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Seiphoori, A. et al. (2014). Geotechnique 64, No. 9, 721–734 [http://dx.doi.org/10.1680/geot.14.P.017]

721

Water retention behaviour and microstructural evolution of MX-80bentonite during wetting and drying cycles

A. SEIPHOORI�, A . FERRARI� and L. LALOUI�†

MX-80 bentonite used in engineered barrier systems would be subjected to wetting and drying cycles.To assess the response of the material under such circumstances, a comprehensive experimentalcharacterisation of the water retention behaviour of compacted MX-80 granular bentonite wasperformed in this study. A new methodology is proposed to investigate this behaviour under a constantvolume condition for specimens prepared at different dry densities. The material was subjected todifferent hydraulic paths, including cyclic variations of the water content. As a result, an irreversiblemodification of the retention behaviour was observed when the material approached a fully saturatedstate during the first main wetting, and a new hydraulic domain was consequently created. The waterretention capacity of the material increased as a result of such modification. Microstructuralobservations were performed at different stages of the hydraulic paths to relate the permanent changein the retention behaviour to the evolution of the fabric during the wetting and drying cycles. A cleartransition from a double-structured to a single-structured fabric, followed by a permanent change ofthe microfabric, was found following the first wetting. Available data on the hydration of smectiteparticles were used to relate the microstructural evolution to the change in the water retentionproperties. This correlation shows the evolution of the active porosity at the particle level within themicrostructure, which consequently affects the macroscopic response of the bentonite in terms of itswater retention behaviour.

KEYWORDS: clays; expansive soils; fabric/structure of soils; partial saturation; particle-scale behaviour;radioactive waste disposal

INTRODUCTIONBentonites are under consideration as buffer and backfillingmaterials in deep geological repositories for the storage ofhigh-level radioactive waste (HLW) (e.g. Bucher & Muller-Vonmoos, 1989; Gens, 2010; Ferrari et al., 2014). In theSwiss concept for the disposal of HLW, bentonite will beused in a repository in the form of compacted blocks tosupport the canisters that contain the waste and in granularform around the canisters to fill the gaps between thecanisters and the host rock formation (Nagra, 2009).

Granular bentonite is usually manufactured and emplacedat its hygroscopic water content (w ¼ 5–6% at a relativehumidity (RH) ¼ 35–45%) with a high total suction in therange of 100–200 MPa (e.g. Pusch, 1992; Plotze & Weber,2007; Villar, 2007). The bentonite buffer is emplaced at atarget dry density and will be under confined conditions.The progressive saturation of the buffer upon uptake ofwater from the host rock will be followed by the expansionof the bentonite and the consequent filling of technologicalgaps in the systems. This swelling occurs at the boundariesnear the surrounding host rock, where the wetting will firstoccur. A density gradient may be established, with lowerdensities in the areas where the bentonite can expand andwith higher densities in the internal areas where the bento-nite is compressed due to the swelling pressure developed in

the external layers. For instance, for compacted bentoniteblocks emplaced at a target dry density of 1.6 Mg/m3, thefinal dry densities after dismantling the Febex in-situ testwere reported to be in the range of 1.4–1.7 Mg/m3 (Villar etal., 2005).

The bentonite barrier would also be subjected to wettingand drying cycles due to the environmental conditions andthe heat generated from the waste canisters. The crucialaspect of the hydration behaviour of the bentonite materialin HLW repositories is the relation between suction and theamount of water stored in the material, which is known asthe water retention curve.

In the case of highly swelling geomaterials, such asbentonite, significant volume changes occur due to variationsof the water content if the material is free to expand. Theinfluence of volume constraints on the water retention be-haviour of swelling geomaterials, particularly at low suctionvalues, has been analysed (e.g. Al-Mukhtar et al., 1999;Lloret et al., 2003; Lloret & Villar, 2007). Villar (2007) hasreported an imperceptible influence of the dry density on thewater retention curves of Febex and MX-80 bentonites,presented in terms of water content, for total suctions greaterthan 10 MPa. In the lower range, a higher water content wasobtained for the less dense samples for a given suctionvalue. Such observations can be explained by introducing aregion where the adsorptive storage mechanism is control-ling the water retention behaviour of bentonite-based materi-als (Romero & Vaunat, 2000).

Despite the importance of the wetting and drying cycleson the water retention behaviour of the bentonite barrierssystems, few studies have addressed this aspect of the be-haviour of bentonite (e.g. Dueck, 2004; Villar, 2007). Speci-fically, there are no experimental data available on thebehaviour of bentonite during a transition from wetting todrying paths (or vice versa) in terms of scanning curves.

Manuscript received 26 January 2014; revised manuscript accepted 23July 2014. Published online ahead of print 22 September 2014.Discussion on this paper closes on 1 February 2015, for further detailssee p. ii.� Ecole Polytechnique Federale de Lausanne (EPFL), School ofArchitecture, Civil and Environmental Engineering, (ENAC),Laboratory for Soil Mechanics (LMS), EPFL-ENAC-LMS, Lausanne,Switzerland.† King Abdulaziz University, Jeddah, Saudi Arabia.

Downloaded by [ Massachusetts Institute of Technology] on [10/11/19]. Copyright © ICE Publishing, all rights reserved.

In addition, swelling materials may undergo significantfabric change during hydration (e.g. Al-Mukhtar et al., 1996;Pusch & Schomburg, 1999; Cui et al., 2002; Simms &Yanful, 2002; Romero et al., 2005; Delage & Cui, 2007;Romero & Simms, 2008). The effect of different mechanical/hydraulic stress paths on the microstructures of bentonite-based materials has been the subject of much recent work(e.g. Hoffmann et al., 2007; Lloret & Villar, 2007; Romeroet al., 2011; Della Vecchia et al., 2013). However, a systema-tic analysis of the microstructural evolution of compactedbentonite is required when, in particular, the material issubjected to wetting–drying cycles.

Although the hydration mechanisms of the smectite miner-als composing the bentonite material and the evolution ofthe interlayer distance have been addressed (Saiyouri et al.,1998, 2000; Villar, 2007; Villar et al., 2012; Bestel, 2014),considerable uncertainties exist over whether these activeinterlayer properties could be influencing the macroscopicresponse of the material in terms of the water retentionbehaviour.

Data on the hydration of smectite particles have alreadybeen used to interpret phenomena such as the ageing of thebentonite after compaction (e.g. Delage et al., 2006), but tothe knowledge of the authors, these data have not yet beenintegrated into the analysis of the water retention behaviourof bentonites subject to wetting–drying cycles.

