International audienceThis paper describes the swelling properties of two highly compacted clays, natural, untreated Wyoming montmorillonite (MX80) and Fourges smectite (FoCa7), saturated with Na and Ca, respectively.The initially compacted samples were hydrated by subjecting them to different suction pressures in a range between 100 MPa and 1 kPa. For each equilibrium state, the volume change (swelling) and water content (hydration) were measured. The samples were then studied by X-ray diffraction using a transmission device to determine interlayer distance and particle size, in order to clarify both the swelling and hydration mechanisms. The distances between clay layers ranged between 10 and 21.6 Å, i.e. corresponding to between 0 and 4 water layers. Upon hydration, the particle size decreased from 350 and 100 clay layers per particle to 10 layers per particle when the suction pressure decreased from 100 MPa to 1 kPa for MX80 and FoCa7, respectively. The first swelling stage is described as being an insertion of water molecules between the layers. Then a division of the initial particles into particles of smaller size with increasingly large inter-particle distances was observed. Observations by transmission electronic microscopy confirmed these results

A compacted smectite clay was studied in order to understand the swelling phenomenon. The in#uence of the sample initial state on the swelling mechanisms was demonstrated. A complete physical}chemical and mineralogical characterization of the clay was achieved using X-ray by re#exion and thermal methods. The X-ray di!raction by transmission method allowed to describe the various levels of the hydrated clay organization and the microstructure evolution. The pore distribution and the water partition are shown. Other important parameters: water retention, inter-layer distances, particle size, number of layers per particle were examined. This information gives the ability to follow the swelling which occurred in two steps, and to calculate the swelling potential versus the water content, or the void ratio. In application to this study, a water transfer in unsaturated clay was studied with the laws of continuous medium mechanics. A "nite di!erence program was used to examine two cases: prevented and free swelling in an oedometer. In order to take into account the material deformation, the swelling potential was introduced in the di!usion coe$cients. The simulation results of the water transfer obtained on this basis, were in good agreement with the experiments.

RI~SUMI~: Des poudres d'6chantillons de smectites satur6es Mg 2+ ont 6t6 r6hydrat6es jusqu'h obtention d'une hydratation maximale. Les microscopies 61ectroniques ~t balayage et ~ transmission ont permis de caract6riser l'organisation des argiles, en particulier l'arrangement, la taille et la structure cristalline des particules. La diffusion aux petits angles des rayons X a 6t6 utilis6e pour quantifier la microstructure des mat6riaux. Les r6sultats obtenus montrent que les teneurs en eau atteintes/t saturation ne sont pas directement reli6es ?a des variations de la distance interfoliaire. Le m6canisme de l'hydratation est en revanche correl6 ~ la taille des particules et ~ celle des agr6gats de particules. I1 est montr6 que l'extension des feuillets dans le plan a, b joue un r61e direct sur la taille et la forme aussi bien des particules que des agr6gats de particules, lnfine, il apparaSt que pour mieux comprendre le m6canisme du gonflement maximal des argiles, il devient n6cessaire de prendre en compte tousles niveaux d'organisation et que la contribution de l'espace interfoliaire dans l'6valuation de l'eau totale peut &re, pour les smectites-Mg relativement faible.A B S T R ACT: Powdered 2:1 Mg-clay samples were rehydrated up to the maximum hydration. Both scanning and transmission electron microscopy were used to characterize their structural organization i.e. particle arrangement, texture and crystal structure. Low-angle X-ray scattering experiments were carried out to quantify the microstructure of the system. The results showed that sample water contents reached at saturation are not directly related to changes in layer distance. The hydration mechanism is rather correlated to particle size and particle aggregate size. It is also shown that the a, b plane extension of the layers influences particle size and shape. Finally, it appears that for a better understanding of clay swelling mechanisms, at high water contents, it is necessary to take into account all organization levels and that the contribution of interlayer space to the total water content, for Mg-smectites, is small.

RI~SUMt~: L'~volution structurale et texturale ainsi que le comportement hydrique de deux montmorillonites-Ca (Wyoming et grecque) ont ~t~ &udi~s par microscopic ~lectronique fi transmission (MET) et plus pr~cis~ment par analyse de la diffusion aux petits angles des rayons X (DPAX). L'~tude au MET a permis de montrer qu'/l faible contrainte, le mat~riau est form~ par un r~seau de particules d~formables d~limitant des pores de grandes dimensions (~2 #m). L'6tude en DPAX des m~mes syst~mes soumis fi une contrainte de type succion a montr~ que: 1. Lors de la premiere dessiceation il y a d'abord d~formation du r~seau sans regroupement des particules (p <~ 1 bar), puis regroupement des particules face-face pour former des particules de plus grande ~palsseur (p > 1 bar). On montre aussi que l'eau est essentiellement extraite de la porosit~ interparticulaire. 2. Lors de la r~hydratation, on met en ~vidence qu'apr~s dessiccation le syst6me perd une grande partie de son aptitude ~, reprendre de l'eau et que cette perte est d'autant plus importante que le niveau de dessiccation pr~alable a &~ ~lev6. Ceci est dO au falt qu'une partie de la porosit~ est r~duite fi des espaces ayant la propri~t~ de l'espace interfoliaire.ABSTRACT: The structural and textural changes and swelling behaviour of two Camontmor]llonites (Greek and Wyoming) were studied by TEM and small-angle XRD. When examined by TEM at low stress, the texture was a network of particles enclosing large-size pores (~ 2 #m). XRD examination of the clays under suction showed that: 1. When first dried, the network changed configuration without coalescence of the particles (p ~< 1 bar). Afterwards, the particles became bound face-to-face and formed thicker groupings (p > 1 bar). Water was also found to be derived essentially from inter-particle porosity. 2. During rehydration the desiccated system had lost, to a great extent, its ability to recover water, this loss being greater the higher the preliminary desiccation. This is due to the fact that porosity was confined in part to spaces which had the same property as the interlayer space.Au niveau de la structure des sols, il est clair que les argiles jouent un r61e sp6cifique qui n'est pas seulement dfi fi leur propri6t6, commune fi beaucoup de constituants des sols, de pouvoir capter et retenir l'eau. Le fait important est que cette prise, ou ce d6part d'eau, s'accompagne g6n6ralement d'une modification du volume apparent et qu'elle se traduit, fi diff6rentes 6chelles, par une r6organisation du mat6riau (

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