prite

heji Unf Ed

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centrations. Salinity effects of inltrating solutions on swelling pressure and hydraulic conductivity of tested sam-lts obtained show that the swelling pressure of GMZ01 bentonite decreases withnltrating solutions, while the degree of the impact decreases with the increase of

ity, goontonite

waste (HLW). During the construction and long-term operation of a cantly lower swelling pressure (~2 MPa) was obtained with low ionic

ect of synthetic seawa-g the important role of

al composition of pore

Engineering Geology 166 (2013) 7480

Contents lists available at ScienceDirect

Engineering

j ourna l homepage: www.e lsinuence the swelling pressure of bentonite. Karnland et al. (2006)found that the swelling pressure ofMX-80 bentonite decreases as the sa-linity of pore water increases. Conrmation was made by Castellanos

uid has signicant inuence on the hydraulic conductivity of compactedbentonite. The hydraulic conductivity of Wyoming Nabentonite in-creases with increase of concentration of the inltrating solutiontions (Herbert et al., 2008). This can affect the physical and chemicalproperties of bentonite, such as the mineralogical composition andswelling capacity etc.

Previous studies show that salt content of pore uid can signicantly

sure than de-ionized water did. Moreover, the effter was found depending on bentonite, evidencinsoil mineralogy in this process.

Literature reports also show that the chemicgeological repository, compacted bentonite can work as an effectivebarrier, protecting the canister and restricting the transfer of radionu-clide released from the waste packages after possible failure of canister(Wersin et al., 2007). Meanwhile, interaction can take place betweencompacted bentonite and groundwater of certain chemical composi-

concentration solutions and the lowest one (b1 MPa) was recordedwith high saline brines. Based on investigation of the inuence of syn-thetic seawater on the swelling pressure of ve common bentonites:Kunigel-VI, Volclay, Kunibond, Neokunibond and MX-80, Komime et al.(2009) also conrmed that synthetic seawater gave lower swelling pres- Corresponding author at: Key Laboratory of GeotechnicofMinistry of Education, Tongji University, Shanghai 200092fax: +86 21 6598 2384.

E-mail address: ye_tju@tongji.edu.cn (W.-M. Ye).

0013-7952/$ see front matter 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.enggeo.2013.09.001has been considered asgineering barrier in deepevel radioactive nuclear

ion content on the behavior of MX-80 bentonite. Observations showedthat the swelling pressure of MX-80 bentonite reaches the highestvalue (N4 MPa) when it was hydrated with de-ionized water, a signi-buffer/backll material for construction of engeological repository for disposal of high-lSwelling pressureHydraulic conductivityDDL

1. Introduction

Due to its low hydraulic conductivsorption properties etc., compacted bedraulic conductivity of GMZ01bentonite increaseswith the increase of solution concentrations. Comparison showsthat the impact of NaCl solutions on the swelling pressure and hydraulic conductivity is higher than that of CaCl2solutions at same concentrations. This may be explained by the impact of cation types on the microstructure ofbentonite.

2013 Elsevier B.V. All rights reserved.

d swelling capacity and

et al. (2008) on the FEBEX bentonite: an increase in salt concentrationdecreases the swelling pressure, but this decrease is less signicant incase of high density. Herbert et al. (2008) investigated the inuence ofSalt solutionGMZ01 bentoniteBuffer/backll materials

concentrations. Moreover, swelling pressure reaches stability more rapidly in case of high concentrations. The hy-Keywords: ples were investigated. Resuincreasing concentration of iInuence of salt solutions on the swellingconductivity of compacted GMZ01 benton

Zhu Chun-Ming a, Ye Wei-Min a,b,, Chen Yong-Gui a, Ca Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education, Tongb United Research Center for Urban Environment and Sustainable Development, the Ministry oc Laboratoire Navier, Ecole des Ponts ParisTech, France

a b s t r a c ta r t i c l e i n f o

Article history:Received 16 January 2013Received in revised form 30 August 2013Accepted 2 September 2013Available online 6 September 2013

