About the state parameters of active clays

The common use of active clays, such as bentonites, as hydraulic and contaminant barriers for landfill and soil remediation applications, including the final disposal of nuclear waste, needs to be supported by adequate theoretical models that take into account the physical, chemical and mechanical coupled phenomena. The elementary particle (platelet) of a bentonite is characterized by a high total specific surface (S) and a permanent or fixed negative electric surface charge (sigma) that interacts with the ions in solution at different concentration (cs) in the pore water. This interaction is the primary factor that determines the final fabric of the solid skeleton and, consequently, most of the bentonite properties in terms of both mechanical behavior and solute transport. The tactoid formation or flocculation is the primary phenomenon that influences bentonite behavior and can be quantified with appropriate state parameters, such as the effective specific surface, Seff, the average number of platelets per tactoid, Nl,AV, and the electric fixed-charge concentration of the solid skeleton, csk,0. On the basis of the aforementioned state parameters, a theoretical hydro-chemico-mechanical framework has been developed. The validity of this framework has been tested, within the present paper, by comparison of its predictions with some of the available experimental results on bentonites with more than 70% montmorillonite content, in terms of hydraulic conductivity, swelling pressure and osmotic efficiency versus void ratio and ion concentrations of the pore solution.