State a N D Location of Water a D S O R B E D on Clay Minerals: Consequences of the Hydration

-The application of the Frenkel-Halsey-Hiil (FHH) formalism to the water desorption isotherms obtained for the whole range of the activity of water with the pressure membrane device (0.98 < aw < 1) and with the desiccator (0 < aw < 0.98) gives information concerning the nature and the relative importance of the 2 mechanisms involved in the dehydration-hydration processes: adsorption and capillary condensation. The state and location of water are described in each domain. An equation that gives the thickness t of the film of water adsorbed on the walls of pores versus the activity of water is developed. This t-curve is used to get, from the desorption isotherm, the pore size distribution curve of the studied hydrated materials. Then concepts of surface and fabric of clay pastes are discussed as a function of hydration and a mechanism is proposed to explain swelling and shrinkage of finely divided materials. Three kinds of surfaces, related to the aggregate fabric, are defined as a function of their capacity to adsorb water. Each kind of surface is determined by a specific technique: the total surface area (St) by ethylene glycol adsorption, the external surface area of particles (S~) by nitrogen adsorption and the external surface area of aggregates (Se) by hydraulic conductivity measurements. As a consequence it is only with completely dispersed clays that swelling is a function of St. With unwell-dispersed clays, water adsorption, which induces swelling, successively occurs on St, S s and Se surfaces. Key W o r d s ~ l a y Paste Fabric, Hydration, State and Location of Water, Swelling and Shrinkage Mechanisms, t-Curve. I N T R O D U C T I O N The unique propert ies of clays mixed with water have permi t ted their use s ince the ancient t imes to make domes t ic objects and beautiful pottery. Al though the phys icochemica l proper t ies of clays as a funct ion of water content have been known for a long t ime, it is a mat ter o f fact that the mechan i sms involved in their hydrat ion, swell ing and shrinkage are still not comple te ly unders tood. The aim of this paper is to analyze data obta ined by the senior author and his coworkers for a bet ter unders tanding o f the state and location o f water retained by clay materials. The states of water are character ized by the m e c h a n i s m (adsorption or capi l lary condensat ion) involved in the water re tent ion phenomenon , and by the wa t e r ca t i on -c l ay structure interact ions that de termine the ar rangement of water molecules with respect to solid surfaces, exchangeable cat ions and other water molecules . The locat ion of water refers to sites or surfaces where water molecules are adsorbed and pores where water is condensed. Determina t ion o f the affinity of clays for water will be d iscussed in the first part. At tent ion will be focused on the me thod deve loped to obtain a water desorpt ion i so therm for the whole range of the activity o f water (0 < aw < 1). In the second part, we will show how the applicat ion of the FHH formal i sm to the water desorpt ion i so therm gives informat ion concern ing the nature and the relative impor tance of the 2 mechan i sms involved in the dehydra t ion-hydra t ion process: adsorpt ion and capi l lary condensat ion. In the third part, the state and location o f water will be descr ibed in each domain where 1 retent ion m e c h a n i s m is p redominant. In the fourth part, the concepts of surface and fabric o f clay will be d iscussed as a funct ion of hydrat ion and a m e c h a n i s m will be p roposed to explain the swel l ing and shr inkage phenomena . Also, an overv iew will be presen ted concern ing the effect o f the structure of adsorbed water on the hydrat ion process and, as a consequence , on the swel l ing p h e n o m e n o n of clay materials. As oxides are c o m m o n l y found in soils, they will be also taken into a c c o u n t . C A R R Y I N G O U T OF W A T E R S O R P T I O N I S O T H E R M S F O R 0 < aw < 1 The water sorpt ion i so therms give, for a given temperature, the amount of water retained by clay materials as a funct ion of the activity of wa te r (aw). It is not poss ible to obtain a water sorpt ion i so therm for 0 < aw < 1 per forming only 1 exper iment . Two me thods are needed according to the range of aw considered. For reasons o f e c o n o m y o f space only typical data will be repor ted here. The reader may refer for more details to another paper by Prost (1990). Carry ing out Water Sorpt ion Iso therms for 0 < aw < 0.98 Water sorpt ion is general ly pe r fo rmed in a desiccator where aw is control led by mixtures o f H2SO4 + Copyright 9 1998, The Clay Minerals Society 117 1 18 Prost, Koutit, Benchara and Huard Clays and Clay Minerals