Homogenization of a granular bentonite material upon saturation: an analysis based on pore structure evolutions

Granular bentonite has been considered as candidate sealing material of various technological gaps in deep geological repository of high-level radioactive waste (HLW). Large inter-pellet pores inside the granular bentonite potentially work as preferential migration pathways of water, gas and radionuclides. During the long-term operation process, the inter-pellet pores will be filled by swollen pellets with the infiltration of groundwater, but it is still unclear how the inter-pellet pores evolve upon saturation and whether the mixture can reach a full homogenization. In this study, a series of constant-volume hydration tests and mercury intrusion porosimetry (MIP) tests were performed on bentonite pellet mixture and compacted bentonite powder. Results revealed that the development curve of swelling pressure for bentonite pellet mixture exhibited a double-peak shape. The first peak value of the pellet mixture was lower than that of the compacted powder at a given dry density, but their final swelling pressures were similar. At the initial hydration stage, the water content, dry density and degree of saturation profiles along the sample height showed obvious gradients, resulting in an apparent heterogeneity of the pore structures. As hydration proceeded, the gradients in the water content and dry density profiles tended to decrease. Simultaneously, the pellet mixture gradually evolved from a trimodal porosity to a bimodal one. The pellet mixture and the compacted powder both exhibited a similar bimodal porosity at nearly saturated state. The homogenization process of the pellet mixture was quantitatively analyzed by a heterogeneity coefficient related to microstructural evolutions.