RESUME: Nous proposons l'&ude du m~canisme de gonflement d'une smectite, bas6e sur l'exploitation des diagrammes de diffusion aux petits angles obtenus avec la source de rayons X synchrotron de Lure (Orsay). Cette technique a permis d'&udier le gonflement, en fonction de la temperature, d'une montmorillonite Na, cn suspension dans l'eau et concentr& ~ 20~o.L'analyse des diagrammes est ~ffectu~e par une m&hode bas6e sur la comparaison des courbes exp~rimentales fi des courbes th6oriques. Les mod61es de structure permettant le calcul des diagrammes th6oriques fait appel fi la notion de particule (d6finie comme un empilement de feuillets parall~les) et de structure interne de la particule (d6finie par un ensemble de translations di affect~es des probabilit& pi qui d&erminent la position mutuelle des feuillets).Les r~sultats montrent qu'fi basse temp6rature, les particules de tr& grande taille sont constitutes par la superposition de sous empilements, comportant 4 ft. 5 feuillets, hydrates d'une mani~re homog~ne et se d~sorientant mutuellement pour former la structure du gel. On explique ainsi que la structure du gel form6 est correl& avec celle du solide initial et que le ph~nom~ne de gonflement est quasi r~versible.
International audienceWhen a geosynthetic clay liner (GCL) containing sodium bentonite is brought into contact with fluids containing other cations, the latter may exchange with the sodium present between clay layers. This modification of clay surface chemistry may change the clay microstructure and hence its hydraulic conductivity. The influence of clay surface chemistry on microstructure and permeability, after prolonged contact between two GCLs (a natural sodium bentonite GCL and a sodium-activated calcium bentonite GCL) and different fluids in oedometer cells, was investigated using exchangeable-cation analysis, small-angle x-ray scattering, and transmission electron microscopy. Results suggest that calcium carbonate in the bentonite, formed during activation of the calcium bentonite, may redissolve during contact with a dilute permeant, releasing calcium ions that exchange with sodium in the clay. This exchange leads to obliteration of a so-called "gel" phase (beneficial in terms of low permeability) and to the development of a more permeable "hydrated-solid" phase. Sodium replacement by calcium during GCL contact with a 0.1 M CaCl2 solution was found to be virtually complete, with or without GCL prehydration with dilute water. No gel phase was observed in these samples. When in contact with real leachate, however, a gel phase appeared, especially when GCL samples were prehydrated. A correlation was observed between the level of hydraulic conductivity and the relative proportions of gel phase and clay interlayer occupation by sodium
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.
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