Multi-scale insights into gas transport in granular bentonite at partially saturated states

Knowledge gaps in gas transport through partially saturated granular bentonite (GB), characterised by an extended particle size distribution up to several millimetres, limit the long-term safety assessment of engineered barriers. Thus, gas transport in GB samples, starting from as-compacted states and including subsequent loading, progressive saturation and their combination, was investigated through variations in the effective gas permeability (Kg · eff). The microstructure of the samples was characterised using combined mercury intrusion porosimetry and X-ray micro-computed tomography, providing multi-scale insights into Kg · eff in relation to pore size distribution, effective connectivity, density heterogeneity and gas pathway development. A higher as-compacted degree of saturation, mechanical loading and progressive saturation each reduced Kg · eff by promoting the reduction, isolation and closure of inter-granular pores. Gas pressurisation also induced enlargement and reconnection of inter-granular pores in as-compacted states and, after progressive saturation, the formation of highly connected fissure-like features that enhanced gas flow, whereas prior loading constrained these responses. A gas permeability model incorporating granular-type microstructural evolution was developed to capture the main experimental trends. Overall, compared with powdered bentonite, the granular-type microstructure of GB facilitates controlled gas release, supporting its suitability for engineered barrier applications.