During the long-termoperaticompositions can affect the bswelling pressure and permehas an initial dry density of 1.al andUnderground Engineering, China. Tel.:+862165983729;

ghts reserved.essure and hydraulic

n Bao a, Cui Yu-Jun a,c

iversity, Shanghai 200092ucation, Shanghai 200092

f a deep geological repository, inltration of groundwaterwith different chemicalr/backll properties of compacted bentonite. Using a newly developed apparatus,ity testswere carried out on densely compactedGMZ01 bentonite samples,whichg/m3,with de-ionizedwater aswell as NaCl and CaCl2 solutions at different con-

Geology

ev ie r .com/ locate /enggeo(Studds et al., 1998). This observation was conrmed by Villar (2005),who found that the hydraulic conductivity ofMX-80 bentonite inltratedwith pore water with a salinity of 0.5% was 135% higher than that withde-ionized water. The hydraulic conductivity of a bentonitesand mix-ture increased 6 times, when inltration uid changed from de-ionizedwater to 16 g/L salt solution (Mata, 2003). A possible explanation to

these observations is that the salinity of inltrating solutions inuencesthe swelling of aggregates, and in turn, changes the microstructure ofbentonite, resulting in changing of the hydraulic conductivity (Puschet al., 1990; Suzuki et al., 2005).

The program for deep geological disposal of high-level radioactivewaste in Chinawas launched in themiddle of 1980s. Based on a nation-wide survey, Beishan, in Gansu province, China, has been selected as oneof the potential disposal sites. Related eld work including geologicaland hydrogeological investigations has been carried out. Results showthat the total dissolved solids (TDS), which is rich in Na+ and Ca2+,in the groundwater in Yemaquan, Beishan area, changes from 2 g/Lto 80 g/L. The main chemical compound is ClSO4Na, followed byClSO4NaCa (Guo et al., 2001). This large variability in terms of chem-ical compositions of groundwater justies the study on the inuence of

75C.-M. Zhu et al. / Engineering Geology 166 (2013) 7480inltrating liquid on the swelling pressure and hydraulic conductivity ofcompacted GMZ01 bentonite.

In this study, the swelling pressure and hydraulic conductivity ofcompacted GMZ01 bentonite (1.7 Mg/m3 dry density) were investigat-ed with different inltrating solutions: de-ionized water and solutionsof sodium chloride (NaCl) and calcium chloride (CaCl2) at different con-centrations. The results obtained were analyzed in terms of microstruc-ture changes.

2. Experimental investigations

2.1. Materials

The GMZ01 bentonite studied was taken from GaoMiaoZi (GMZ)in the Inner Mongolia Autonomous Region, 300 km northwest fromBeijing, China (Ye et al., 2009). It is a light gray powder, dominated bymontmorillonite (75.4% in mass). As a Nabentonite, its basic physicaland chemical properties are presented in Table 1 (Wen, 2006). A highcation exchange capacity and adsorption ability can be identied (Yeet al., 2010, 2012).

The salts NaCl and CaCl2 used in this test were of analytical grade,corresponding to a purity of 99%.

2.2. Test apparatus

The experimental setup for the swelling pressure and hydraulic con-ductivity test with salt solutions is shown in Fig. 1. It is composed of fourparts: a testing cell, a pressurevolume controller, a fresh/saline waterconversion device and a data logger (Fig. 1(b)).

The testing cell contains a basement, a metallic sample ring, two po-rous stones, a stainless steel piston, a top cover, a pressure sensor andfour screws for xing all parts together (Fig. 1(a)). Two outlets aredesigned in the basement, one is connected to the pressurevolumecontroller and the second is used for air expulsion. A load sensor isplaced between the top cover and the stainless steel piston for monitor-ing the swelling pressure. The pressurevolume controller (032 MPato an accuracy of 1 kPa; 0200 cm3 to an accuracy of 1 mm3) is

Table 1Basic physical and chemical properties of GMZ01 bentonite (Wen, 2006).

Property Description

Specic gravity of soil grain 2.66pH 8.689.86Liquid limit (%) 276Plastic limit (%) 37Total specic surface area (m2/g) 597Cation exchange capacity (mmol/100 g) 77.3Main exchanged cation (mmol/100 g) Na+ (43.36), Ca2+ (29.14),

Mg2+ (12.33), K+ (2.51)Main minerals Montmorillonite (75.4%),

Quartz (11.7%),Feldspar (4.3%),Cristobalite (7.3%)employed for application of a stable injection water pressure and mea-surement of the volume of water injected.

