On the relationship between fabric and hydraulic conductivity of Enhanced Bentonites

Available experimental evidence on the permeability of enhanced bentonites, which comprise natural smectitic clays amended with polymers or organic compounds, has been critically interpreted through a theoretical framework based on the use of a modified form of the Kozeny-Carman equation, to relate the measured hydraulic conductivity to the clay fabric. Based on this interpretation, the average number of lamellae per tactoid, Nl,AV, which reflects the clay fabric via the separation of the pore space between conductive and non-accessible interlayer pores, has been determined for four enhanced bentonites, viz., Multiswellable Bentonites (MSBs), Dense Prehydrated GCLs (DPH-GCLs), HYPER Clays (HCs), and Bentonite Polymer Composites (BPCs). The variation in the so determined Nl,AV values due to a change in the ionic strength of the permeant solution has then been characterised. While intergranular pore clogging by sodium polyacrylate has been shown to be the primary factor controlling the conductive porosity of BPC upon exposure to concentrated electrolyte solutions, preservation of a dispersed clay fabric via intercalation of sodium carboxymethyl cellulose (Na-CMC) between the montmorillonite unit layers has been shown to affect the behaviour of DPH-GCLs. As with BPC, the calculated values of Nl,AV for MSB have consistently been found to be greater than the expected values for natural (unamended) bentonites. However, in the case of MSBs, the decrease in the conductive porosity can be ascribed to the greater compressibility of the solid skeleton, which results from mixing with propylene carbonate.