Experimental study on shrinkage characteristics of bentonite under different initial dry densities and water contents

Bentonite serves as a buffer/backfill material in deep geological repositories for high-level nuclear waste. From compaction to on-site installation, bentonite blocks undergo drying-induced shrinkage that can compromise the engineering barrier's mechanical integrity and radionuclide containment. In this paper, initially unsaturated bentonite is taken as the research object, and the indoor constant temperature drying test is carried out. The effects of initial dry density and initial water content on water evaporation, shrinkage deformation, and fracture evolution of bentonite are studied, respectively. The experimental results show that the initial dry density and initial water content have a significant effect on the water evaporation, shrinkage deformation, and crack evolution of bentonite. The larger the initial dry density or the smaller the initial water content, the less the water loss of bentonite in the evaporation process, the smaller the shrinkage deformation, and the fewer the number of cracks on the surface of bentonite. The shrinkage geometric factor can quantitatively describe the proportion of axial strain in the total volumetric strain. After drying, the shrinkage geometric factors of all compacted bentonite samples are between 1 and 3, which indicates that the axial strain accounts for a large proportion of the total volumetric strain, and the shrinkage deformation exhibits anisotropy. The effects of initial dry density and initial water content on water evaporation, shrinkage deformation, and fracture evolution of bentonite are closely related to their effects on inter-aggregate pores.