Time- and density-dependent microstructure features of compacted bentonite

Pre-compacted bentonite bricks are often considered as sealing/backfill elements in deep geological repositories for high level radioactive waste. A good understanding of their microstructure changes upon hydration is essential as the microstructure changes are directly related to the macroscopic hydro-mechanical behaviour. In this study, the microstructure features of the compacted MX80 bentonite used as a sealing material in a field experiment were characterized by means of both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Emphasis was put on the effects of final dry density (density after swelling) and hydration time. The results obtained show that the changes in soil porosity upon swelling are mainly due to the increase in large-pores of about 50 µm diameter and medium-pores of 1 µm diameter. In addition, the microstructure changed over time due to the water re-distribution that occurred among each level of pores: the volume of both the large-pores and small-pores decreased along as the volume of the medium-pores increased. A uniform microstructure can be then expected in the long term. Furthermore, it was observed that the higher the final dry density, the slower the microstructure changes.