Five-year thermo-hydro-mechanical and chemical evolution of compacted bentonite: Physical and mineralogical analysis

To help describe and understand the mechanisms and factors governing the geochemical evolution and mineralogical alteration of compacted bentonite under hydraulic and thermal gradients similar to those experienced by engineered barriers in radioactive waste repositories, an experimental and modelling programme was carried out. Six thermo-hydraulic tests were performed in instrumented cylindrical cells (10 × 10 cm) using compacted Wyoming-type bentonite. The bottom of the cells was initially heated at 90 °C and once the relative humidity inside the bentonite stabilised, hydration of the bentonite was started through the top surface and lasted for different periods of time (1, 2.5 and 5 years of hydration). The top surface was maintained at 20 °C during the whole test duration to induce a thermal gradient. Half of the cells were hydrated with water reproducing saline groundwater and the other half with synthetic dilute glacial water. The temperature of the heater was increased to 110 °C after ∼1 year of hydration. At termination, the bentonite blocks were extracted from the cells and cut into sections to obtain samples for different postmortem determinations. The thermo-hydraulic evolution during the TH tests, final state of the bentonite and its mineralogical and geochemical changes were simulated with a thermo-hydro-chemical (THC) model. After 2.5 years of hydration, full saturation and hydraulic equilibrium had been reached in the cells saturated with glacial water, while the saturation process was somewhat slower when saline water was used. Despite the high degree of water saturation, gradients of water content and dry density along the blocks were observed in all the tests, which is interpreted as a consequence of the irreversibility of the initial swelling deformation. Overall, the bentonite experienced minimal mineralogical changes after the tests, especially when glacial water was used, and the changes were mainly limited to the heater contact and, to a lesser extent, the hydration surface. Precipitation of halite close to the heater was relevant in the tests hydrated with saline water, whereas the amount of gypsum close to the heater decreased in favour of that of anhydrite, irrespective of the kind of hydration water. The montmorillonite layer charge slightly decreased after the TH treatment. This is considered an indication that only minor, non-relevant modifications in the montmorillonite crystal-chemistry took place during the 5-year operation. Comparison between the experimental measurements and modelling results showed an overall good agreement. A companion paper details the geochemical and transport processes experimentally observed and simulated with the THC model. • Thermo-hydraulic conditions of compacted bentonite in radioactive waste repository reproduced. • Effect of salinity, hydration time on hydro-mechanical, mineralogical evolution of bentonite analysed. • Water content and dry density gradients persisted even after full saturation. • Minimal changes in mineralogy, mainly at heater contact and hydration surface. • No significant modifications in montmorillonite crystal-chemistry, but decrease of layer charge.