Swelling behavior is an important property in achieving the low-permeability sealing function of buffer material. In this study, free swelling tests were conducted on a potential buffer material under various near field environmental conditions. The potential buffer material used is a locally available clayey material known as Zhisin clay. Experimental results show that Zhisin clay is a temperature-sensitive Ca-bentonite. The maximum swelling strain decreases with increasing hydrothermal temperatures due to lattice contraction. Dry heating of Zhisin clay causes reduction in its cation exchange capacity, which in turn results in decreases of swelling strain. The effects of gamma radiation on the physical and chemical properties of Zhisin clay are found to be similar to those of dry heating. The swell of Zhisin clay under simulated underground geochemistry is affected by both the type of electrolyte and the concentration of solution.