ARTICLE

Engineering properties and microstructural characteristics ofcement-stabilized zinc-contaminated kaolinYan-Jun Du, Ning-Jun Jiang, Song-Yu Liu, Fei Jin, Devendra Narain Singh, and Anand J. Puppala

Abstract: This paper presents details of a study that deals with determination of engineering properties, identification of phasesof major hydration products, and microstructural characteristics of a zinc-contaminated (referred to as Zn-contaminated in thispaper) kaolin clay when it is stabilized by a cement additive. Investigations were carried out with respect to the effect of the levelof zinc (Zn) concentration on the overall soil properties including Atterberg limits, water content, pH, stress–strain character-istics, unconfined compressive strength, and secant modulus. In addition, X-ray diffraction, scanning electron microscopy, andmercury intrusion porosimetry studies were conducted to understand the mechanisms controlling the changes in engineeringproperties of the stabilized kaolin clay. The study reveals that the level of Zn concentration has a considerable influence on theengineering properties, phases of hydration products formed, and microstructural characteristics of the stabilized kaolin clay.These changes are attributed to the retardant effect of Zn on the hydration and pozzolanic reactions, which in turn alters thephases of hydration products and cementation structure – bonding of the soils. Theoretical simulation of the pore-size distri-bution curves demonstrates that the cement-stabilized kaolin exhibits bimodal type when the Zn concentration is less than 2%,whereas it displays unimodal type when the Zn concentration is 2%. With an increase in the Zn concentration, the characteristicsof the interaggregate pores in terms of volume and mean diameter change considerably, whereas those of intra-aggregate poresremain nearly unchanged.

Key words: heavy metals, microstructure, modulus, pore-size distribution, strength.

Résumé : Cet article présente les détails d’une étude visant a déterminer les propriétés d’ingénierie, a identifier les phases desproduits d’hydratation majeurs, et a identifier les caractéristiques microstructurales d’une argile kaolin contaminée par du zinclorsqu’elle est stabilisée par un additif cimentaire. Des investigations ont été réalisées afin d’évaluer l’effet de la concentrationde zinc sur les propriétés globales du sol, incluant les limites d’Atterberg, la teneur en eau, le pH, les caractéristiques decontraintes–déformations, la résistance a la compression non confinée, et le module sécant. De plus, des études par diffractiondes rayons-x, microscopie électronique a balayage et porosimétrie par intrusion au mercure ont été réalisées pour comprendreles mécanismes contrôlant les changements de propriétés d’ingénierie dans l’argile kaolin stabilisée. L’étude révèle que laconcentration de zinc a une influence considérable sur les propriétés d’ingénierie, sur les phases d’hydratation formées et sur lescaractéristiques de la microstructure de l’argile kaolin stabilisée. Ces changements sont attribués a l’effet retardant du zinc surles réactions d’hydratation et pouzzolaniques, qui a leur tour altèrent les phases des produits d’hydratation et la structurecimentaire et les liens des sols. Une simulation théorique des courbes de distribution de la taille des pores démontre que le kaolinstabilisé par le ciment présente un type bimodal lorsque la concentration de zinc est inférieure a 2 %, tandis qu’il présente untype uni-modal lorsque la concentration de zinc est de 2 %. Lorsque la concentration de zinc augmente, les caractéristiques despores inter-agrégats change considérablement en termes de volume et de diamètre moyen, alors que celles des pores intra-agrégats demeurent presque inchangées. [Traduit par la Rédaction]

Mots-clés : métaux lourds, microstructure, module, distribution de la taille des pores, résistance.

IntroductionWith rapid industrialization and urbanization in China, land

contamination has become a serious environmental issue. It isreported that a huge number of lands in China are contaminatedby heavy metals including zinc (Zn), lead (Pb), cadmium (Cd), andarsenic (As) (Xie and Li 2010). The contaminated lands are inevita-bly posing a potential threat to human health and the surround-ing environment. As such, remediation of the contaminated landsis of a great concern to Chinese professionals and planners.

Solidification–stabilization (S–S) has been used extensively forremediation of the heavy metal contaminated soils by earlier re-

searchers. This process is responsible for lowering the soil perme-ability, which results in a reduction of leaching of contaminants,and enhancing its strength (Chen et al. 2009; US EPA 2010). Ordi-nary Portland cement (OPC) is a commonly utilized binder in theS–S remediation and soil stabilization practice (Paria and Yuet2006; Liu et al. 2012). By S–S, heavy metals are converted to stableforms and are entrapped in the solid cementitious matrix of thestabilized contaminated soils (Paria and Yuet 2006). Strength, per-meability, and leaching properties of the S–S treated soils havebeen extensively studied by earlier researchers (Al-Tabbaa andKing 1998; Yilmaz et al. 2003; Antemir et al. 2010; Kogbara and

Received 11 May 2013. Accepted 16 December 2013.

Y.-J. Du, N.-J. Jiang,* and S.-Y. Liu. Institute of Geotechnical Engineering, Southeast University, Nanjing 210096, China.F. Jin. Engineering Department, Cambridge University, Trumpington Street, Cambridge CB2 1PZ, UK.D.N. Singh. Department of Civil Engineering, Indian Institute of Technology, Bombay, Mumbai 400076, India.A.J. Puppala. Department of Civil Engineering, University of Texas at Arlington, Arlington, TX 76019, USA.Corresponding author: Yan-Jun Du (e-mail: duyanjun@seu.edu.cn).*Present address: Engineering Department, Cambridge University, Trumpington Street, Cambridge CB2 1PZ, UK.

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Can. Geotech. J. 51: 289–302 (2014) dx.doi.org/10.1139/cgj-2013-0177 Published at www.nrcresearchpress.com/cgj on 17 December 2013.

