Experimental and modeling study on long-term alteration of compacted bentonite with alkaline groundwater

The experimental and modeling studies were conducted on alteration of bentonite buffer materials. The dissolution rate of montmorillonite in, the diffusivity of hydroxide ions in and the hydraulic conductivity of compacted sand–bentonite mixtures were experimentally determined and formulated. Dissolution of montmorillonite followed the linear dependence on time under the employed experimental conditions of aOH-aOH- (activity of hydroxide ions) of 0.04–0.57 mol dm−3 and temperatures of 50–170 °C. The slope gave the dissolution rate, RA (Mg m−3 s−1). The dissolution rate was a function of pH and temperature, T   (K), and expressed as RA=3.5(aOH-)1.4exp(-51000/RT), where R is the gas constant. The effective diffusivity, De (m2 s−1), of hydroxide ions was found to be in the order of 10−10–10−11 m2 s−1 at 10–90 °C and expressed as De = 5.0 × 10−7ε2.1 exp(−18 600/RT) where ε is the porosity. The dependence of hydraulic conductivity, K (m s−1), of the sand–bentonite mixtures on the effective montmorillonite dry density, ρmont (Mg m−3), and on the ionic strength of the permeant, I (mol dm−3), was identified as K = 1.2 × 10−7I1.510−4.2ρmont.A PHREEQC-based, coupled mass-transport/chemical-reaction code (MC-BENT) was developed for predicting hydraulic conductivity of the bentonite buffer by using the formulae for the dissolution rate, the effective diffusivity and the hydraulic conductivity. This code was able to reproduce observed changes in concentrations of major species and montmorillonite contents in the lab-scale experiments on the bentonite alteration, which is indicative of partial verification of our calculation.