One reason for this application gap could be the lack ofan efficient technique for the analysis of the water retentionbehaviour of highly swelling bentonite subjected to wettingand drying cycles with a precise control of the void ratio.The technique must provide enough data points along thewater retention curve to enable a more accurate represent-ation of the material behaviour, in particular when thescanning paths are presented.

In this context, this paper presents the results from acomprehensive experimental campaign to analyse the waterretention behaviour of compacted MX-80 granular bentonite.A new methodology called ‘Microcell’ was developed toobtain water retention curves with high resolution andreproducibility. The method provides measurements of thetotal suction of swelling bentonite under constant volumecondition with a precise control of degree of saturation ofthe specimen. The proposed method is a fast technique thatenables the determination of the water retention behaviourof the same specimen in a given compaction state followingwetting–drying cycles.

The Microcell was also designed to provide representativesamples for microstructural investigations during the hydrau-lic path using mercury intrusion porosimetry (MIP) analysisand scanning electron microscopy (SEM) observations. Forthis reason, the device is designed so that the freeze-dryingof the sample can be directly performed inside the cell to fixthe material structure and eliminate the pore water. Theseparticular features enable the analysis of the microstructureof the compacted bentonite at different points along thewater retention domain during wetting and drying cycles. Inthis way, it is possible to analyse how the evolution of themicrostructure would influence the macroscopic response ofthe material in terms of the water retention behaviour.

METHODOLOGYThe experimental methods for analysing the water reten-

tion behaviour rely on the measurement or control of suctionin different ranges. Total suction refers to the total potentialof pore water in unsaturated geomaterials. Several techniqueshave been developed to measure the total suction of speci-mens with controlled water content, such as the filter papertechnique (Houston et al., 1994), thermocouple psychrom-

eters (Spanner, 1951) and chilled-mirror hygrometers (Geeet al., 1992). Total suction control techniques are mainlybased on the control of the relative humidity and tempera-ture of a closed system in which the material is enclosed. Inthis way, the soil liquid phase potential is applied throughthe migration of water molecules in the vapour phase from areference system of a certain potential to the soil matrixuntil equilibrium is achieved (Romero, 2001). In each of themethods mentioned above, the water content correspondingto the measured (or controlled) suction is recorded to gener-ate points along the water retention curve. The obtainedretention path describes either a wetting or drying curvedepending on the initial suction value and the imposedsuction variation. For swelling materials, controlled totalsuction (vapour transfer) techniques with the application ofdifferent salt solutions have been actively used in numerousrecent studies (e.g. Dueck, 2004; Tang & Cui, 2005; Villar,2007).

In this paper, an improved methodology, the Microcell, ispresented for the determination of water retention behaviour.The Microcell provides determination of the air entry valueand the total suction at fully saturated state for a givencompaction void ratio. It enables the maintenance of aconstant volume condition during the hydration of highlyswelling materials, while the total suction at each hydrationstage can be measured by using a dew point potentiometer.

(a)

(b)

Tray

Drawer

Microcell containingthe compacted

specimen

Compactedspecimen30 mm

Mesh # 200

Microcell and filters

Threaded lid withholes

Fig. 1. (a) Microcell components; (b) WP4C dew point chilled-mirror potentiometer

722 SEIPHOORI, FERRARI AND LALOUI


The Microcell is a rigid cell made of brass that holdsspecimens 7 mm high with a diameter of 30 mm (Fig. 1(a)).The cell consists of two parts that are connected by a threadlid to provide constant volume conditions during the hydra-tion process. The top and bottom of the cell are perforated toprovide for the possibility of water exchange in vapour orliquid form with the surrounding environment. Filters madeof a standard ASTM steel mesh number 200 (opening equalto 75 �m) are placed at the top and bottom of the specimen toprevent migration of the fine material particles. The mesh wasshown to have a negligible retention capacity that does notinfluence the measurement of the specimen’s water content.

The measurement of total suction is performed using theWP4C (Decagon Device Inc., 2002) dew point chilled-mirrorpotentiometer (e.g. Leong et al., 2003; Cardoso et al.,2007). The Microcell has been designed to be fit directlyinto the device by placing it on the tray of the potentiometerdrawer (Fig. 1(b)).

The relative humidity in the measurement chamber of thedevice is controlled by the vapour from the specimen passingthrough the holes of the Microcell. The relative humidity isthen measured by the device, which reads the dew pointtemperature at which condensation occurs on the chilledmirror. The device also controls the temperature of the speci-men (18–408C) during the measurement. The total suction(ł) is then calculated according to the psychrometric law

ł ¼ �rwRT

Mw

ln(RH) (1)

where R is the universal gas constant (i.e. 8.3143 J/mol K),rw is the density of water, Mw is the molecular mass ofwater, RH is the relative humidity and T is the absolutetemperature of the specimen. The Microcell was first testedusing various saturated salt solutions of known water poten-tials, and the corresponding suction values were read withthe psychrometer. This procedure made it possible to verifythat the cell does not alter the total suction measurement.

In the proposed methodology, specimens are prepared bypouring and/or statically compacting the material at thetarget dry density and water content directly into the Micro-cell using an ad hoc compaction mould. The Microcell isthen closed, and the initial total suction is read with thepsychrometer. A wetting path is then initiated by placing theMicrocell in a controlled temperature chamber with animposed relative humidity of 100% to allow the specimen toabsorb water through the vapour phase. The evolution of thewater content is monitored through continuous weighing ofthe Microcell using a precision balance. The process ishalted once the specimen reaches the target water contentfor the current step. The cell is then sealed and cured for3 days at a controlled temperature to ensure the homogenisa-tion of the water in the specimen. The degree of saturationof the specimen at step i as a function of the transferredwater mass can be calculated through the following ex-pression

Sir ¼ Si�1

r þ Gs

e

˜Miw

M s

� �(2)

where ˜Miw is the incremental water mass transferred in

vapour form, Ms is the solid mass of the specimen, Gs is thespecific gravity of the material and e is the target void ratioat which the specimen was compacted. After curing, theMicrocell is inserted in the psychrometer, and the corre-sponding total suction is measured.