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Al-Tabbaa 2011; Du et al. 2012). The compression characteristics ofthe cement-stabilized heavy metal contaminated soils have beeninvestigated by Wei et al. (2012), wherever such soils are used asbearing stratums. These studies indicate that the contaminantconcentration, binder content, and curing time have significanteffects on the aforementioned properties of the S–S soils. More-over, the effects of wet and dry cycling, freeze and thaw cycling,acid rain, carbonation, and sulfate attack on the durability of suchsoils have also been investigated by various researchers (Puppalaet al. 2005; Malviya and Chaudhary 2006; Chen et al. 2009; Antemiret al. 2010; Chittoori and Puppala 2011; Du et al. 2012; Jiang et al.2012; Chittoori et al. 2013). The results demonstrate that the com-plex external environment could result in a considerable changein the pore profile and strength–leaching properties of the stabi-lized soils.

The speciation of zinc (Zn) in the contaminated soils includesfree ion (Zn2+), zinc carbonate (ZnCO3), zinc sulfate (ZnSO4), zincsulfide (ZnS), and zinc complex ions (Paria and Yuet 2006). It isindicated that Zn has a retardant effect on the cement hydrationof cement-based S–S treated industrial wastes and heavy metalsludge (Yousuf et al. 1995; Olmo et al. 2001; Wei et al. 2012), whichin turn could result in decrease in strength and increase in leach-ability of Zn (Gervais and Ouki 2002; Minocha et al. 2003). How-ever, very few researches have comprehensively addressed theinfluence of Zn concentration on the basic and engineering prop-erties of cement-based S–S treated Zn-contaminated soils, includ-ing Atterberg limits, water content, pH, unconfined compressivestrength (qu), and secant modulus (E50). Besides, studies on themicrostructural characteristics (i.e., clay mineralogy, fabric, andpore-size distribution) of cement-stabilized Zn-contaminated soilsand their intrinsic connection with engineering properties havebeen noticed to be quite limited.

This paper presents details of investigations on engineeringbehavior, formation of various phases of the major hydrationproducts, and microstructural characteristics of Zn-contaminatedkaolin clay stabilized by cement. The effects of Zn concentrationon Atterberg limits, water content, pH, stress–strain characteris-tics, qu, and E50 were studied. In addition, changes in the phases ofmajor hydration products and microstructural characteristics ofthe stabilized soils, as Zn concentration increases, were investi-gated by resorting to X-ray diffraction (XRD), scanning electronmicroscopy (SEM), and mercury intrusion porosimetry (MIP).Based on these results, an effort has been made to interlink themicrostructural characteristics of the soil, which are altered bythe evolution of hydration products, with the overall engineeringproperties. It is believed that such investigations would be quiteuseful to facilitate the design of remediation of heavy metal con-taminated soils by using cement-based S–S technique.

Materials and methods

Raw materialsKaolin clay was used in this study because its particles have

limited surface activity and the mineral present in it (i.e., kaolin-ite) is chemically less reactive with the pore water as compared toother minerals such as montmorillonite. In addition, kaolin clayhas a relatively stable structure and its shrinkage and swelling aremuch less significant as compared to montmorillonite. The basicproperties of the kaolin clay used in this study are listed in Table 1.Based on the Unified Soil Classification System (ASTM (2011)D2487), the soil is classified as a low plasticity clay (CL). The X-raydiffraction results indicate that this clay is composed of kaolinite(96%) and quartz (4%). Locally produced cement (equivalent toPortland cement type I) was used as a binder. The chemical com-positions of the cement are listed in Table 2. Concentrated zincnitrate, Zn(NO3)2, solutions were prepared by dissolving predeter-mined weights of Zn(NO3)2·6H2O powder (chemical analytical re-agent, Sinopharm Chemical Reagent Co. Ltd.) in deionized water.

The basic reason for selecting nitrate anion is that it is inert to thecement hydration (Cuisinier et al. 2011). Furthermore, when com-pared to phosphate anions, nitrate anions have much less effecton the engineering characteristics of the clayey soils (Sridharanet al. 1987).

Sample preparationTo prepare Zn-contaminated soil samples, predetermined vol-

ume of concentrated Zn(NO3)2 solution was added into the air-dried kaolin clay, until the water content of about 59% wasachieved. The soil and solution were mixed thoroughly for about5 min, by using an electronic mixer to obtain homogenous slurry(hereinafter referred to as initial kaolin slurry). The cement pow-der was poured into the initial kaolin slurry and the mixture wasmixed thoroughly for another 10 min to achieve homogeneity.Later the mixture was poured into a PVC mold (inner diameter50 mm and height 100 mm). Due care was taken to remove trappedair bubbles from the mixture during the placement into themolds. The molds were wrapped with polythene caps to minimizemoisture loss, and the specimens were cured under controlledambient conditions (22 °C and relative humidity of 95%). After 7and (or) 28 days of curing, specimens were extruded from themolds using a hydraulic jack, and unconfined compression tests(UCTs) were conducted to determine their crushing strength.

The concentration of Zn(NO3)2 was varied (0%, 0.01%, 0.02%,0.05%, 0.1%, 0.2%, 0.5%, 1.0%, and 2.0%, on oven-dry soil weightbasis) and the cement contents were set as 8%, 12%, 15%, and 18%(on oven-dry soil weight basis), as recommended by Bates et al.(2000). The levels of Zn concentration presented in this study aretypically encountered in contaminated lands of China urban areasas reported by Liao et al. (2011), and are consistent with those ofmetals contaminated industrial soils (Voglar and Leštan 2010). Inaddition, the US EPA soil screening levels of Zn concentration forresidential and commercial–industrial scenarios are 2.36% (i.e.,23 600 mg/kg) and 34% (i.e., 340 000 mg/kg), respectively (US EPA2002). For SEM and MIP tests, soils (of approximately 1 cm3) wereretrieved from the carefully hand-broken identical specimens.The freeze-drying technique, the most appropriate method for

Table 1. Physicochemical properties of kaolin claytested.

Property Characteristic

Specific gravity, Gs 2.65Plastic limit, wp (%) 23.0Liquid limit, wL (%) 34.5Grain-size distribution (%)a

Clay (<0.005 mm) 33.0Silt (0.005–0.075 mm) 63.3Sand (0.075–2 mm) 3.7

pH 8.7aMeasured using a laser particle size analyzer Master-

sizer 2000.

