Abstract: In China, Gaomiaozi (GMZ) bentonite has been chosen as a potential buffer/backfill material for deeply buried geological disposal system to isolate the high-level radioactive wastes (HLW). A series of laboratory tests (including swelling pressure test, swelling deformation test and compression test) on two GMZ Na-bentonites (called as GMZ001 and GMZ07) is performed at different initial dry densities. The test results show that GMZ001 has larger swelling pressure than GMZ07 at the same void ratio and is harder to be compressed than GMZ07. On the other hand, the microstructural features of two GMZ Na-bentonites in saturated state are investigated by combining the mercury intrusion porosimetry (MIP) tests and scanning electronic microscope (SEM) observations. The test results reveal that GMZ07 has more inter-aggregate pores than GMZ001 at the same void ratio, and the aggregates of GMZ001 is characterized by more strong hydration compared to GMZ07. The difference in mechanical behavior of two GMZ Na-bentonites is due to differences in the grain size and montmorillonite content. Furthermore, based on the concept of the montmorillonite void ratio, the results of swelling test on two GMZ bentonites under fully saturated condition can be expressed by a unique line, and the predictions of the swelling pressure and deformation are conducted for two bentonites.

Key words: Gaomiaozi Na-bentonite, unsaturated soil, swelling, microstructure, montmorillonite void ratio