Temperature and alkaline solution effects on the hydro-mechanical behaviours of GMZ bentonite pellet mixtures

Bentonite pellet mixture emplaced in a high-level radioactive waste (HLW) repository may suffer decay heat released by nuclides and alkaline solutions produced by concrete degradation, leading to complex hydro-mechanical behaviours different to those of compacted bentonite blocks. In this paper, swelling pressure and hydraulic conductivity tests were conducted on bentonite pellet mixtures infiltrated with distilled water and NaOH solutions (0.1, 0.5 and 1.0 M) at different temperatures (20, 40 and 60 °C). At selected hydration times, water contents, dry densities, pore size distributions of lower, middle and upper layers of the specimens were measured. Mineralogies and microfabrics of the bottom part of the specimens were also studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed that with increasing NaOH concentration (0–0.5 M) and temperature, more montmorillonites were dissolved and more large pores or even cracks generated in the specimen, leading to deteriorated swelling pressure and increased hydraulic conductivity. As the NaOH concentration further increased to 1.0 M, large pores and cracks were clogged by precipitates generated during dissolution of montmorillonite, resulting in decreased hydraulic conductivity. The migration of water/solution was significantly retarded by combinedly increasing NaOH concentration and temperature, due to the reduced adsorption and capillary ability of the bentonite, pore clogging near the specimen bottom as well as chemical consumption of pore water during the dissolution of montmorillonite and other minerals.