Table 2. Oxide chemistry of cement tested.

Oxide chemistrya Characteristic (%)

Calcium oxide (CaO) 49.7Aluminium oxide (Al2O3) 9.87Magnesium oxide (MgO) 2.06Potassium oxide (K2O) 0.75Silicon oxide (SiO2) 22.6Ferric oxide (Fe2O3) 3.50Sulphate oxide (SO3) 3.84Sodium oxide (N2O) 0.24Loss on ignitionb 6.19

aMineral composition was analyzed by X-ray fluores-cence method using ARL9800XP+ XRF spectrometry.

bValue of loss on ignition is referenced to 950 °C.

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dehydrating soil specimens for SEM and MIP tests, was employedto dehydrate the soils, as suggested by earlier researchers (Penumaduand Dean 2000; Li and Zhang 2009; Gumaste and Singh 2010). Liquidnitrogen with a boiling point of –195 °C was used to freeze the soilspecimens, after which the specimens were placed in a freezingunit with a vacuum chamber, and were dried by sublimation ofthe frozen water at a temperature of –80 °C. The freeze-dryingapparatus used in this study was the XIANOU-18N freeze-drier.

Testing methodsThe unconfined compression test (UCT) was conducted on the

stabilized kaolin samples, by fixing the strain rate as 1%/min, asper ASTM (2008) D 4219. From these tests, E50, which is defined asthe secant modulus of elasticity, obtained from a stress–straincurve corresponding to half of qu (Kitazume and Terashi 2001),were obtained. A certain quantity of the stabilized kaolin wassampled from the broken UCT specimens and was immediatelysubjected to various tests to determine the Atterberg limits, watercontent, and pH. XRD was also performed when the sample wasair dried, ground, and sieved (<0.075 mm). The plastic limit (wP)and liquid limit (wL) tests were performed as per ASTM (2010)D4318. Fragments from the broken UCT specimens were crushedand passed through a 425 �m sieve to ensure the homogeneity ofthe samples. Then the sieved soils were used for the plastic limitand liquid limit measurements using the hand-rolling methodand multi-point liquid limit method, respectively. The gravimet-ric water content, defined as the ratio of the weight of the water tothat of the total solids (including soil and cement), was deter-mined by heating the soil in an oven at a constant temperature of105 °C for 24 h (Chew et al. 2004). Two cases were considered forthe measurement of the water content: (i) initial kaolin slurry; and(ii) soils immediately sampled from the broken UCT specimens.The pH values of the initial kaolin slurry and concentratedZn(NO3)2 solution were measured by placing a HORIBA D-54 pHmeter into the initial kaolin slurry and prepared solution, respec-tively. The pH measurement for the stabilized soils was conductedas per ASTM (2001) D4972. Fragments from the broken UCT spec-imens were air-dried, crushed, and passed through a 2 mm sieve.A quantity of 10 g of the sieved soil and 10 mL of distilled waterwere poured into a glass container. They were then mixed thor-oughly and left to stand for 1 h. pH values were obtained by plac-ing the HORIBA D-54 pH meter into the supernatant.

X-ray diffraction analysis was performed on these samplesby using a Rigaku D/Max-2500 X-ray diffractometer. A Cu-K�(� = 1.540538 Å) X-ray tube with an input voltage of 40 kV and acurrent of 20 mA was employed. The samples were scanned fortwo-theta (2�) value ranging between 5° to 60° with a step lengthof 0.02° and a scanning rate of 2°/min. The SEM analysis of thesesamples was conducted by using a LEO1530VP scanning electronmicroscope. Furthermore, the MIP was conducted by using anAutoPore IV 9510 mercury intrusion porosimeter. The capillarypressure equation has been employed to compute the pore diam-eter as expressed by (Mitchell and Soga 2005):

(1) d � �4� cos�

p

where d is pore diameter; � is the surface tension of intrudingmercury (4.84 × 10−4 N/mm at 25 °C); � is the contact angle (135° inthis study); p is applied pressure of mercury intrusion (maximum413 MPa in this study).

Triplicate samples were tested for UCT, water contents, and pH,and the average values of these parameters were reported here. Itis observed that for higher Zn concentrations (0.2% and 0.5%),corresponding to 7 and 28 days of curing, cement hydration andpozzolanic reactions were remarkably retarded and hence the soilspecimens broke easily during extrusion. Therefore, such sampleswere discarded for UCTs and were tested only for Atterberg limits,pH, and XRD. For the sake of completeness, the Zn concentration,cement content, and curing time of the stabilized soils, subjectedto various tests, are summarized in Table 3.

Experimental results

Atterberg limits and water contentFigure 1 shows the effect of Zn concentration on wL, wp, and

plasticity index (Ip). It can be observed that wL and wP of the stabi-lized uncontaminated kaolin is about 60% higher than those ob-tained for the pure kaolin. The reason for higher wL and wP of thestabilized uncontaminated kaolin is the presence of a certain frac-tion of water trapped within the intra-aggregate pores, whichwould result in considerable increase in the Atterberg limits ofthe low plasticity clays (Bergado et al. 1996; Locat et al. 1996; Chewet al. 2004). The formation of the intra-aggregate pores is attrib-uted to the filling of large pore space by calcium silicate hydrate(C–S–H) and ettringite (Chew et al. 2004), as discussed in detail inthe section titled “Pore-size distribution”.

Figure 1 also reveals that wL and wP dropped dramatically withan increase in Zn concentration, after 7 and 28 days of curing.When Zn concentration is ≥0.5%, the values of wL and wP areapproximately the same as that of the original kaolin. As far as theeffect of curing time is concerned, it is observed that when Znconcentration is <0.5%, wL and wP increase with curing time,which is mainly due to the development of hydration and pozzo-lanic reactions over the curing time. However, no significantchanges could be observed with respect to the values of plasticity,Ip, at different curing times. Further investigations need to beconducted on the effect of curing time on the plasticity of cement-stabilized Zn-contaminated kaolin clays.

