STATE AND LOCATION OF WATER ADSORBED ON CLAY-MINERALS - CONSEQUENCES OF THE HYDRATION AND SWELLING-SHRINKAGE PHENOMENA

The application of the Frenkel-Halsey-Hill (FHH) :formalism to the wat er desorption isotherms obtained for the whole range of the activity o f water with the pressure membrane device (0.98 < a(w) < 1) and with t he desiccator (0 < a(w) < U.98) gives information concerning the natur e and the relative importance of the 2 mechanisms involved in the dehy dration-hydration processes: adsorption and capillary condensation. Th e state and location of water are described in each domain. An equatio n that gives the thickness t of the film of water adsorbed on the wall s of ports 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 m echanism is proposed to explain swelling and shrinkage of finely divid ed materials. Three kinds of surfaces, related to the aggregate fabric , are defined as a function of their capacity to adsorb water. Each ki nd of surface is determined by a specific technique: the total surface area (S-t) by ethylene glycol adsorption, the external surface area o f particles (S-e) by nitrogen adsorption and the external surface area of aggregates (S-s) by hydraulic conductivity measurements. As a cons equence it is only with completely dispersed clays that swelling is a function of S-t. With unwell-dispersed clays, water adsorption, which induces swelling, successively occurs on S-t, S-s and S-e surfaces.