The retention curve for the drying paths is obtained by airdrying the specimen in the Microcell under laboratory con-ditions (T ¼ 228C, RH ¼ 34%). These conditions correspondto an applied suction of 148 MPa (see equation (1)). To

apply a higher total suction, the specimens are placed in adesiccator with a saturated salt solution (corresponding to atotal suction of up to 301 MPa for saturated lithium chloride(LiCl) solutions). The drying process is performed in acontrolled way by continuous weighing of the Microcell.The drying is halted once the target water content isreached. The Microcell is then sealed, and the total suctionis read after a curing time of 3 days. During the dryingprocess, it is possible to compute the degree of saturationwhile the specimen remains in contact with the cell walls.For the tested MX-80 bentonite, preliminary testing allowedassessment of the water content values at which the speci-mens lost contact with the cell (see the ‘Void ratio depen-dency of the water retention curves’ subsection below).

To study the microstructural features of the material andthe evolution along the wetting and drying paths, MIP testsand SEM observations were performed on specimens atdifferent steps of the retention analysis. Because those tech-niques are destructive, several specimens were prepared, andthese specimens followed the same hydraulic path until theyreached the point at which the structural features wereobserved. MIP and SEM analyses require dry specimens; topreserve the structural features during the dehydration,freeze-drying was carried out by immersion of the Microcellcontaining the specimen directly in liquid nitrogen (boilingpoint of –1968C), and subsequent sublimation under appliedvacuum (0.06 mbar) was used to eliminate the frozen porewater at a controlled temperature of –528C.

This feature of the Microcell allows freezing of thestructure at a given swelling pressure, reached during thehydraulic path, and avoids change of porosity due to stressrelease upon the opening of the cell; this feature is ofparticular interest for fully saturated specimens that mayreach very high swelling pressures. In this study, the MIPtests were carried out using a Thermo Electron Corporationsporosimeter device that attained a maximum intrusion pres-sure of 400 MPa (corresponding to an entrance pore sizediameter of approximately 4 nm). The results are presentedin terms of the cumulative intruded void ratio (eHG) andpore size density function (PSD ¼ –˜eHG/˜(log d )) plottedagainst the entrance pore diameter (d ), where eHG is theequivalent void ratio obtained by the intruded mercury. SEMobservations were carried out using a Carl Zeiss Merlin HR-SEM system.

WATER RETENTION BEHAVIOUR OF MX-80BENTONITETested material

Tests were performed on compacted MX-80 (Wyoming)granular bentonite. The index properties and characteristicsof the material are summarised in Table 1. The liquid limitwas obtained by the cone penetration method (BSI, 1990);Gs was measured using a fluid displacement technique in apycnometer using a non-polar liquid. The tested bentonitecontains 85% sodium (Na)-smectite clay, which reflects thewell-known high swelling capacity of the material. Theapparent grain size distribution of the granular bentonitebefore compaction is shown in Fig. 2. Specimens wereprepared at different void ratios. The corresponding massrequired to reach the target void ratio was then calculatedconsidering the volume of the specimen. Different grain sizefractions (in terms of dry mass) corresponding to Fig. 2were selected and mixed in order to obtain the sameapparent grain size distribution for all specimens. Thisprocess was performed for each single specimen. In thisway, the highest void ratio attainable by simply pouring thematerial with the selected grain size distribution is 0.83(rd ¼ 1.50 Mg/m3). Specimens with lower void ratios of

MX-80 BENTONITE DURING WETTING AND DRYING CYCLES 723


e ¼ 0.66 (rd ¼ 1.65 Mg/m3) and e ¼ 0.53 (rd ¼ 1.80 Mg/m3)were obtained by static compaction.

The microstructural features of the material in the as-compacted state were investigated by combining the MIPanalysis and the SEM observations. All specimens for theseanalyses were prepared at the same water content of 5%.

In Fig. 3, the PSD of the material at the as-compactedstate for a void ratio of e ¼ 0.53 is presented along with thePSD function of a single grain. The SEM photomicrographsof an isolated bentonite grain and its surface are shown inFig. 4, revealing a well-iso-oriented clay particle arrange-ment. The grain exhibits a single-mode porosity with amodal value of d ¼ 12 nm. This PSD of the single grainwould suggest that there is a marginal inter-aggregate poros-ity within the grains. However, this inter-aggregate porositywas reported to be significant by Hoffmann et al. (2007) forFebex bentonite pellets. This difference between MX-80bentonite grains and Febex pellets is well explained con-sidering the differences in the production process – in particular, the fact that they are both prepared from powders

– but the lower water content used for the production ofpellets facilitates the formation of aggregates.

The as-compacted specimen shows a bimodal PSD. Poreswith characteristic diameters in the range of 300 nm–400 �mare clearly detected. The modal value of this family of poresis directly related to the compaction effort and was measuredto be d ¼ 1.5 �m for e ¼ 0.53 (Fig. 3) and d ¼ 15 �m fore ¼ 0.83. The modal value for the peak observed for thelower pore dimension is situated at d ¼ 17 nm, and thecharacteristic diameters for these pores are d , 300 nm. Inthis pore domain, the compacted granular bentonite shows awider distribution of pore diameters with respect to the PSDobserved for the single grain. Within this peak, a localmaximum can be recognised for the diameter correspondingto the modal value of the grain PSD (d ¼ 12 nm). SEMphotomicrographs can help in understanding the increaseddiameter range for these pores for the granular mixture withrespect to the single grain. Figs 5(a) and 5(b) show an

Table 1. Index properties and characteristics of the tested MX-80 bentonite

Smectite content:�%

Specific surfacearea, s:� m2/g

Specific gravity,Gs

Liquid limit,wL: %

Plastic limit,wP: %

Hygroscopic water content underlaboratory conditions (T ¼ 228C,

RH ¼ 34%), whg: %

85 523 2.74 420 65 5.4

� Data from Plotze & Weber (2007).

1001010·1

Curvature number, 1·30Distortion number, 18·38

CC

C

U

�

�

0·01Grain size: mm

0

20

40

60

80

100

Per

cent

pas

sing

: %

Gravel sizeSand sizeSilt size

Fig. 2. Apparent grain size distribution for the tested MX-80granular bentonite

200 mμ

3 mμ

Fig. 4. SEM photomicrographs of a single MX-80 grain withe 0.28 (rrd ¼ 2.13 Mg/m3) and w ¼ 5%. Note: the void ratio ofthe bentonite grain was obtained by a liquid displacementtechnique using a non-polar liquid (Peron et al., 2007; Seiphoori,2014)

10�3 10�2 10�1 100 101 102 103 104

Pore size diameter, : md μ

0

0·10

0·20

0·30

PS

D

As compacted, 0·53e �

Grain, 0·28e �

1·5 mμ

�20 nm12 nm

Fig. 3. Different microstructural levels in poured and compactedMX-80 granular bentonite



overview of the compacted material (e ¼ 0.53). Grains areclearly visible as distinct units. The grains are coated withaggregates composed of finer clay particles, which occupythe inter-grain space (Fig. 5(c)).

