Geochemical and mineralogical changes in compacted MX-80 bentonite submitted to heat and water gradients

Abstract

A 20-cm high column of MX-80 bentonite compacted at dry density 1.72 g/cm 3 with an initial water content of 16% was submitted to heating and hydration by opposite ends for 496 days (TH test). The temperature at the bottom of the column was set at 140 °C and on top at 30 °C, and deionised water was injected on top at a pressure of 0.01 MPa. Upon dismantling, water content, dry density, mineralogy, specific surface area, cationic exchange capacity, content of exchangeable cations, and concentration of soluble salts and pH of aqueous extracts were determined in different positions along the bentonite column. In addition, the pore water composition was modelled with a geochemical software. The test tried to simulate the conditions of an engineered barrier in a deep geological repository for high-level radioactive waste. The water intake and distribution of water content and dry density along the bentonite were conditioned by the thermal gradient. Liquid water did not penetrate into the column beyond the area in which the temperature was higher than 100 °C. A convection cell was formed above this area, and liquid water loaded with ions and moving by advection evaporated towards cooler bentonite as it reached the area where the temperature was too high. In this area the precipitation of mineral phases took place. Advection, interlayer exchange and dissolution/precipitation processes conditioned the composition of the pore water along the column. In most of the column the pore water was Na–SO 4 2−, and changed to Na–Cl type near the heater. The overall changes in cation content of the pore water could be explained by changes in the smectite interlayer and mineral phases equilibrium. The TH treatment did not cause significant alteration of the smectite or the other mineral phases of the bentonite.