Particle size evolution of granular bentonite in hydro-mechanical paths

Abstract

Granular bentonite (GB) is a candidate material for engineered barriers in geological disposal of radioactive waste. Previous studies have focused on the hydro-mechanical (HM) behaviour of GB and the resulting evolution of pore size distribution, whereas this study additionally examines changes in its particle size distribution, which spans from micrometres to several millimetres. During wetting under unstressed conditions for pouring GB, coarse and high-density granules disaggregated, while fine grains aggregated, resulting in larger-sized and lower-density aggregates. Particle swelling upon wetting caused a significant decrease in the dry density of GB on pouring, indicating that wetter GB required greater compaction energy to achieve a specified dry density. Changes in particle size distribution after pouring and compaction also impacted the microstructure of the samples, directly influencing their subsequent HM behaviour, which was examined through the particle size evolution after different loading and wetting paths. The initial water content conditioned granule behaviour and its breakage upon loading. At low water content, stiff granules prompted breakage, thereby increasing sample compressibility. Conversely, aggregate sticking during loading at elevated water content protected the soft granules from breakage and reduced sample compression. The aggregation and expansion of aggregates dominated the HM response to further wetting, contributing to the swelling of samples even under high stress. These particle-scale insights into the evolution of the material's initial conditions and their influence on microstructural and HM behaviour are expected to help in guiding the evaluation of GB barriers' HM stability and permeability during service.