Water infiltration and swelling pressure development in GMZ bentonite pellet mixtures with consideration of temperature effects

Abstract

Bentonite pellet mixture (BPM) is considered as a potential buffer/backfill material for geological repository of radioactive waste. Once emplaced in a repository, the initially discrete BPM with heterogeneous porosity would be subjected to groundwater infiltrated from surrounding rock and decay heat released from nuclides, leading to temperature depended processes of water infiltration and swelling pressure development. In this study, Gaomiaozi (GMZ) BPMs were hydrated with distilled water under constant volume condition at temperatures 20, 40, 60 and 80 °C. The water content profiles, dry density profiles and swelling pressure were periodically measured and recorded, respectively. Results indicate that, both water infiltration and swelling pressure development were accelerated by increasing temperature. Due to local swelling formation that brining water ahead and prevailing vapour diffusion through the large interconnected inter-pellet voids, the water infiltration through the BPM was anomalous super-diffusion at non-Boltzmann scale. Accordingly, a non-Boltzmann variable based water infiltration model was developed and verified by the experimental data. The non-Boltzmann variable was inversely related to water content, dry density and montmorillonite content, whereas positively related to temperature. It was proved to be able to qualitatively reflect the mobility of water for different unsaturated soils with different water contents. In addition, a comprehensive explanation of temperature effects on swelling pressure was put forward, stating that the variation of swelling pressure with temperature depended on the competitions among the thermal-induced responses of hydration pressure, water pressure and osmotic pressure. The competitions could be controlled by the clay mineralogy, pore water properties, electrolyte properties and the clay-water-electrolyte interactions at micro scale, and by dry density, thermal mode and drainage condition at macro scale.