As salt solutions cannot be directly used in the pressure/volumecontroller, a fresh/saline water conversion device ( in Fig. 1(b)) isdesigned. It is made of Plexiglas, one end is connected to the pressurevolume controller and the other end is connected to the basement. De-ionizedwater and salt solution can be lled in the two parts respectively,which are separated by the silicone oil kept between them.

2.3. Test procedures

2.3.1. Sample preparationAccording to the target cylindrical sample with a height of 10 mm, a

diameter of 50 mm and a dry density of 1.70 Mg/m3 to be compacted,GMZ01 bentonite powder at an initial water content of 10.76% wasweighted (37 g) and put into a cylindrical column. Compaction loadwas applied through a piston at a rate of 0.4 kN/min to a maximumvalue of 48 kN. Then, the maximum load was kept for 1 h. After that,the sample was immediately put into the testing cell ( in Fig. 1(a))with the metallic sample ring for the swelling pressure and hydraulicconductivity test.

De-ionized water and 8 solutions at desired saline concentrations(Table 2) were employed for the inltration tests.

2.3.2. Swelling pressure testsAfter the compacted GMZ01 bentonite sample was introduced into

the testing apparatus as shown in Fig. 1, solutions at different concen-trationswere inltrated into the sample through thewater/salt convert-er under a pressure of 100 kPa. Air-bubbles in the test system wereexhausted. The temperature was maintained at 20 1 C. The volumeof injected solution and the evolution of swelling pressure wererecorded. When the sample was saturated (which was characterized bythe stabilization of swelling pressure, Villar and Lloret, 2004), the swell-ing pressure test was considered as completed. The constant-volumemethod was employed for determination of the swelling pressure ofcompacted samples tested.

2.3.3. Hydraulic conductivity testsThe constant hydraulic head method was employed for the determi-

nation of saturated hydraulic conductivity. After completion of the swell-ing pressure test mentioned above, the hydraulic conductivity test wasconducted on the same sample. For this purpose, the injection pressurewas increased to 1 MPa and was maintained during the whole test.The volume of solution injected was recorded by the volume/pressurecontroller. When the volume of inltration solution injected reached astable state, the test was stopped. Based on the results obtained, thehydraulic conductivity was determined using Darcy's Law.

3. Test results and discussions

3.1. Swelling pressure

3.1.1. Impact of concentrationInuences of concentration of inltration solutions on the swelling

pressure of compacted GMZ01 bentonite are presented in Figs. 2 and3. It can be observed that the swelling pressure decreases from5.11 MPa (de-ionized water) to 3.06 MPa (2.0 M NaCl solution) and3.6 MPa (2.0 M CaCl2 solution). Namely, the swelling pressure of thecompacted GMZ01 bentonite decreases as the concentration of the inl-tration solutions increases. This conclusion is consistentwith the resultsreported by different researchers (Karnland et al., 2006; Castellanoset al., 2008; Herbert et al., 2008; Komime et al., 2009; Siddiqua et al.,2011; Lee et al., 2012).

Figs. 2 and 3 also present that the evolution curves of swellingpressure of samples inltrated with low concentration solutions are

double-peak shaped. Namely, the swelling pressure increases at the

matic diagram ensor 5Top cover 6Ring 7Basement 8

12Pressure/volume controller 13Data logger

76 C.-M. Zhu et al. / Engineering Geology 166 (2013) 7480(a) Sche1Sample 2Porous stone 3Piston 4Load s

Valve 9Salt solution 10Nut 11Silicon oilbeginning stage of hydration and rapidly reaches its rst peak, whichfollowed by an intermediate period where the swelling pressure de-creases. After that, the swelling pressure increases again and reachesits nal steady-state value (the second peak). This observation is in ac-cordance with the results reported by Villar and Lloret (2008) and Yeet al. (2012). However, when the concentration of salt concentrationsincreased to a relatively high level (N0.5 M), the double-peak curvesof swelling pressure faded to single-peak ones. This phenomenon

(b) Pictures Testing cell 2 Fresh/saline water conversion device 3 Pressure-volume controller 4Data

logger

Fig. 1. Setup for swelling pressure and saturated hydraulic conductivity test.

Table 2Tests and selected solutions.