A dominant pore size of 1.5 �m can be identified in Fig.5(d) as the characteristic dimension of the pores between theaggregates or between the grains and the aggregates. Thisfamily of pores is referred to hereafter as macroporosity.The micropores are defined here as pores between thebentonite particles within the grains and aggregates. Inter-particle porosity within the bentonite aggregates seems toprevail with respect to the inter-particle porosity within thegrains (Fig. 3) in the sense that the compacted granularbentonite (assemblage of grains and aggregates) exhibits awider distribution of pore diameters with respect to singlegrains in the vicinity of the modal value d ¼ 20 nm. Anindicative value of 300 nm can be assumed to differentiatemicroporosity from macroporosity. This value correspondswell to the diameter suggested by Romero et al. (2011) fromthe analysis of the retention properties of compacted MX-80bentonite.

A further level of porosity that refers to the intra-particle(or inter-layer) pores cannot be intruded by the MIP and isnot visible in the SEM observations. The characteristicdiameters for these pores are strongly dependent on thehydration of the smectite layers and can be in the range of1.26–2.16 nm (Saiyouri et al., 1998).

Void ratio dependency of water retention curvesThe water retention curves of the MX-80 granular bento-

nite at three different void ratios are presented in Fig. 6.The curves follow a wetting path starting from a total

suction imposed in desiccators supplied with saturated saltsolutions (initial imposed total suction in the range of 100–300 MPa). The difference in the starting conditions in termsof water content is justified by the fast absorption of waterby the bentonite once the material is taken out of thedesiccator for the preparation of the specimens. The wettingwas performed using the procedure already detailed underthe ‘Methodology’ section and was stopped once the speci-men did not exhibit any further absorption of water. Thewetting paths for the three void ratios lay on the same mainwetting path (Fig. 6(a)).

Figure 7 shows the evolution of the degree of saturationover time for a specimen in the Microcell prepared ate ¼ 0.83 and Sr ¼ 0.70 and placed inside a chamber with100% relative humidity. The graph highlights that the mater-ial continuously absorbs water through the vapour phaseuntil saturation is reached. For all the tested conditions, thecomputed degree of saturation at the end of the wetting pathwas approximately 1.00 � 0.005. This observation, togetherwith the results in Fig. 6, shows that the accuracy of themethod in following the wetting process and in assessing thedegree of saturation is very high.

The drying paths were run up to a total suction of200 MPa. In the plane of total suction against water content,the drying paths align in a unique trend (Fig. 6(a)). The twowell-identified main wetting and drying curves delimit allpossible hydraulic states for the confined MX-80 granularbentonite. The hysteresis behaviour of the water retentioncurves is easily observable because the degree of saturationreached in the drying paths is always higher than the degreeof saturation obtained during the main wetting process for agiven total suction.

The total suction values for which the fully saturated

(a) (b)

(c) (d)

50 mμ

Bentonite grain

100 mμ

Assemblage

3 mμ

Aggregate

3 mμ

Inter-aggregatepore

Fig. 5. SEM photomicrographs of MX-80 granular bentonite in the compacted state with e 0.53 (rrd 1.80 Mg/m3) and w 5%



condition is reached depend on the void ratio (Fig. 8); ahigher void ratio corresponds to a higher water content anda lower total suction at which saturation occurs.

The drying paths in terms of the degree of saturation (Fig.6(b)) are depicted until the point at which the lateral contactof the specimen is lost. For each compaction void ratio, thispoint is obtained from complementary tests performed onspecimens that followed the same hydraulic paths and forwhich the Microcells were opened after each equalisationstep and inspected by image analysis to detect if the speci-men had lost lateral contact with the cell due to shrinkage.

The air entry values can be identified by observing theplane of the degree of saturation plotted against totalsuction. The air entry value is taken as the total suctionvalue at which a degree of saturation equal to 0.95 isreached in a main drying path. The evolution of the air entryvalue as a function of the void ratio is depicted in Fig. 8.

To investigate the reproducibility of the obtained results,the water retention curves at the three considered void ratios(e ¼ 0.83, 0.65 and 0.53) were repeated along the wettingpath, and the results are shown in Fig. 9. The second seriesof tests resulted in excellent agreement with the first series.This result confirms the high resolution and good repeatabil-ity of the method.

Dueck (2004) reported data on the retention behaviour ofMX-80 under free volume conditions in terms of watercontent plotted against relative humidity at a controlledtemperature. The method considers a controlled total suctiontechnique that involves placing several specimens in glassjars above the surface of different salt solutions. The equiva-lent total suction values have been computed using equation(1), and the results are reported in Fig. 10 along with theretention domain identified by the Microcell method. Theretention curves of MX-80 bentonite reported by Delage etal. (2006) and Villar (2007) and obtained under constantvolume conditions (void ratios of e ¼ 0.56 and e ¼ 0.76,respectively) are plotted in this figure as well.

The figure shows that the retention domain obtained withthe proposed technique is in good agreement with the dataobtained under the constant volume conditions, and thesedata are consistent with the data of the free water retentioncurve reported by Dueck (2004).

The effect of the confinement is discussed later in thepaper. At this stage, note that the Microcell method provides

e 0·83�

e 0·66�

e 0·53�

10010

e 0·83�

e 0·66�

e 0·53�

100101Total suction, : MPa

(b)ψ

0

0·2

0·4

0·6

0·8

1·0

Deg

ree

ofsa

tura

tion,

Sr


(a)ψ

0

0·05

0·10

0·15

0·20

0·25

0·30

0·35

Wa

ter

cont

ent,

w

Fig. 6. Effect of the void ratio on the water retention behaviour ofMX-80 granular bentonite: (a) water content plotted against totalsuction, (b) degree of saturation plotted against total suction

00·65

0·70

0·75

0·80

0·85

0·90

0·95

1·00

Deg

ree

ofsa

tura

tion,

Sr

e 0·83�

Final calculated1·005Sr �

Starting 0·702Sr �

252015105Time: days

Fig. 7. Evolution of the degree of saturation with respect to timefor a specimen inside the Microcell placed in an RH 100%environment

1·00·90·80·70·60·5

Sr 1·0, wetting path�

Sr 0·95, drying path�

ψ 425·69 exp( 4·96 ), 0·97� � �e R 2

ψ 248·21 exp( 4·78 ), 0·99� � �e R 2

0·4

Void ratio, e

1

10

100

Tota

l suc

tion,

: MP

aψ

Fig. 8. Evolution of the air entry and suction at which the fullsaturation is reached as a function of the void ratio



a precise representation of the retention behaviour based onthe method’s ability to utilise a large number of experimen-tal points along the wetting and drying paths, whereas in thecase of the glass jar method, the number of data points islimited to the number of salt solutions used to control thetotal suction. The glass jar method involves a slow transferof water to or from the specimen and the solution throughpure diffusion, and equilibrium is usually obtained after along time (on the order of months), particularly for the mainwetting path, whereas the Microcell method allows eachpoint to be obtained within a few days.