Figure 2 shows the variation of the water content measuredimmediately after conducting the UCT on the samples. It can beobserved that as compared to the initial kaolin slurry (i.e., nocement), water content of the stabilized kaolin drops remarkablywith increasing cement content and curing time, while the effectof the cement content is more significant. The higher reduction ofwater content at higher cement content can mainly be attributedto the higher amount of cement clinkers in the stabilized samples

Table 3. Zinc concentration, cement content, and curing time for various tests.

Test type Zinc concentration (%)Cementcontent (%)

Curingtime (days)

Atterberg limits 0, 0.02, 0.1, 0.2, 0.5, 1 8 7, 28Water content 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5 8, 12, 15, 18 7, 28UCT 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5 8, 12, 15, 18 7, 28pH 0, 0.2, 2 8, 12, 18 7, 28XRD 0, 0.02, 0.2, 2 12 28SEM 0, 0.02, 0.2, 2 12 28MIP 0, 0.02, 0.2, 2 12 28

Note: UCT, unconfined compression test; XRD, X-ray diffraction; SEM, scanning electron microscopy; MIP,mercury intrusion porosimetry.

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that have triggered greater extent of hydration and pozzolanicreactions (Bergado et al. 1996; Shen et al. 2003, 2008; Horpibulsuket al. 2005). Furthermore, it is evident that smaller change ofwater content is observed at Zn concentration of 0.5%, as com-pared to that observed for the pure kaolin (with Zn concentrationequal to 0). In this study, because the initial gravimetric water

content is almost the same for all specimens, with the same ce-ment content (i.e., same amount of cement clinkers) and curingtime, a higher water content at Zn concentration of 0.5% reflectsthat a lesser amount of water and cement clinkers are involved inthe hydration and pozzolanic reactions. In other words, the de-grees of hydration and pozzolanic reactions are lower.

The effects of Zn concentration on the aforementioned Atterberglimits and water content, as well as other properties including pH,stress–strain behavior, qu, phases of hydration products, and mi-crostructural characteristics of the stabilized kaolin samples, arediscussed in the following sections.

Soil pHFigure 3a presents the measured pH of Zn(NO3)2 solution and

the initial kaolin slurry. The pH of Zn(NO3)2 solution decreasesfrom 8.8 to 5.6 when Zn concentration increases from 0% to 1%,which is attributed to the hydrolysis of aqueous Zn (Du andHayashi 2006), as expressed by the following chemical equations:

(2) Zn2 H2O ¡ Zn�OH� H

(3) Zn�OH� H2O ¡ Zn�OH�2 H

It is evident that with an increase in the aqueous Zn concentra-tion in the solution, the concentration of hydrogen ions (H+) in thesolution increases. Consequently, the pH of the initial kaolinslurry drops (see Fig. 3a).

Fig. 1. Effect of Zn concentration on Atterberg limits of the cement-stabilized kaolin clays: (a) 7 days of curing and (b) 28 days of curing.

Fig. 2. Water content measured immediately after UCT of thecement-stabilized kaolin clays.

Fig. 3. Measured pH for the cases of (a) Zn(NO3)2 solution and initialkaolin slurry and (b) cement-stabilized kaolin clays.

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Figure 3b shows the measured pH of the pure kaolin, and theZn-contaminated kaolin samples stabilized with 8%, 12%, and 18%cement after 7 and 28 days of curing. It can be observed that thepH of all the specimens decreases with curing time regardless ofthe cement content and Zn concentration. This could be attrib-uted to the time-dependent pozzolanic reaction, which consumesportlandite and water to form additional hydration products suchas C–S–H and calcium aluminate hydrate (C–A–H) (Chew et al.2004). In addition, pH is significantly lower at high Zn concentra-tions as compared to the lower concentrations. When Zn concen-tration increases from 0% to 2%, pH decreases from 11.6–12.3 to9.7–11.2, respectively.

Stress–strain behaviorFigure 4 depicts typical stress–strain curves for the specimens of

kaolin stabilized with 18% cement. It can be observed that whenZn concentration is <0.02%, the stabilized kaolin exhibits brittlecharacteristic in terms of rapid drop in the post-peak stress withan increase in strain, which is similar to that exhibited by thestructured natural clays. At higher Zn concentrations (≥0.02%),the stabilized kaolin behaves like a ductile material, exhibiting agradual drop in the post-peak stress with strain, which is akin tothat of remolded natural clays.

Unconfined compressive strength and secant modulusFigure 5 shows the effect of Zn concentration on the values of qu

at 7 and 28 days of curing. It is evident from this figure that for agiven curing time and cement content, qu decreases with an in-crease in Zn concentration. For instance, after 7 and 28 days of

curing, qu decreases approximately by 1 to 8 times and 4 to 6 timeswhen Zn concentration increases from 0% to 0.2% and 0% to 0.5%,respectively. Nevertheless, when Zn concentration is higher than0.1%, the effect of cement content and curing time on qu becomesvery moderate. This suggests that for a relatively low Zn concen-tration (<0.1%) in the contaminated soils, increasing cement con-tent and curing time would be quite effective to enhance theirstrength; while alternative binders or additives need to be consid-ered to satisfy the strength requirement for higher concentra-tions of Zn. When Zn concentration is below 0.1%, qu increasesnoticeably with cement content and curing time.

Figure 6 presents the relationship between the pH and qu for thestabilized soils. The dashed lines represent an increasing trend ofqu with an increase in pH. From the trend line corresponding to28 days of curing, it can be deduced that the stabilized soils wouldexhibit low strength for pH ≤10.8, whereas qu increases consider-ably for pH >10.8. This observation is consistent with that re-ported in literature (Chrysochoou et al. 2010) for the dredgedmaterials stabilized by various binders (see Fig. 6). This could beattributed to the fact that pH >10.8 is required for thermodynamicstability of C–S–H (Stronach and Glasser 1997). In cementitioussystems, at pH <10.8, C–S–H is instable and hence strength wouldbe significantly reduced.

