Hydraulic Conductivity of Bentonite–Polymer Composite Geosynthetic Clay Liners Permeated with Coal Combustion Product Leachates

Experiments were conducted to evaluate the hydraulic conductivity of geosynthetic clay liners (GCLs) containing bentonite–polymer composites (BPCs) permeated with coal combustion product (CCP) leachates. Eight synthetic CCP leachates were used: the five characteristic CCP leachates from a leachate database and three based on actual leachate chemistries. Hydraulic conductivity tests were conducted on non-prehydrated and prehydrated BPC GCL specimens at an effective stress of 20 kPa. Prehydration was achieved by contact with a subgrade for 60 days or by permeation with deionized (DI) water. Comparisons are made to the hydraulic conductivity and swell index of conventional sodium bentonite (NaB) GCLs permeated with the same leachates. The hydraulic conductivity of BPC GCLs with low polymer loading ($<1.9\%$) to the CCP leachates is similar to that of NaB GCLs, with hydraulic conductivity related inversely to swell index and directly to ionic strength. For higher polymer loading ($\ge 1.9\%$), the hydraulic conductivity of BPC GCLs appears to be controlled by the polymer hydrogel and is not related directly to the swell index of the BPC or ionic strength of the leachate. Higher hydraulic conductivity of BPC GCLs is associated with greater polymer elution and nonuniformity of the polymer within the GCL. Subgrade hydration had a modest beneficial effect on the hydraulic conductivity of a BPC GCL with low polymer loading ($<1.9\%$). Prehydration by permeation with DI water resulted in low hydraulic conductivity ($4.3\times {10}^{-11}\mathrm{m}/\mathrm{s}$) for this same GCL with one of the strongest leachates.