Retention behaviour in wetting and drying cyclesIt is commonly agreed that, for a given void ratio, all

possible hydraulic states lie between the main wetting andmain drying curves, and the scanning curves describe thetransition from the main wetting to main drying conditions

or vice versa. The cyclic water retention behaviour is animportant aspect to be considered when cyclic variations ofthe suction are expected, as in the case of the use ofbentonite for engineered barriers. To the knowledge of theauthors, there are no experimental data showing the scanningpaths for compacted MX-80 bentonite. For this reason, aseries of tests was carried out with the aim of investigatingthe water retention behaviour in wetting and drying cycles.Tests were performed on specimens compacted at e ¼ 0.53(rd ¼ 1.80 Mg/m3). The obtained results are depicted in Fig.11. The compacted specimen was wetted in steps (pathA–B–C–D) until full saturation was reached (point D). Themain drying was then initiated until a suction value of95 MPa was achieved (path D–E). At this stage, a wettingpath (E–F) was imposed, again reaching full saturation.Drying was then initiated once more until a suction of174.5 MPa was reached (point G), and a final wetting wasperformed until a degree of saturation of 0.95 was obtained(point H).

The first (D–E) and second (F–G) drying paths arealigned, whereas after the first wetting (A–D), the materialdoes not follow the initial wetting curve, and a new mainwetting curve is created (G9–H). Before this new mainwetting path is reached, scanning curves can be clearlyobserved (paths E–E9 and G–G9). These results suggest thatduring the first wetting, the material undergoes a significantand irreversible change in its retention behaviour. After thefirst saturation (path A–D), the retention capacity of thematerial increases, in the sense that more water can bestored in the bentonite for a given suction in a wettingpath. This process results in an improvement in the reten-tion behaviour of the bentonite used as a buffer materialand has a strong impact on the performance of engineeredbarriers.

In the next section, the ability of the Microcell to providerepresentative specimens for microstructural investigations isused to perform a systematic analysis of the evolution of thefabric of the material during the wetting and drying cycles.

MICROSTRUCTURAL EVOLUTION OF MX-80BENTONITE DURING WETTING–DRYING CYCLINGMicrostructural observations

MIP tests were carried out at different stages of thehydraulic path depicted in Fig. 11. Because MIP is adestructive technique, several specimens were prepared under

0

0·05

0·10

0·15

0·20

0·25

0·30

0·35

0·40

1 10 100

Wa

ter

cont

ent,

w

Total suction, : MPaψ

Water retention domainof MX-80 bentonite(present study)

Suction control, free volume condition(Dueck, 2004)Suction contol, wetting path 0·76(Villar, 2007)

e �

Suction control, wetting path 0·56(Delage ., 2006)

eet al

�

Fig. 10. Comparison of the water retention curves obtained by theMicrocell with other techniques of total suction control

10010

Firstseries

Secondseries

1


0

0·2

0·4

0·6

0·8

1·0

Deg

ree

ofsa

tura

tion,

Sr

e 0·83�

e � 0·66

e � 0·53

Fig. 9. Reproducibility of the determination of the retentionbehaviour using the Microcell for different void ratios

10010


0·04

0·06

0·08

0·10

0·12

0·14

0·16

0·18

0·20

Wa

ter

cont

ent,

w

First wetting

First drying

Secondwetting

Second drying

Thirdwetting

0·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

1·0

Deg

ree

ofsa

tura

tion,

Sr

Initial state

Final state

A

B

C

E

F

G

H

E'

G'

D

Fig. 11. Water retention behaviour of highly compacted MX-80granular bentonite during cyclic variation of the total suction



the same initial conditions (e ¼ 0.53, w0 ¼ 0.05) and used inthis investigation, which followed the same wetting anddrying path until the conditions were reached for an MIPtest. Fig. 12(a) shows the states during the main wetting pathfor which the MIP tests were performed, while Figs 12(b)

and 12(c) present the corresponding cumulative void ratios(intruded void ratio by mercury) and the PSD functions,respectively. As discussed, the material in the as-compactedstate (A) exhibits a clear bimodal pore size distribution withthe inter-bentonite assemblage pores (macropores) and

100

A, as compacted

B, at 0·62Sr �

D, fully saturated

(b)

Deg

ree

ofsa

tura

tion,

Sr

Cum

ula

tive

void

ra

tio,e

Hg

10010

Total suction, : MPa(d)

ψ

(e)

10�3 10�2 10�1 1 00 101 102 103

Pore size diameter, : m(f)

d μ

10�3 10�2 10�1 1 00 101 102 103

Pore size diameter, : m(c)

d μ

0

0·1

0·2

0·3

0·4P

SD

1·5 mμ

20 nm

A

0

0·1

0·2

0·3

0·4

PS

D

D, fully saturated

E, at 0·64Sr �

G, end of drying

20 nm

0·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

1·0

A

D

G

E

Main drying

B

10

Total suction, : MPa(a)

ψ

0·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

1·0

Deg

ree

ofsa

tura

tion,

Sr

A

D

Main wettingB

B

D

C

0

0·1

0·2

0·3

0·4

0·5

0·6

Cum

ula

tive

void

ra

tio,e

Hg

AB

D

0

0·1

0·2

0·3

0·4

0·5

0·6

e 0·53�

G

D

E

D, E and G

MacroporesMicropores

e 0·53�

MacroporesMicropores

Delimiting lined 300 nm�

Fig. 12. Microstructural evolutions of compacted granular bentonite at e 0.53 (rrd 1.80 Mg/m3) during different phases ofthe wetting and drying paths



the intra-grain along with the intra-aggregate pores thatconstitute the micropores.