Figure 7 presents the variation of E50 with Zn concentration. It isevident from the figure that, for a given curing time and cementcontent, E50 generally decreases with an increase in Zn concentra-tion, which is consistent with the changes occurring in the valueof qu. Furthermore, when the cement content increases from 8%

Fig. 4. Stress–strain curves of the Zn-contaminated kaolin claysstabilized with 18% cement: (a) 7 days of curing and (b) 28 days ofcuring.

Fig. 5. Effect of Zn concentration on the unconfined compressivestrength (qu) of the cement-stabilized kaolin clays: (a) 7 days ofcuring and (b) 28 days of curing.

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to 12%, E50 increases considerably, whereas this did not makemuch difference for a cement content increase from 12% to 18%.The magnitude of E50 corresponding to 28 days has been found tobe about 1 to 4.5 times higher than that corresponding to 7 days.

Fig. 6. Relationship between pH and unconfined compressivestrength (qu) of the cement-stabilized kaolin clays after 7 and 28 daysof curing.

Fig. 7. Effect of Zn concentration on the secant modulus (E50) of thecement-stabilized kaolin clays: (a) 7 days of curing and (b) 28 days ofcuring.

Fig. 8. Relationship between secant modulus (E50) and unconfinedcompressive strength (qu) of the stabilized kaolin clays with Znconcentrations ranging from 0% to 0.2% (7 days of curing) and 0% to0.5% (28 days of curing).

Fig. 9. X-ray diffractograms of the untreated and cement-stabilizedkaolin clays at Zn concentrations of 0%, 0.02%, 0.2%, and 2% on asemi-logarithmic scale (cement content = 12%, curing time = 28 days).

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As reported in the literature (Kitazume and Terashi 2001), qu andE50 of the cement-stabilized soils exhibit an excellent correlation,which can be expressed as:

(4) E50 � qu

where is a dimensionless coefficient. The value of varies from70 to 250 for the cement-stabilized Zn-contaminated kaolin clays,as shown in Fig. 8, which is a wide range (115 to 150) for cement-stabilized Bangkok clay (Lorenzo and Bergado 2006), and 100 to200 for cement-stabilized dredged marine sediments (Tang et al.2000).

X-ray diffraction analysisFigure 9 shows X-ray diffractograms of the stabilized kaolin

with Zn concentrations of 0%, 0.02%, 0.2%, and 2% along with 12%cement, cured for 28 days. Diffractogram for pure kaolin is alsosuperimposed in the figure as a reference. It is evident from thefigure that irrespective of Zn concentration, presence of kaoliniteand quartz is identified in the cement-stabilized and untreatedkaolin clays. Portlandite (CH) has been identified at two-theta ofabout 32.3° (2.81 Å) when Zn concentration is ≤0.02%, which dis-appears when Zn concentration is higher than 0.02%. The absenceof portlandite can be attributed to: (i) the retardant effect of Zn oncement hydration — because portlandite is one of the major hy-dration products, any retarded cement hydration would lead to areduced quantity and even absence of portlandite in the soil ma-trices; and (ii) chemical reactions where portlandite is involved infixing Zn. As indicated by Lo et al. (2000), when cement is used asa binder to solidify Zn-rich sludge, portlandite is ready to reactwith Zn to form new compounds. One of the compounds formed,

due to the chemical reactions between portlandite and Zn, is cal-cium zincate, CaZn2(OH)6·2H2O, as discussed in detail in the fol-lowing and was also reported by Lo et al. (2000).

To scrutinize the evolution of major hydration products, en-larged images are plotted in Figs. 10–12. It is evident that C–S–Hand ettringite (AFt) are present in both 0% and 0.02% Zn concen-trations, corresponding to a two-theta of about 31.9° (2.81 Å) and9.1° (9.72 Å), respectively; whereas these peaks could be identifiedonly in the case of 0 Zn concentration, corresponding to two-thetaof 27.3° (3.27 Å) and 18.9° (4.70 Å), respectively (see Figs. 10 and 11).When Zn concentration is higher than 0.02%, C–S–H and ettring-ite could not be identified, which indicates that their formationhas been considerably retarded due to the presence of relativelyhigh Zn concentrations. The evolution of hydrogarnet (C3AH6)exhibits a slightly different pattern, as shown in Fig. 11. Theintensity of the C3AH6 peak is greater in the case of 0.02% Znconcentration as compared to 0% Zn concentration. In contrast,the development of CaZn2(OH)6·2H2O exhibits an oppositetrend as compared to that of C–S–H and ettringite. In otherwords, CaZn2(OH)6·2H2O could only be identified under rela-tively high Zn concentration, i.e., 0.2% and 2%, and the intensityof CaZn2(OH)6·2H2O reached a maximum at the highest Zn con-centration (2%) (Fig. 12). Furthermore, evidence for the retardationof cement hydration, due to the presence of Zn in the kaolin, isconfirmed by the existence of belite (C2S) corresponding to thehighest Zn concentration (2%) (Fig. 13). This is because belite is oneof the cement clinkers and its content in the unhydrated cementusually ranges from 10% to 40% (Bensted and Barnes 2002). Thepresence of belite in the cement-stabilized Zn-contaminated soilsconfirmed by the XRD analysis indicates that a certain amount of

Fig. 10. X-ray diffractograms showing the effect of Zn concentration on the formation of C–S–H (cement content = 12%, curing time = 28 days).

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cement clinker has not yet been hydrated due to the retardationeffect of Zn additive.

