EXPOSURE OF BENTONITE TO SALT SOLUTION - OSMOTIC AND MECHANICAL EFFECTS

The mechanical behaviour of clays is strongly influenced by pore fluid composition. Moreover, exposure to fluids different from their pore f luid may produce a transient stage during which the clays undergo noti ceable volume changes. The aim of this paper is to investigate these p henomena and their causes. To this end, a large number of water-satura ted specimens of Ponza bentonite (mainly composed of Namontmorillonite ) were exposed, alternately, to distilled water and to saturated NaCl, KCl or CaCl2 solutions in the course of laboratory direct shear tests and oedometer tests. Exposure to any of the three electrolytes produc ed consolidation of the specimens, a large decrease in deformability a nd an equally large increase in residual shear strength. At equilibriu m, the mechanical behaviour became very similar to that exhibited by s pecimens that had been directly prepared with the appropriate salt sol ution as pore fluid. For both types of specimen (i.e. those prepared w ith the electrolyte and those exposed to it) NaCl effects were reversi ble when the specimens were re-exposed to water, while KCl and CaCl2 e ffects persisted even after some months of continuous testing. The exp erimental results were consistent with the hypothesis that changes in the thickness of the diffuse double layer were produced by ions diffus ing into or out of the clay. X-ray diffraction analysis showed that, i n the cases of KCl and CaCl2, the inward diffusion had produced ion ex change. Hence, the decrease of the diffuse double layer was permanent, under the given experimental conditions, and it rendered subsequent o smotic phenomena practically negligible.