The PSD function at point B, where the bentonite specimenhas reached a degree of saturation of 0.62, shows a decreasein the macropore volume without a change in the modal valuewith respect to the initial state (point A). The microporesexhibit a negligible change, implying that the intruded part ofthe micropore by mercury has not changed remarkably frompoint A to point B. The cumulative void ratio curve and thePSD function of the material in the fully saturated condition(plot D in Fig. 12(b) and Fig. 12(c)) indicate a significantreduction of the macropores due to the further hydration ofthe bentonite assemblages. The cumulative volume of themicropores and the PSD value at the peak were also increasedfor point D, indicating an increase in the intruded part of themicropore at this point.

The states of the material during the drying path forwhich the microstructural analyses were performed areshown in Fig. 12(d). Figs 12(e) and 12(f) present thecumulative void ratio and the PSD functions, respectively.The PSD function at point E, when the specimen hasreached a degree of saturation of 0.64, exhibits a singleporosity distribution similar to the one obtained at fullsaturation (point D). The cumulative void ratio at point Eremains almost identical to the one obtained at point D. Thisobservation indicates a similar structure for the material atpoints D and E.

Point G (Fig. 12(d)) describes the state at the end of thelast drying path after the wetting and drying cycles. Fig.12(f) shows that the material at point G still exhibits asingle porosity structure analogous to the porosity structureobtained at points D and E. However, a slight increase inthe intruded volume can be observed, as presented in Fig.12(e).

This observation reveals a significant and permanentmodification of the bentonite structure with respect to theas-compacted state. This structural alteration is also percep-tible in the SEM analyses. In Fig. 13, the SEM photomicro-graphs of the bentonite sample at point G are presented. Fig.13(a) shows that the bentonite grains remain detectable,while the aggregates exhibit a high degree of expansioncompared to the as-compacted state of the material (Fig. 5).A further observation at the inter-assemblage space (Fig.13(c)) demonstrates that the aggregates have expanded andfilled the macropores, forming a more homogeneous andcompacted structure. This structural modification seems tooccur when the material approaches a fully saturated state(point D) and remains permanent during the subsequentwetting and drying cycles. This observation is associatedwith the change of the microstructure due to the inclusion ofwater molecules in bentonite particles during hydration. Aninsight into the water retention behaviour of the material inthe first wetting path would help to specify the suctiondomain at which this structural modification is triggered.Available data on the hydration mechanism of the smectiteparticles are used in the next section to interpret the macro-scopic water retention behaviour in the first wetting path.

Hydration mechanism and particle subdivisionIn smectite minerals, hydration is governed by the pro-

gressive placement of layers of water molecules along thesurface of the elementary smectite platelets, starting withone layer under dry conditions and reaching a maximum offour layers for very low suction (Saiyouri et al., 1998;Bestel, 2014).

Saiyouri et al. (1998) analysed the hydration mechanismof the smectite particles in compacted MX-80 bentonite,wetted in the unconfined condition, using X-ray diffraction.

The evolution of the average interlayer distance, correspond-ing interlayer water and the number of elementary sheets persmectite particle were described as a function of the totalsuction (Figs 14(a) and 14(b)). The smectite mineral hasonly one water layer up to approximately 50 MPa of totalsuction. The second water layer is absorbed at a lowersuction, and the third water layer is added at a suction valueof less than 7 MPa. At the same time, the number of layersper particle decreases from a few hundreds to ten when thetotal suction decreases to below 3 MPa.

Bestel (2014) measured the evolution of the interlayerdistance as a function of the water content using a neutrondiffraction analysis. These results are plotted in Fig. 14(a)using the water retention curves obtained in the currentresearch in terms of the total suction (Fig. 6).

50 mμ

(a)

5 mμ

(b)

200 nm

(c)

Fig. 13. SEM photomicrographs of MX-80 granular bentoniteafter wetting/drying cycling corresponding to point G in Fig.12(d)



To specify the domain for which the particle-scale hydra-tion mechanism could impose structural modifications, asdiscussed in the earlier subsection on ‘Microstructural ob-servations’, a water retention test was performed on a speci-men compacted to the same void ratio of e ¼ 0.53, aspresented in Fig. 14(c). The results of the previous test (Fig.11) are also shown in this plot.

This water retention test aims to evaluate the hysteresisresponse of the material at three different points along thefirst wetting path. When the material reached degrees ofsaturation of 0.38, 0.62 and 0.88 (points A9, B and C), adrying–wetting path was performed and resulted in a scan-ning path.

The material exhibits a completely reversible retentionbehaviour in the A–A9–A0 path without a hysteresis cycle.The corresponding positions of point A9 in Figs 14(a) and14(b) show that the smectite particles only have one layer ofwater with the maximum number of sheets per particle. Thisinitial hydration is believed to occur on the mineral surfaceand around the exchangeable cations (Na+ for MX-80 bento-nite) in the interlayer space (e.g. Sposito & Prost, 1982).

Later, a drying and wetting cycle was performed at pointB, where the material reached a degree of saturation of0.62. The scanning curve returned to the initial wetting pathby exhibiting hysteresis behaviour in the B–B9–B0 path. Theinterlayer state of the material at point B in Fig. 14(a)indicates that the material is near a transition to two layersof water. Based on Fig. 14(b), the first subdivision isexpected when the total suction reaches a value of less than50 MPa. However, an analysis of the MIP results in terms ofthe cumulative void ratio (Fig. 12(b)) indicates an increasein the non-intruded pores that is not necessarily related tothe subdivision of the particles but could be associated withparticle expansion during hydration. Such a mechanismcould result in a rearrangement/reduction of inter-particlepores and an increase in non-intruded pores. The preserva-tion of the main wetting path, despite the reduction inmacropores, implies that the water retention behaviour inthis suction domain is exclusively controlled by the micro-pores.

Finally, a drying and wetting path was performed at pointC, where the material reached a degree of saturation of0.88. The scanning path again reaches the initial wettingpath with a hysteresis loop (path C–C9–C0). Figs 14(a) and14(b) show that the material has adsorbed the second waterlayer at point C, and the subdivision of the smectite particleis possible due to further hydration. However, this confinedconfiguration could have prevented the subdivision of all theparticles during hydration. It was previously noted that theparticle hydration mechanism results reported by Saiyouri etal. (1998) are associated with hydration under an unconfinedcondition, whereas the bentonite specimens in the currentstudy are confined using the Microcell. Villar et al. (2012)have also reported the influence of confinement on theinterlayer distance at lower suctions.

