Hydraulic conductivity and microstructure changes of compacted bentonite/sand mixture during hydration

Compacted bentonite-based materials are often considered as sealing/backfill materials in deep geological repository for high level radioactive waste. A good understanding of their hydration process is essential as this process is directly related to over-pack corrosion and nuclide migration. In this study, the unsaturated hydraulic properties of MX80 bentonite/sand mixture were characterized by carrying out a series of experiments including water retention test, infiltration test as well as microstructure observation. It was found that with suction decrease under constant volume condition, the hydraulic conductivity decreased followed by an increase after a suction threshold. At suctions higher than 12.6 MPa, hydration led to progressive large-pore clogging by exfoliation of clay particles. On the contrary, when saturation was approached (suction lower than 4.2 MPa), the large-pore quantity increased due to the creation of two-dimensional pores. It was also observed that the soil hydraulic conductivity changed following the same tendency as the large-pore quantity during hydration. In other words, water transfer was primarily governed by the network of large-pores.