pH and time dependent microstructure features of compacted bentonite-sand mixture

Compacted bentonite-sand mixture is considered as a potential sealing material for deep geological disposal of high-level radioactive waste. In the long run, the intruding alkaline plumes, as a result of concrete degradation, can alter the microstructure of the mixture, potentially affecting the sealing performance. In this study, Compacted bentonite-sand (40/60) mixture samples were hydrated by deionised water and alkaline solutions with varying pHs (8.8, 11.5, 13.5) for different durations under constant-volume conditions. The resulting microstructural changes were subsequently investigated by Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscope (SEM). Results showed that after hydration for one week, for the sample hydrated with deionised water, a tri-modal porosity was observed, defining different populations for intra-aggregate pores, inter-particle pores, and inter-aggregate pores. With the increase of pH, the intruded void ratio was significantly reduced, mainly due to the shrinkage of the diffuse double layer. The uniformity increased for the intra-aggregate pores while decreased for the meso and macro pores (inter-particle and inter-aggregate pores). Additionally, in the case of pH = 13.5, the intruded void ratio of the samples decreased over time, with further increase of void ratio and pore uniformity for the intra-aggregate pores and the decrease of size for the macropore population. These phenomena suggest that the alkaline attack resulted in montmorillonite dissolution, further modifying the macro-pores. This is confirmed by the SEM observations.