Abstract：Deep landfill of nuclear waste has become a key issue for the safe use of nuclear energy in the world. As a key sealing material for separating the nuclear waste from outside world at a deep underground point，the properties of swelling clay have attracted more and more attentions from geotechnical engineers. The volume change characteristics of densely compacted expansive bentonite under different hydration controlling suctions are studied by using mercury intrusion porosimetry(MIP) and scanning electron microscope(SEM) techniques in a comparison way. The osmotic technique was used for controlling suction. Tests on different densities of MX80 clay were carried out. The achieved results show that：(1) swelling potential of compacted bentonite during hydration depends on the initial compaction density；(2) the denser the clay is compacted，the higher the potential of expansion is；and (3) the volumetric swelling of the compacted clay is mainly derived from the expansion of the pores inter-aggregations.