Scanning electron microscope analysisFigure 14 shows the microstructures of the kaolin stabilized

with cement content of 12% after 28 days of curing. The reticulateand needle-shaped products in Fig. 14a represent C–S–H andettringite, respectively, as reported by earlier researcher (Benstedand Barnes 2002); this covers the surface of the soil aggregatestogether with a tiny amount of C–S–H gels. From Fig. 14a, it can beobserved that when no Zn is present in the stabilized kaolin, themorphology of C–S–H and ettringite is well developed and thequantity is relatively large. This demonstrates the flocculated na-ture of the fabric and fine network of reticulation in the matrix ofthe stabilized kaolin samples. The flocculated fabric could be at-tributed to the cation-exchange process, which results in replace-ment of calcium ions with readily exchangeable ions such aspotassium and sodium (Locat et al. 1990; Du et al. 1999; Chew et al.2004; Shen et al. 2008). The formation of a fine network of reticu-lation is attributed to the well-developed formation of reticulateand needle-shaped hydration products, which occupy the largepores and hence in turn enhance cementation bonding of soilaggregates (Locat et al. 1996; Chew et al. 2004).

When Zn concentration is 0.02%, as shown in Fig. 14b, it isobserved that less reticulate and needle-shaped products areformed. While a large quantity of cubic crystallized hydrationproduct, C3AH6 is formed, which is confirmed by the XRD analysismentioned above (see Fig. 11). This observation can be substanti-ated by the occurrence of Zn substitution of aluminium inettringite, as suggested by previous works (Gougar et al. 1996;

Chrysochoou and Dermatas 2006). In the CaO–A12O3–H2O system,as the stability of C3AH6 is higher than that of ettringite due to thepresence of Zn2+ (Faucon et al. 1997), tricalcium aluminate (C3A),one of the cement clinker constituents, easily gets transformed toC3AH6 instead of ettringite in the cement hydration reactions.

Figure 14c shows the microstructure of the stabilized soils withZn concentration of 0.2%, which is quite different as compared toFigs. 14a and 14b. The fine network of reticulation is almost invis-ible. Instead, the surface of kaolin clay clusters is covered by smallparticles, which is identified as CaZn2(OH)6·2H2O by the XRD anal-ysis shown in Fig. 12. In addition, presence of a small amount ofreticulate C–S–H and needle-shaped ettringite could be observed(see Fig. 14c), which otherwise could not be identified in the X-raydiffractograms due to the poor crystallinity or relatively smallquantity. When Zn concentration reaches 2%, it has been foundthat surfaces of the clay clusters are almost fully covered byCaZn2(OH)6·2H2O crystals, as depicted in Fig. 14d. No C–S–H orettringite could be identified in Fig. 14d, which indicates that thecement hydration and pozzolanic reactions were significantly re-tarded at high Zn concentrations.

Pore-size distributionNagaraj et al. (1990) and Gumaste and Singh (2013) have pro-

posed three types of pores in clays, namely, intra-aggregate, inter-aggregate, and large enclosed pores. Delage et al. (2006) havereported that the radii of intra-aggregate and interaggregatepores are around 0.02 and 2.0 �m, respectively, for a compactedbentonite. Li and Zhang (2009) have reported that the thresholdradius for intra-aggregate and interaggregate pores is 2.0 �m for acompacted completely decomposed granitic soil. Horpibulsuk

Fig. 11. X-ray diffractograms showing the effect of Zn concentration on the formation of ettringite (AFt) and hydrogarnet (C3AH6) (cementcontent = 12%, curing time = 28 days).

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et al. (2009) have reported that for the cement and fly-ash stabi-lized silty clays, pore diameters of 0.01 and 10 �m are thresholdsbetween intra-aggregate and interaggregate pores, and interag-gregate and air pores, respectively.

Unimodal pore-size distributions (PSDs) are frequently ob-served in coarse granular soils as reported by earlier researchers(Zhang and Li 2010). For compacted soils, residual soils, and struc-tured soils, bimodal PSDs are often observed (Delage et al. 2006; Liand Zhang 2009). In this study, PSD curves of the stabilized soilsare plotted in Fig. 15. The y-axis is plotted as f(D) (f(D) = dV/dlogD,where V is the volume of mercury intruded at a given pressureincrement corresponding to pores having a diameter of D in 1 g ofthe dry soil), as reported by Gumaste and Singh (2013). The areasbounded by two given pore diameters represent the volume ofpores that are distributed between these two diameters. FromFig. 15, it can be seen that when Zn concentration increases from0% to 2%, the pore diameter corresponding to the peak of themeasured f(D) increases from 0.6 to 2 �m. This phenomenon isattributed to the retardant effect of Zn on the evolution of hydra-tion products of C–S–H and ettringite, which could fill large poresspace.

To identify whether the PSD curves of the soils exhibit uni-modal or bimodal types, peak analysis (PA) was utilized to simu-late all peaks. In PA, a Gaussian distribution function was appliedto fit each peak of the PSD curves. Therefore, the PSD curves canbe written as:

(5) f(D) � �i�1

n

fi(D) � �i�1

n

ai1

�2��i

e��(logD��i)2/2�i

2�

where n is the number of peaks in PSD curves on a logarithmicscale (1 and 2 for the unimodal and bimodal types, respectively);ai is pore volume in the 1 g dry soil covered by the fitted curve offi(D) (mL/g); �i is standard deviation on a logarithmic scale; �i is themean pore diameter in the fitted curve of fi(D) on a logarithmicscale (�m).

A detailed discussion on the mathematical approach to eq. (5)for unimodal and bimodal PSDs is reported in the literature (Liand Zhang 2009). Results of PA are shown in Fig. 15, from which itcan be observed that PSD curves of the stabilized soils have bi-modal characteristics for Zn concentration <2%. However, whenZn concentration is equal to 2%, only a unimodal curve is ob-served. Delage et al. (2006) and Li and Zhang (2009) have indicatedthat the pore diameters corresponding to dual peaks in PSDcurves represent intra-aggregate and interaggregate pores, re-spectively. The fact that simulated PSD curves displayed unimodaltype distribution when Zn concentration is 2% indicates that highZn concentration retards formation of intra-aggregate pores,which results from filling of the large pores by hydration prod-ucts. Table 4 lists the changes occurring in the parameters of a1, a2,�1, and �2 when Zn concentration increases from 0% to 2%. It canbe observed that �1 varies from –0.93 to –0.88, which correspondsto a slight change in the mean diameter of an intra-aggregate pore(from 0.11 to 0.13 �m, which is an 18% increase) when Zn concen-tration increases from 0% to 2%. In contrast, �2 increases steadilyfrom –0.231 to 0.285, which corresponds to a notable increase inthe mean diameter of an interaggregate pore (from 0.59 to1.93 �m, which is a 230% increase). Besides, when Zn concentra-tion increases from 0% to 2%, a1, which corresponds to the volumeof an intra-aggregate pore covered by the simulated f1(D), slightly

Fig. 12. X-ray diffractograms showing the effect of Zn concentration on the formation of calcium zinc hydrate (CaZn2(OH)62H2O) (cementcontent = 12%, curing time = 28 days).

