Sensitivity of bentonite swelling to pore water activity

Abstract

The deformability of bentonites is strongly conditioned by their salinity conditions. Consequently, the chemomechanical coupling must be taken into account to characterise correctly the deformational behaviour of these materials. Some models have been developed that account for this coupling. These models, which usually assume a double-porosity approach to characterise the structure of compacted bentonite, idealise the geochemical system, commonly using a simplified definition of the activity of the species dissolved in the aqueous solutions present in the bentonite pores. In this paper, the significance of the formulation used to calculate the activities is analysed. As a sensitivity exercise, the extended Debye–Hückel model is applied to estimate the activity coefficients of the species in the macrostructure. The exercise is conducted by simulating vertical free-swelling tests under different salinity conditions. It is shown that when the salinity conditions are not negligible, the results are improved using this illustrative model compared with the case in which the activity of the species was assumed equal to their concentration. Although the formulation of the activity coefficients could be improved for higher-salinity conditions, this study highlights the sensitivity of the bentonite mechanical behaviour to the activity definition.