Effects of heat and humidity gradients on MX-80 bentonite geochemistry and mineralogy

Abstract To simulate the conditions of a sealing material in a nuclear waste repository a 20-cm long column of MX-80 bentonite compacted at a nominal dry density of 1.7 g/cm 3 with a water content of 17% was tested in a thermo-hydraulic cell. On top of the column a hydration surface simulated the groundwater coming from the host rock and at the bottom a heater simulated the waste canister. The test comprised two phases: a heating phase (lasting 324 days) and a ‘heating + hydration’ phase (1510 days). The temperatures at the ends of the column were set during the last phase to 30 °C and 140 °C. The thermo-hydraulic treatment resulted in major changes related to the location of the bentonite along the column. These changes led to important gradients along the column with respect to the physical state (water content, dry density) and geochemistry of the bentonite. Due to the dissolution of low-solubility species and to the loss of exchangeable positions in the smectite, the content of soluble salts in the pore water increased with respect to the original one. The solubilised ions were transported; sodium, calcium, magnesium and sulphate having a similar mobility, which was in turn lower than that of potassium and chloride. The cationic exchange complex was also modified. Smectite dissolution processes (probably colloidal) occurred, particularly in the more hydrated areas. Consequently, an overall decrease of the smectite content with respect to the initial value took place, especially in the most hydrated areas, where the percentage of interestratified illite also increased. On the other hand, the content of cristobalite and calcite increased.