Mechanical effects of aqueous solutions of inorganic acids and bases on a natural active clay

The mechanical properties of clays are strongly affected by the pore fluid's chemical composition. Experimental and theoretical analyses have investigated the mechanical effects of exposing clays to aqueous solutions with different salt concentrations and to organic solvents, highlighting the different role of the various cations. With the exception of clay suspensions, no attention has been paid to the effects of pore fluid pH variations on the mechanical behaviour of clays, although clay may be exposed to inorganic acids or bases in many environmental conditions and man-induced situations. The aim of this paper is to investigate how the mechanical behaviour of active clays is affected by pore fluid acidity and alkalinity. Only aqueous solutions of inorganic acids and bases are considered in this study; the rheology of clay suspensions is not investigated. The tests were performed on a natural Na-bentonite, and focused on the influence of pore fluid acidity/alkalinity on the liquid limit, one-dimensional compressibility, permeability and residual strength. Any clay lattice dissolution was found to have negligible mechanical effects. All mechanical effects were reversible, provided the clay was subsequently exposed to an appropriate cycle of acid or alkaline solutions. H⁺ protons revealed a strong aptitude for adsorption even at low concentrations, causing the desorption of previously adsorbed cations, large volume compression, greater stiffness and residual shear strength. In contrast, exposure to NaOH produced swelling at low concentrations and compression at high concentrations. As a result, the mechanical properties of active clay are strongly affected by even relatively small variations in pore fluid pH. The experimental results are consistent with the concepts of cation exchange on permanently charged surface sites and acid–base reactions on variably charged sites (mainly located at clay edges), and with the assumption of a variable diffuse double layer thickness depending on pore fluid ionic strength. Appropriate sequences of chemical exposures highlight the significant effects of acid–base reactions at particle edges with respect to cation exchange.