Sample Solutions

1 De-ionized water2 0.1 M NaCl3 0.5 M NaCl4 1.0 M NaCl5 2.0 M NaCl6 0.1 M CaCl27 0.5 M CaCl28 1.0 M CaCl29 2.0 M CaCl2

Fig. 2. Inuence of concentrations of NaCl solutions on the swelling pressure of GMZ01bentonite.

indicates that the concentration of solutions signicantly inuences theswelling properties of the compacted GMZ01 bentonite.

This observation can be explained by the swelling process of

water in the interlayer space between the TOT montmorillonite units.This amount of water may be not enough to form any DDL in thesmall interlayer space (Pusch and Yong, 2006), but it can be enoughto form some DDL in the interquasicrystal pores. Consequently, thestructure of bentonite may partially rebuilt after the initial collapseand swells again, leading to an increase of swelling pressure up to amaximum constant value corresponding to the second peak (stage IIIin Fig. 4).

In case of hydrating with relatively higher concentration solutions(N0.5 M), swelling in stages I and II are similar to that of hydratingwith low concentrations. It can also be observed that, for all the tests,it takes about 10 h for swelling pressure to reach its rst peak. This phe-nomenon may be explained that development of swelling pressure atthis stage probably is governed by the matric suction dissipation notthe chemical composition of the inltrating solutions (Rao et al.,2006). According to the DDL theory, the DDL thickness varies inverselywith the square root of the concentration (Tripathy et al., 2004). Hence,higher concentrations will cause a reduction in the DDL thickness andconsequently a decrease of the repulsive forces between clay particles(Yong and Warkentin, 1975; Mitchell, 1976). As a result, the materialsundergo lower swelling (Karnland, 1997; Mata, 2003; Castellanoset al., 2008). Therefore, after collapse of the soil skeleton, high concen-tration of solutions diminishes the diffuse double-layer swelling. Conse-

Fig. 3. Inuence of concentration of CaCl2 solutions on the swelling pressure of GMZ01bentonite.

77C.-M. Zhu et al. / Engineering Geology 166 (2013) 7480compacted bentonite described in Fig. 4. Commonly, swelling of benton-ite exposed towater or electrolytes is primarily on account of twomech-anisms: the crystalline swelling and the diffuse double-layer swelling(Madsen, 1989; Savage, 2005). The crystalline swelling is caused by thehydration of exchangeable cations (K+, Na+, Ca2+ and Mg2+) betweenmontmorillonite unit layers that have a structurewith one alumina octa-hedral sheet sandwiched between two silica tetrahedral sheets (TOT).After the adsorption of maximumnumber of hydrates, surface hydrationbecomes less signicant and diffuse double-layer repulsion becomes thegoverning swelling mechanism (Bradbury and Baeyens, 2003).

In case of hydrating with low concentration solutions, after com-pletion of the crystalline swelling, swelling pressure reaches its rstpeak (stage I in Fig. 4). Followed by the swelling of aggregates, thisinduces the collapse of the soil skeleton under conned conditions,which characterized by the thick quasicrystals split into thinner onesand ll into the macro-pores (inter-aggregate pores), resulting in thedropping of the swelling pressure (from stages I to II in Fig. 4).Then, the diffuse double-layer repulsion dominates the swelling. Fordensely compacted bentonite, there is only a small amount of adsorbedFig. 4. Conceptual diagrams of constant-volume swelling process and microstructurequently, swelling pressure will almost not increase and directly reachesits nal stable value. Furthermore, this swelling pressure is mainly in-duced by the crystalline swelling, while the double layer repulsionmakes little contributions.

3.1.2. Impact of cation typesThe inuence of cation types on nal swelling pressure is shown in

Fig. 5. It can be observed that, for a given concentration, the swellingpressure of compacted GMZ01 bentonite hydrated with NaCl solutionsis lower than that with CaCl2 solutions. The difference depends on theconcentration of solutions. For low concentrations of salt solutions, thedifference is small. On the contrary, for higher concentrations, the swell-ing pressure with the low-valence salt (NaCl) solutions is much lowerthan that with high-valence salt (CaCl2) solutions. This phenomenonsuggests that theweakening effect of Na+ on swelling pressure is great-er than that of Ca2+.

