Laboratory assessment of semi-permeable properties of a natural sodium bentonite

The relevance of the semipermeable properties of bentonites, which affect both their transport processes and mechanical behaviour, has been assessed through the experimental determination of three parameters: the reflection coefficient, the osmotic effective diffusion coefficient, and the swell coefficient. Two multi-stage tests were conducted on a natural sodium bentonite, while varying both the specimen void ratio, e, and the solute concentration, c_s, of the equilibrium sodium chloride (NaCl) solutions. The measured phenomenological parameters were interpreted through a mechanistic model, in which the electric charge of clay particles is taken into account via a single material parameter, c'_sk0, referred to as the "solid charge coefficient". A constant value of c'_sk0 = 110 mM was found to provide an accurate interpretation of the experimental data, at least within the investigated range of bentonite void ratios (3.33 ≤ e ≤ 4.18) and NaCl concentrations of the external bulk solutions (5 ≤ c_s ≤ 90 mM). The results support the hypothesis that both chemical osmosis and swelling pressure are macroscopic manifestations of the same interactions, which occur at the microscopic scale between the clay particles and the ions contained in the pore solution, and that both of them can be modelled through a single theoretical framework.