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decreases from 0.232 to 0.199 mL/g (i.e., 14% decrease); whereas a2,which corresponds to the volume of an interaggregate pore cov-ered by the simulated f2(D), increases considerably (from 0.077 to0.191 mL/g, which is a 150% increase). The variation of �1, �2, a1,and a2, with Zn concentration (see Table 4), indicates that when Znconcentration increases, the magnitude of the mean diameterand volume of the interaggregate pores (�2 and a2, respectively)increases noticeably, while for the intra-aggregate pores (�1 and a1,respectively) these changes are minor.

DiscussionAs explained earlier, the presence of Zn has hindered the evo-

lution of hydration products such as calcium silicate hydrate(C–S–H) and ettringite. This change is due to the retardant effect ofZn on the cement hydration and pozzolanic reactions in a cemen-titious system. It can be seen from Table 5 that with an increase inZn concentration, intensities of C–S–H and ettringite decrease,implying that the quantity of these two hydration products withinthe stabilized soil reduces. These compounds formed are verysmall in magnitude and hence were not detected by the XRDstudies when Zn concentration is ≥0.2%. This evident change sub-sequently hampers aggregation and cementation of soil particlesinto larger size clusters. As a result, the Atterberg limits decreasewith an increase in Zn concentration (see Fig. 1). Meanwhile, arelatively higher water content is observed for samples with 0.5%Zn concentration than that of uncontaminated samples (see

Fig. 2), indicating less water has been consumed in the hydrationand pozzolanic reactions for the Zn-contaminated samples.

Because the presence of Zn retards the hydration reaction, for-mation of portlandite is therefore hindered, which is substantiatedby its reduced XRD intensity with increasing Zn concentration asshown in Table 5. As a result, soil pH decreases when Zn concentra-tion increases (see Fig. 3b), as portlandite would provide a highalkaline (equilibrium pH equals �12.5) circumstance (Du et al.2012). The decrease in soil pH is also attributed to the reduced pHof Zn(NO3)2 solution with an increase in Zn concentration (seeFig. 3a), which in turn brings down pH of the initial kaolin slurry(i.e., homogenous mixture of Zn(NO3)2 solution and air-driedkaolin) from 6.1 to 4.6 when Zn concentration increases from0% to 1%.

When Zn concentration increases to 0.02%, formation of C3AH6is observed, which is identified by the XRD analysis as depicted inFig. 11. The SEM image shows the existence of C3AH6 in the stabi-lized soil matrix, and this could result in a reduced degree ofentanglement of ettringite to some extent (see Fig. 14b). The pres-ence of C3AH6 as well as hindered formation of C–S–H and ettring-ite, therefore, weaken the cementation bonding of the soil particles.Accordingly, a notable change in the brittleness–ductility character-istics (i.e., stress–strain behavior) is observed for cement-stabilizedZn-contaminated kaolin samples. Meanwhile, because the quantityof hydration products (e.g., C–S–H) in cement-stabilized soils is posi-tively correlated with the strength (Chew et al. 2004), the retardedformation of hydration products caused by Zn contamination wouldlead to lower strengths and modulus. The aforementioned change inthe mechanical properties can be substantiated from Figs. 4, 5, and 7and also from the findings of the earlier researchers (Minocha et al.2003; Wei et al. 2012).

Under relatively high Zn concentration (≥0.2%), CaZn2(OH)6·2H2Ois formed and its intensity increases with increasing Zn concentra-tion, which is substantiated from Fig. 12 and Table 5. The forma-tion of CaZn2(OH)6·2H2O remarkably hinders the formation ofhydration products due to its coating effect on the cement grains,hence creating a barrier that separates cement grains from water(Yousuf et al. 1995; Wei et al. 2012). Accordingly, the soils exhibitporous structures, which is substantiated from the SEM images(Figs. 14c and 14d).

The development of porous structures with increasing Zn con-centrations of the soils, is further illustrated by the increased porediameters corresponding to the peak of the measured f(D) as de-picted in Fig. 15, and also by the elevated magnitudes of the meandiameter and volume of the interaggregate pores as shown inFigs. 16a, 16b, and 16c, and Table 4. Since the interaggregate poresin the stabilized soil matrix represent pores between the largeclay–cement clusters, more pores between clay–cement clusterswould be left unfilled when the formation of hydration productsis hindered. However, the change in the quantity of hydrationproducts does not considerably affect the filling of the pores in theclay–cement clusters. As a result, the magnitudes of the meandiameter and volume of the intra-aggregate pores change margin-ally, as illustrated in Figs. 16a, 16b, and 16c and Table 4. When Znconcentration reaches the highest (2%), only interaggregate poresare observed in the cement-stabilized kaolin sample (see Fig. 16d),which reflects that the soil exhibits a highly weakened cementa-tion bonding and porous structure. Hence, the soil displays veryductile characteristics and very low strengths and modulus,which is substantiated from Figs. 4, 5, and 7.

In summary, the presence of Zn and its concentration levelshave resulted in considerable changes in physicochemical andmechanical properties (Atterberg limits, water content, pH,strength, and modulus) of the cement-stabilized kaolin. The im-pacts to these engineering characteristics are attributed to thenegative influence of Zn on the cement hydration and pozzolanicreactions in the stabilized soils, which is substantiated by acomprehensive investigation of phases of hydration products and

Fig. 13. X-ray diffractograms showing the effect of Zn concentrationon the formation of belite (C2S) on a semi-logarithmic scale (cementcontent = 12%, curing time = 28 days).