It is generally recognized that cation exchange is an important factorinuencing the claywater interaction (Abdullah et al., 1999). The cat-ion exchange is mainly controlled by the type, valence, concentrationof compacted GMZ01 bentonite (Modied from Villar, 2002; Suzuki et al., 2005).

sw

particles in the relatively large pores in the upper part of the soil sample.

Fig. 6. Inuence of concentration of NaCl solutions on the hydraulic conductivity of GMZ01bentonite.

78 C.-M. Zhu et al. / Engineering Geology 166 (2013) 7480and size of cations (Mata, 2003). The higher the valence, the higher isthe replacing capacity of the cation. For cations with same valence, thereplacing capacity increases with the size of cation (Laine andKarttunen, 2010). A typical order for cation exchanging capacity is:Na+ b K+ b Mg2+ b Ca2+ (Mitchell, 1976; Pusch, 2001; Mata, 2003).So, when Nabentonite inltrated with calcium solutions, sodium willbe replaced by calcium (Muurinen and Lehikonen, 1999; Mata et al.,2005), resulting in the transformation from the Nabentonite to a calci-um bentonite (Montes-H and Geraud, 2004; Montes-H et al., 2005).

In present study,when the compactedGMZ01bentonite samplewasinltratedwith CaCl2 solutions, someNa+ in the bentonitewas gradual-ly replaced by Ca2+. Correspondingly, part of the Nabentonite is thentransformed into a kind of Cabentonite, in which Ca2+ becomes themain exchangeable cation. It is generally admitted that the hydrationforces of Na+ and Ca2+ ions are different. The number of interlamellarhydrated depends on the relative humidity and the density of thecompacted bentonite. Provided that there is no geometrical restraint,14 interlamellar hydrate layers can be formed depends on the relativehumidity. Under constant volume conditions, no more than twointerlamellar hydrate layers can be formed in the densely compactedGMZ01 bentonite sample (1.70 Mg/m3) tested in this study. When theadsorbed cation is Ca2+, the thicknesses of interlamellar of the rstand the second hydrates are 3.89 and 2.75 , respectively. While forthe adsorbed cation Na+, the thicknesses of interlamellar of the rstand the second hydrates are 3.03 and 3.23 , respectively (Pusch

Fig. 5. Comparison of inuence of cation types on the swelling pressure of GMZ01bentonite.and Yong, 2006). This suggests that Cabentonite has large basal spacethan that of Nabentonite. Moreover, when Na+ is in the interlamellar,the coupling to water molecules is probably weak and the cations rela-tively free to move. While in Casmectite, the cations are strongly hy-drated and the interlamellar complexes are rigid and stable. Thismeans that the swelling pressure is higher for Casmectite than thatfor Nasmectite with high bulk densities (Pusch and Yong, 2006).Therefore, the swelling pressure of bentonite inltratedwith CaCl2 solu-tions is higher than that of bentonite inltrated with NaCl solutions.

3.2. Hydraulic conductivity

3.2.1. Impact of concentrationsThe inuence of de-ionized water and concentration of salt solu-

tions (NaCl and CaCl2) on the evolution of hydraulic conductivity ofcompacted GMZ01 bentonite is shown in Figs. 6 and 7, respectively. Itis observed that the hydraulic conductivity decreases gradually overtime and becomes stable after 250 hour inltration. This decrease canbe possibly attributed to the clay particle movement (Ye et al., 2012).The hydrated clay particles could move with inltration at the begin-ning of test and this movement would lead to accumulation of clayAs a result, the hydraulic conductivity decreases. When this processended, the measured hydraulic conductivity becomes stable. Themaximum value changes from 2.1 1013 m/s (inltrated with de-ionized water) to 8.2 1013 m/s (with 2.0 M NaCl solution) and4.5 1013 m/s (with 2.0 M CaCl2 solution), respectively. The resultsindicate that the hydraulic conductivity increases with the increase ofconcentration of inltrating solutions. This observation is consistentwith that reported by other researchers (Studds et al., 1998; Dixon,2000; Mata, 2003; Suzuki et al., 2005; Villar, 2005; Karnland et al., 2006).