As a result of further hydration, the material reached afully saturated state followed by a permanent structuralmodification and the creation of a new wetting path, asdetailed in the earlier subsection ‘Microstructural observa-tions’. The main subdivision of the smectite particles for theapplied path seems to occur when the material has reachedpoint D. This modification at the particle level consequentlyresults in a macroscopic irreversibility in terms of the waterretention behaviour. Further subdivision is prevented at pointD by the application of a constant volume condition, despitethe tendency for further hydration. As a result, the totalsuction remains at 20.6 MPa. In the subsection on ‘Influenceof the confinement on microstructural evolution’ below, it isshown that a further reduction of the total suction can beachieved if a volume change is provided.

Microstructural and nanostructural void ratio evolutionThe definition of the micropores from the MIP tests was

based on the assumption of a delimiting pore size of 300 nmto differentiate the micropores from the macropores. Thetotal void ratio could then be decomposed as e ¼ em + eM,where em is the microstructural void ratio and eM is themacrostructural void ratio. The microstructural void ratio em

is defined as the ratio of the micropore volume to the solid

10001001010·10·01

10001001010·10·010·001Total suction, : MPa

(a)ψ


(b)ψ

100·2

0·3

0·4

0·5

0·6

0·7

0·8

0·9

1·0

Deg

ree

ofsa

tura

tion,

Sr

e 0·53, first test�

e � 0·53, second test

InitialReversiblebehaviour

Hysteresisbehaviour

B

A

C

D

E

G

0·001Total suction, : MPa

(c)ψ

0

100

200

300

400

Num

ber

ofsh

eets

per

part

icle

0

1

2

3A

vera

ge d

ista

nce:

nm

X-ray diffraction analysis (Saiyouri ., 1998)et al

Neutron diffraction analysis (Bestel, 2014)

Layer 1Layer 2

Layer 3Layer 4

B

D

Subdivision insmectite particles

CA�

A�

AA�B�

C�

AA�BCDC�

B�

Fig. 14. (a), (b) Average interparticle distance in the smectite mineral and the number of sheets per particle plotted againsttotal suction. (c) Water retention behaviour of highly compacted bentonite under the constant volume condition



volume. This subsection aims to investigate the evolution ofthis parameter during the hydraulic path applied in theearlier subsection ‘Microstructural observations’.

The evolution of the microstructural void ratio withrespect to the total suction is presented in Fig. 15. Themicrostructural void ratio increases during wetting from aninitial value of 0.3 (corresponding to a degree of saturationof 0.27) to 0.53 in the fully saturated state. After drying toa degree of saturation of 0.64, no appreciable change inthe microstructural void ratio occurs. After performing awetting–drying cycle, the material was dried to a degree ofsaturation of 0.4, while the microstructural void ratio re-mained almost constant. It is also possible to introduce ananostructural void ratio en, which is defined as the ratio ofthe volume of the non-intruded pores to the solid volume(e.g. Nowamooz & Masrouri, 2009) and is associated withthe interlayer pore network within the smectite particles. Thenon-intruded pore structure of the material was completelyattributed to the microstructure in some studies (e.g. Lloretet al., 2003; Delage et al., 2006). This part of the micro-pores (interlayer pores) can be referred to as the active partof the microstructure to which the swelling and osmoticphenomena are associated. Fig. 15 shows the evolution ofthe nanostructural void ratio as a function of the totalsuction computed from the cumulative void ratio curves inFig. 12. The increase in the nanostructural void ratio in-dicates the progressive inclusion of the water moleculesinside the interlayers when the total suction decreases. Thisalso indicates an irreversible behaviour after full saturation.This irreversible change of the nanostructural void ratio isbelieved to be associated with the subdivision of the smec-tite particles. It is noted that, even if the process of increas-ing the interlayer distance is reversible, the subdivision ofsmectite particles is irreversible. In Fig. 16, the SEM photo-micrographs of the material in the as-compacted state arecompared with the one after the wetting–drying cycle. Themodification of the microstructure resulting from subdivisionof the particles is observable in these images.

This evolution of the microporosity influences the macro-porosity and, at a certain level, imposes an irreversiblechange in the macroscopic response of the material in termsof the water retention behaviour. On the basis of theseobservations, the assumption of a reversible response for themicrostructure, often used in describing the behaviour of

double-structured clays (e.g. Gens & Alonso, 1992; Alonsoet al., 1999; Airo Farulla et al., 2010), may not be appro-priate for highly active geomaterials.

Influence of the confinement on microstructural evolutionIn the earlier subsection ‘Hydration mechanism and parti-

cle subdivision’, it was mentioned that the further hydrationand subdividing of the particles were prevented by maintain-ing a constant volume condition. If a volume change isallowed, the hydration process will be followed by thefurther inclusion of water molecules, subdivision, and adecrease in the total suction.

A complementary test to observe the unconfined waterretention behaviour of MX-80 bentonite was performed, andthe results are presented in Fig. 17 in terms of the watercontent and the total suction. The specimen was prepared atan initial void ratio of e ¼ 0.83. The water retention behav-iour of the material in the unconfined condition was deter-mined based on a controlled wetting–drying process and themeasurement of the total suction with the psychrometer.

The water retention curves of the material under confinedconditions are also shown in Fig. 17. For the poured speci-men at e ¼ 0.83 under confined and unconfined conditions,both water retention curves are aligned until the total suctionat which the confined specimen reaches the fully saturatedstate (ł ¼ 4.85 MPa). At lower suction values, there is adivergence in the confined and unconfined retention behav-iour because the unconfined material is able to adsorb wateras the suction decreases, whereas this possibility is hindered

100

Sr 0·27�

10Total suction, : MPaψ

0

0·1

0·2

0·3

0·4

0·5

0·6

0·7

Voi

d ra

tio,

and

ee

mn

Subdivision

0·62Microporosity

Nanoporosity

First wetting

1·000·64 0·40

e 0·53�

First main drying After wet–dry cycling

One layer ofinterlayer water

Two layers ofinterlayer water

30 300

Fig. 15. Evolution of the microstructural void ratio and nanos-tructural void ratio with respect to total suction for the compactedbentonite sample

1 mμ

(a)

1 mμ

(b)

Fig. 16. SEM photomicrographs of MX-80 granular bentonite:(a) in the as-compacted state; (b) after the wetting–drying cycle



under the confined condition. The corresponding particlestate of the material in Figs 17(b) and 17(c) shows a signifi-cant subdivision from 150 to 12 sheets per particle when thetotal suction decreases from 6.8 to 2.8 MPa. Fig. 17(a)shows that the retention curve under the free volume condi-tion does not exhibit hysteresis in this range where themaximum subdivision has occurred.

