Abstract: An indoor permeability test was conducted to study the sealing performance of joints between the compacted blocks which were backfilled with bentonite powder (P), bentonite particle-powder mixture (P/P) and bentonite slurry (S), respectively. The swelling stress and hydraulic conductivity were monitored during the permeability test. After the test, the homogenization of blocks and sealing degree of joints were evaluated according to the X-CT scanning test, thermal conductivity test and microstructure test. The results show that: with the increase of hydration time, the bentonite in the core area of the block intrudes into the pores of the joint, and the increase rate of swelling stress in the joint zone is higher than that in the axial direction of the block, which leads to the slowing down of the development speed of the axial swelling stress of the block and produces hysteresis. The joint sealing materials have significant influence on the hydration process of the block system, resulting different swelling stress development rates. The sealing degree of the joint is positively correlated with the initial dry density of the joint filling material. The X-CT scanning images show that the sealing joint and the block have been "welded" together after hydration. By contrast, there is still a gap between the blocks with a blank joint, showing a lower sealing degree. The MIP and SEM tests demonstrate that the pore size distribution of inter-aggregates in the joint area lies in the proportion between 3 μm and 30 μm, and a certain number of large pores with a diameter between 100 μm and 500 μm were observed in the joints sealed with bentonite slurry. Some fissure-like 2-dimensional pores appeared in the blank joint according to the SEM picture. Generally, sealing can significantly improve the expansion ability and homogeneity of the block system, and the impermeability and thermal conductivity of the healing block system are close to that of the intact sample. Joint backfilled with bentonite particle-powder mixture shows a better sealing performance, followed by the joint sealed with bentonite powder and bentonite slurry, and the sealing effect of joint without backfilling is the worst.

Key words: high-level radioactive waste, bentonite block, joint, sealing materials, swelling pressure, permeability