Coupling X-ray CT and MIP to Assess Freeze-Drying Effects on the Structural Arrangement of Granular Bentonite

This study examines the impact of freeze-drying on the microstructure of compacted granular bentonite (GB) before mercury intrusion porosimetry (MIP) analysis. Two states were evaluated: the as- compacted samples (dry density of 1.55 Mg/m³ and 20% water content), with initial matric suction above the air-entry value (AEV), and saturated samples, where matric suction developed during water-undrained unloading but remained below the AEV. In the as-compacted state, freeze-drying had a negligible effect on the pore structure. In contrast, it caused the expansion of macropores in the saturated samples due to the instantaneous removal of matric suction. This process generated fissures that unexpectedly resembled the initial granular fabric, indicating that saturation did not fully erase the original structure. X-ray micro- computed tomography (CT) was used to quantify these effects and to segment the material into structural components, including low-density clay gel, aggregates/granules, and macropores. A correction procedure was introduced to compensate for the expansion observed in freeze-dried saturated samples, enabling the estimation of porosity and dry density. CT-derived values closely matched laboratory measurements when this correction was applied. CT imaging thus proves essential for validating and interpreting MIP results and characterising the evolving microstructure of compacted GB with component-level resolution.