Development of hydraulic conductivity with nal swelling pressureof compacted GMZ01 bentonite testedwas plotted in Fig. 8. Fig. 8 showsthat the hydraulic conductivity decreases with the increase of swellingpressure. This phenomenon can be explained from a microstructurallevel. In case of bentonite hydrating with low concentration solutions,sufcient hydration leads to thick quasicrystals splitting into severalthinner ones and yields clogging of the macro-pores (inter-aggregatepores) (stage III in Fig. 4), which work as main owing-channels.Consequently, the hydraulic conductivity is relatively low. On thecontrary, when the concentration of electrolyte increases, the diffusedouble-layer swelling will be restricted, results in decreasing of theswelling of clay particles. This would lead to decrease of inuence ofhydration on macro-pores (stage II in Fig. 4). As a result, the hydraulicconductivity increases as the concentration of electrolyte increases.Fig. 7. Inuence of concentration of CaCl2 solutions on hydraulic conductivity of GMZ01bentonite.

sw

sw

Fig. 8. Hydraulic conductivity vs. nal swelling pressures of compacted GMZ01 bentonite.

79C.-M. Zhu et al. / Engineering Geology 166 (2013) 74803.2.2. Impact of cation typesComparison of inuence of cation types on hydraulic conductivity of

GMZ01 bentonite is shown in Fig. 9. It appears that the hydraulic con-ductivity of GMZ01 bentonite is found to increase by 3.25 and 1.54times, when the concentration of NaCl and CaCl2 solutions increasefrom 0.1 M to 2.0 M. For a given concentration, the values of hydraulicconductivity with different salt solutions are different. Furthermore,the difference depends on the concentration of the solutions. For lowconcentrations, the difference is insignicant. However, as concentra-tion increases, the difference increases signicantly.

Fig. 9 also shows that the hydraulic conductivity of GMZ01 bentoniteinltrated with NaCl solutions is higher than that inltrated with CaCl2solutions at same concentrations. This may be attributed to that, com-pare to CaCl2 solutions, inltration of NaCl solutions leads to less clog-ging of macro-pores, which results in a relatively higher hydraulicconductivity. This observation agrees with the results reported byPusch (2001) and Castellanos et al. (2008) who also found that the hy-draulic conductivity of bentonite increases faster when sodium is thepredominant cation in the inltrating solution.

4. Conclusions

Inuence of the salinity of inltration solutions on the swelling pres-sure and hydraulic conductivity of compacted GMZ01 bentonite at aninitial dry density of 1.7 Mg/m3 was investigated by means of swellingFig. 9. Comparison of inuence of cation types on hydraulic conductivity of GMZ01bentonite.and permeability tests, in which de-ionized water and NaCl and CaCl2solutions of different concentrations were used.

Results obtained indicate that the salinity of inltrating solutionssignicantly inuences the swelling pressure of GMZ01 bentonite. Theswelling pressure decreases with the increase of concentration of inl-trating solutions and the degree of impact decreases with the increaseof concentration. For a given concentration, the swelling pressure ofGMZ01 bentonite inltrated with CaCl2 solution is higher than thatwith NaCl solution. The higher the concentration of inltrating solutionis, the shorter it takes for the swelling pressure to reach its stable state.

The hydraulic conductivity of GMZ01 bentonite increases with theincrease of concentration of inltrating solutions. For high concentra-tions, the inuence of Na+ on the hydraulic conductivity of GMZ01 ben-tonite is greater than that of Ca2+. This can be explained that inltrationof NaCl solutions induces less clogging of macro-pores, which results ina relatively higher hydraulic conductivity.

Acknowledgments

The authors are grateful to the National Natural Science Foundationof China (Projects No. 41030748, 41272287), China Atomic Energy Au-thority (Project [2011]1051) for the nancial supports. This work wasalso conducted within a Program for Changjiang Scholars and Innova-tive Research Team in University (PCSIRT, IRT1029).

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Influence of salt solutions on the swelling pressure and hydraulic conductivity of compacted GMZ01 bentonite1. Introduction2. Experimental investigations2.1. Materials2.2. Test apparatus2.3. Test procedures2.3.1. Sample preparation2.3.2. Swelling pressure tests2.3.3. Hydraulic conductivity tests

3. Test results and discussions3.1. Swelling pressure3.1.1. Impact of concentration3.1.2. Impact of cation types

3.2. Hydraulic conductivity3.2.1. Impact of concentrations3.2.2. Impact of cation types

4. ConclusionsAcknowledgmentsReferences