Effect of Polymer Amendment on Hydraulic Conductivity of Bentonite in Calcium Chloride Solutions

Abstract

Hydraulic conductivity of polyanionic cellulose (PAC)–amended bentonite (PB) in calcium chloride (CaCl2) solutions was investigated to access its chemical compatibility in vertical cutoff walls application. PB was synthesized by mixing conventional bentonite (CB) powder with PAC (2% dry weight). The specific gravity (Gs), liquid limit (wL), pH, swell index (SI), and cation exchange capacity (CEC) of CB and PB were measured, and hydraulic conductivities and microstructures of the PB and CB filter cakes were evaluated by performing modified fluid loss (MFL) tests and scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS) analyses, respectively. The results showed that PB had higher wL, SI, and CEC but lower Gs and pH than CB; in particular, PB possessed higher SI in CaCl2 solution as compared to CB. Increase in the applied overall pressure and decrease in the CaCl2 concentration resulted in a decreased hydraulic conductivity for both PB and CB. However, the hydraulic conductivity of PB was found to be one to two orders of magnitude lower than that of CB when exposed to the same CaCl2 solutions, indicating superior chemical compatibility of PB. SEM-EDS image analyses demonstrated that polymer formed a three-dimensional net structure between bentonite particles, which could clog the intergranular pore space, resulting in a narrow and tortuous flow path for liquid and low hydraulic conductivity.