Chemico-osmotic transport properties and mechanical behaviour of a natural sodium bentonite

The relevance of the semipermeable properties of bentonites, which affect both their transport processes and mechanical behaviour, was assessed through the determination of two parameters: the reflection coefficient, w, and the swell coefficient, w1. Laboratory tests were conducted, using a new apparatus, with the aim of measuring w and w1 of a natural sodium bentonite, while varying both the bentonite void ratio and the solute concentration of the sodium chloride (NaCl) solutions in contact with the bentonite specimens. The obtained results show that both parameters can be interpreted through a mechanistic model, in which the electric charge of clay particles is taken into account via a single parameter, csk0, that is referred to as the “solid skeleton electric charge concentration”. A constant value of csk,0 = 110 mM was found to provide an accurate interpretation of all the experimental data, at least within the investigated range of bentonite void ratios (e varying from 3.33 to 4.18) and NaCl concentrations of the external bulk solutions (cs varying from 5 mM to 90 mM). The obtained 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.