Pore-scale mechanisms underlying the behavior of enhanced bentonites exposed to aggressive inorganic solutions

Understanding the mechanisms that control the hydraulic and semipermeable membrane behavior of enhanced bentonites (EBs), which comprise natural montmorillonite amended with polymers or organic compounds, is important for assessing the long-term performance of engineered barriers manufactured from these materials. Accordingly, the available experimental evidence for the hydraulic and semipermeable membrane behavior of EBs was critically interpreted through a theoretical framework, which allows the macroscopic transport and swelling properties of chemically active clays to be related to a limited number of intrinsic, state, and fabric parameters. Four commonly evaluated EBs were interpreted, namely, Multiswellable Bentonites (MSBs), Dense Prehydrated GCLs (DPH-GCLs), HYPER Clays (HCs), and Bentonite Polymer Composites (BPCs). Osmotic swelling, which is the primary mechanism for significant swelling and low hydraulic conductivity of unenhanced (natural) sodium bentonite, is not significantly influenced by polymer amendment. The primary mechanism controlling the conductive porosity and the flow path tortuosity upon permeation of BPCs with concentrated electrolyte solutions is intergranular pore clogging by sodium polyacrylate, whereas the mechanism for the same behavior of DPH-GCLs and, probably to a lesser extent, of HCs is preservation of a dispersed clay fabric via intercalation of sodium carboxymethyl cellulose between the montmorillonite unit layers. Similar to BPCs, direct exposure of MSBs to liquids with aggressive chemistries induces the fabric to flocculate to a greater extent than that of prehydrated natural bentonites. However, unlike BPCs, the decrease in the conductive porosity of MSBs is due to a greater compressibility of the solid skeleton rather than to a pore-clogging mechanism.