For lower void ratios (e ¼ 0.66 and e ¼ 0.53), higher totalsuctions are achieved at full saturation under the constantvolume condition. Despite the difference in void ratios, allthe curves are aligned. However, as previously mentioned,the total suction value at full saturation is a function of thevoid ratio: the lower the void ratio is, the higher thecorresponding total suction at full saturation (Fig. 8). Incontrast, the swelling pressure of the bentonite upon fullsaturation is also a function of the void ratio: the lower thevoid ratio is, the higher the swelling pressure at full satura-tion (e.g. Muller-Vonmoos & Kahr, 1983; Lloret & Villar,2007). Such behaviour is a result of the dependence of theosmotic repulsion pressure on the interlayer distance: thehigher the interlayer distance is, the lower the osmoticrepulsion pressure and, accordingly, the swelling pressure(e.g. Tripathy et al., 2004). This means that by increasingthe interlayer distance, both the swelling pressure and totalsuction at full saturation will decrease. In this way, one candefine the swelling pressure as the attempt of the swellinggeomaterial to expand and subdivide at the particle level toinclude more water molecules after reaching full saturationunder constant volume conditions.

SUMMARY AND CONCLUSIONSBentonite used as a buffer barrier in deep geological

repositories would be subjected to wetting and drying cyclesdue to environmental conditions. An improved understandingof the water retention behaviour, microstructural evolution,and the mutual interaction between the microscopic featuresand macroscopic response of bentonite materials is required

to predict the long-term performance of bentonite engineeredbarriers.

To this end, comprehensive experiments were performedto determine the water retention behaviour and microstruc-tural evolution of highly swelling MX-80 granular bentonite.A new methodology called Microcell was proposed to obtainwater retention curves with high resolution and reproduci-bility. The Microcell also provided representative samplesfor MIP and SEM analyses of the compacted bentonitesamples at different stages of the wetting and drying cycles.

An analysis of the water retention behaviour of thegranular bentonite indicated a predominant adsorption mech-anism whereby water was stored inside the micropores. Ahydraulic domain created by the water retention behaviourof the material at the highest void ratio was found to includeall the possible hydraulic paths for the lower void ratios. Anexponential evolution of the air entry value was also ob-tained as a function of the void ratio.

The water retention behaviour of the compacted materialduring wetting and drying cycles revealed an irreversiblemodification of the water retention domain through thecreation of a second wetting path when the material ap-proached full saturation. The new hydraulic domain wasobserved to be permanent for the following wetting anddrying cycles. Complementary tests proved that the firstwetting path is preserved for the wetting and drying cyclesbefore the material reached full saturation. This evidenceindicated a hydraulic range within which the water retentionbehaves reversibly in terms of the preservation of the firstwetting path.

The microstructure of the material was analysed at differ-ent points within the analysed water retention domain. TheMIP analyses indicated a transition from a double- to single-structured material by a progressive elimination of themacropores, while the micropores (including the non-intruded part of the porosity) were observed to increase.However, the intruded part of the micropore did not changewithin the hydration range before the material approached

Free volume, wetting

Free volume, drying

e 0·83�

e 0·66�

e 0·53�

S er 1·0 at 0·66� �

S er 1·0 at 0·53� �

100101 10001001010·10·01Total suction, : MPa

(a)ψ

10001001010·10·010·001Total suction, : MPa

(b)ψ

0·0010

100

200

300

400

Num

ber

ofsh

eets

per

part

icle

0

1

2

3

Ave

rage

dis

tanc

e: n

m

X-ray diffraction analysis (Saiyouri ., 1998)et al

Neutron diffraction analysis (Bestel, 2014)

Layer 1Layer 2

Layer 3Layer 4

Maximum subdivision

b

c

d

c

b

d

0·1

Total suction, : MPa(c)

ψ

0

0·1

0·2

0·3

0·4

0·5

Wa

ter

cont

ent,

w S er 1·0 at 0·83� �

ba

4·85 MPa�c, ψ

1·44 MPa�d, ψ

a

a

Fig. 17. (a) Effect of confinement on the water retention behaviour of bentonite. (b) Interlayer distance evolution withdecreasing suction. (c) Number of sheets per particle in suction decrease and the maximum subdivision under the free volumecondition



full saturation. This hydration range corresponds to thedomain where the first wetting path was preserved. Thetransition to a single-structured pore network progressed bythe elimination of the macropores when the full saturationcondition was reached. An increase in the intruded part ofthe micropore was also observed after this point. The changeof the pore size distribution was determined to be irreversi-ble by analysing the behaviour of the material in the dryingpath. The modification of the water retention behaviour andthe creation of the second wetting path corresponded to theirreversible change in the microstructure. An analysis of theavailable data of the hydration mechanism of the bentoniteclays indicated that the smectite particles undergo a subdivi-sion and splitting due to a progressive inclusion of watermolecules in the interlayers. For the compacted bentoniteanalysed in this research, this mechanism was found to occurat a total suction at which the material approached the fullsaturation condition. The subdividing of the particles isbelieved to modify the microstructure of compacted bento-nite in an irreversible manner. The macroscopic irreversibil-ity of the water retention behaviour observed after the firstwetting was thus a consequence of the irreversible modifica-tion of the microstructure.

Such modification of the hydraulic domain results in asignificant increase in the water retention capacity of thebentonite and therefore has a notable impact on the long-term performance of the buffer material in HLW repositorybarriers. Such irreversibility of the microstructure must betaken into account for the constitutive modelling of highlyswelling geomaterials such as MX-80 bentonites.

ACKNOWLEDGEMENTSThis work was funded by the National Cooperative for

Nuclear Waste Storage of Switzerland (NAGRA) and per-formed as part of the ‘Thermo-hydro-mechanical character-isation of the MX-80 bentonite’ framework programme.Helpful discussions with Dr Mohammad Monfared and MrsDonatella Manca are appreciated. The Microcell was manu-factured at the mechanics workshop of EPFL by LaurentMorier.

NOTATIONd pore diametere void ratio

eHG cumulative intruded void ratioeM macrostructural void ratioem microstructural void ratioen nanostructural void ratioGs specific gravityMs solid mass of specimenMw molecular mass of water

R universal gas constant (i.e. 8.3143 J/mol K)Sr degree of saturation of specimens specific surface areaT temperature

whg hygroscopic water content under laboratory conditionswL liquid limitwP plastic limit

˜Miw incremental water mass transferred in vapour form in step ird dry densityrw density of waterł total suction

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