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microstructural characteristics via XRD, SEM, and MIP tests aswell as peak analyses.

It is noteworthy that this study presents the results of engineer-ing properties at the curing time of 28 days. Indeed, the results areuseful to facilitate formulating the strategies for remediation ofheavy metal contaminated soils by cement-based solidification–stabilization technique, where engineering properties (e.g., strengthand modulus) corresponding to 28 days of curing time are used asthe input parameters. The long-term performance of the cement-stabilized Zn-contaminated soils would depend on both the ce-ment and Zn interactions and external environment that matters.Therefore, further study is suggested on the long-term evolutionof engineering properties and microstructural characteristics. In

addition, this laboratory study uses a single soil (i.e., kaolin clay)and the results are applicable to other types of clayey soilswhose predominant clay mineral is kaolinite. Further valida-tion of the results presented in this study needs to be carried

Fig. 14. SEM images showing the effect of Zn concentration on the phases of major hydration products of the kaolin clays stabilized with 12%cement and cured for 28 days: (a) 0% Zn; (b) 0.02% Zn; (c) 0.2% Zn; (d) 2% Zn.

Fig. 15. Pore-size distribution of the cement-stabilizeduncontaminated and Zn-contaminated kaolin clays (cementcontent = 12%, curing time = 28 days).

Table 4. Parameters obtained from the simulated PSD curves of sta-bilized kaolin (12% cement, 28 days of curing) based on peak analysis.

Zincconcentration(%) n

a1

(mL/g)a2

(mL/g) �1 �2 �1 �2 R2

0 2 0.232 0.077 −0.884 −0.231 0.637 0.139 0.960.02 2 0.217 0.104 −0.925 −0.192 0.680 0.169 0.980.2 2 0.199 0.147 −0.890 0.007 0.680 0.278 0.942 1 — 0.191 — 0.285 — 0.115 0.81

Note: n, number of peaks in the fitted PSD curves; a1, volume of intra-aggregate pore; a2, volume of interaggregate pore; �1, mean diameter of intra-aggregate pore; �2, mean diameter of interaggregate pore; �1, �2, standarddeviation; R, correlation coefficient.

Table 5. Intensity of major hydration products obtained from X-raydiffraction analysis.

Intensity (counts)a

Hydrationproducts 0b 0.02b 0.2b 2b

C–S–H 1608 (27.3°) ND (27.3°) ND ND1440 (31.9°) 1250 (31.9°) ND ND

AFt 899 (9.1°) 829 (9.1°) ND ND422 (18.9°) ND (18.9°) ND ND

CH 1275 (32.3°) 1047 (32.3°) ND NDCZnH ND ND ND (14.2°) 714 (14.2°)

683 (28.6°) 760 (28.6°)

Note: C–S–H, calcium silicate hydrate; AFt, ettringite; CH, portlandite; CZnH,calcium zincate; ND, not detected.

aNumbers in brackets represent the degree of two-theta.bZinc concentrations (%, oven-dry soil weight basis).

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out by using Zn-contaminated field soils with entirely differentclay mineralogy.

Summary and conclusionsThis paper provides a comprehensive summary of the labora-

tory testing program conducted to investigate the basic andengineering properties, phases of major hydration products,and microstructural characteristics of cement-stabilized Zn-contaminated kaolin. Based on the results reported, the follow-ing conclusions can be drawn:

1. Atterberg limits of the cement-stabilized kaolin in the absenceof Zn were significantly higher than those of the pure kaolin. Ithas been observed that with an increase in the Zn concentra-tion, Atterberg limits decreased sharply and were almost thesame as those for the untreated soil when the Zn concentra-tion was 2%. Increases in curing time and cement content ledto a considerable drop in the water content. The pH of thestabilized kaolin decreased remarkably with an increase in theZn concentration.

2. From the stress–strain curves, it has been observed that withan increase in the Zn concentration both magnitudes of qu andE50 decreased. This indicates that the stabilized kaolin switchesover its response from a brittle to ductile material in the pres-ence of Zn.

3. Results from the SEM and XRD analyses reveal that with anincrease in the Zn concentration, the quantities of C–S–H,portlandite, and ettringite decreased and ultimately they dis-appeared. Corresponding to 0.02% Zn concentration, thepresence of cubic-structured C3AH6 was identified, while itdisappeared when the Zn concentration was higher than0.02%. A new product, CaZn2(OH)6·2H2O, has been observed tobe formed in the stabilized kaolin when the Zn concentrationis over 0.2%.

4. When the Zn concentration increased from 0% to 2%, the porediameter corresponding to the peak of the measured f(D) in-creased from 0.6 to 2 �m. The PSD curves of the stabilizedZn-contaminated kaolin can be well simulated by a peak anal-ysis. The simulated curves exhibited bimodal type when theZn concentration was less than 2%, whereas they displayedunimodal type when the Zn concentration was 2%. Peak anal-ysis results demonstrate that with an increase in the Znconcentration, the volume and mean diameters of the in-teraggregate pores increased noticeably, whereas the mag-nitude of the intra-aggregate pores remained nearly unchanged.

5. The significant variations in the engineering properties ofthe cement-stabilized Zn-contaminated kaolin are well inter-preted by the XRD, SEM, and MIP analysis. As such, it can beopined that XRD, SEM, and MIP can be employed successfully,as useful engineering tools, for investigating the influence ofZn contamination on the engineering properties of the cement-stabilized soils.

Acknowledgements

This research is financially supported by the National NaturalScience Foundation of China (Grant No. 51278100 and 41330641),Natural Science Foundation of Jiangsu Province (Grant No. BK2010060and BK2012022), and National High Technology Research and Devel-opment Program of China (Grant No 2013AA06A206). The authorswould like to thank the undergraduate students for carrying out partof the laboratory tests.

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