

From the above results, it appeared that with the additionof 3% slag-cement, no significant alteration occurred to thefabric, however, with the addition of 9%, aggregations andconnectors were formed, all of which, along with granulargrains, contributed to the reduction of the soil’s swell poten-tial.

Granulated blast furnace slag (GBFS) treated samplesThe GBFS was the most effective additive as seen by the

major reduction in the swell pressure values at all three per-centage additions. The GBFS contained 22% of exchange-able Ca2+ and 41.24% of CaO and l0% of exchangeable Na+.The SEM micrographs of the treated samples clearly showedthe formation of aggregations and connectors and the pres-ence of granular grains with the additions of 3, 6, and 9% ofGBFS. It is believed that the development of aggregationshad significantly contributed to the reduction of the swellpercent and swell pressure. The XRD results showed a gen-eral reduction in the clay minerals’ peak intensities, therebyreducing the minerals’ potential for expansion.

Geotechnical significance of resultsThe geotechnical engineer is concerned with the evalua-

tion of the swelling potential of expansive soil and determin-ing the optimum solutions to eliminate or reduce swelling toa reasonable limit. The results of this study formed a frame-work for the key elements controlling the soil’s behavior. Itshowed that the untreated soil consisted of expansive clayminerals including smectite, palygorskite, and illite associ-ated with high and low amounts of Na+ and Ca2+, respec-tively. Further, its fabric was composed of dense claymatrices with no appearance of aggregations or connectors.These conditions are prerequisites for a potentially expan-sive soil, and the information is very useful to thegeotechnical engineer in the preliminary evaluation of thesoil behavior.

With regard to the other aspect concerning the evaluationof soil additives, the results showed that the amounts of Na+,Ca2+, and CaO play a major role in this respect. For exam-ple, copper slag, which contained the highest amount of Na+

and the lowest amounts of Ca2+ and CaO, increased theswelling potential of the soil, whereas GBFS (which has lowamounts of Na+ and relatively high amounts of CaO) re-duced the swelling potential. This is because additions ofcalcium ions contribute to the reduction in intensity ofsmectite, palygorskite, and illite clay minerals and alter thedense clay matrices of the soil to form aggregations of vari-ous sizes, and thereby reduce the swell potential of the soil.Therefore, the determination of the chemical composition ofa soil additive is very crucial in the stabilization process.The results gave a full picture of the key elements involvedin stabilization and the relationship between them.

Conclusions

Based on the results obtained from the investigations pre-sented in this paper, the following conclusions can be drawn.

(1) The swell potential of the untreated soil can be ex-plained by the high percentage of sodium smectite clay min-eral, along with palygorskite and illite. Furthermore, the soilis composed of dense clay matrices with no appearance ofaggregations or connectors, which provide strength to thesoil against swelling.

(2) Granulated blast furnace slag (GBFS) treated samplesexhibited the lowest swell pressure, whereas copper slagtreated samples showed the highest values of swell pressure.The 9% addition of CBPD and slag-cement caused a markedreduction in the swell pressure values. Therefore, copperslag is not recommended for the stabilization of expansivesoils.

(3) A high percentage of Na+ and a low percentage ofCa2+ promotes swelling and vice versa. The chemical com-position of stabilizing materials proved to be a good indica-tor of the effectiveness of these materials.

(4) All additions of GBFS and 9% additions of CBPD andslag-cement resulted in the formation of aggregations orflocculations, which reduced the swelling potential of thesoil.

(5) The XRD results showed a general reduction in allclay minerals’ peak intensities particularly in the case ofCBPD treated samples. However, it appeared that there wasno clear correlation between the reduction in peak intensitiesand the swell potential values.

(6) The swell potential, mineralogical composition, andmicrofabric of the stabilized soil gave a full scale character-ization of the soil behavior. However, performing swell po-tential tests to measure the swell pressure and swell percentof the stabilized soil will be the most reliable method for as-sessing the actual swell potential.

Acknowledgment

The author would like to thank Mr. Nasser Al-Busaidi, anundergraduate student in the Department of Civil Engi-neering, and the technical staff of the Department of EarthSciences (Mr. Saif Al-Mamari and Ms. Samirah Al-Kharousi) and the Department of Civil Engineering (Mr.Yaqoob Al-Alawi) for their help in performing the scanningelectron microscope and X-ray diffraction tests.

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Microfabric and mineralogical studies on the stabilization of an expansive soil using cement by-pass...

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