The volume change behavior of compacted GMZ bentonite: combined effects of temperature and suction

As the buffer/backfill material for geological disposal of high-level radioactive waste, the sealing performance of compacted bentonite is affected by its volume deformation characteristics. During the operation of the geological disposal repository, the suction and temperature of bentonite changes due to the groundwater level and heat released by radioactive waste. On this premise, to study the thermal effect on the volume change of bentonite induced by suction variations, a series of wetting/drying tests were conducted on cubic bentonite samples under controlled temperatures of 20celcius, 40celcius and 60celcius. The corresponding microstructure changes during the test were investigated by the mercury injection porosity (MIP) technique. The results show that the increasing temperature accelerated the change in water content and weakened the water retention capacity of bentonite. Additionally, the volume deformation induced by suction change, regardless of swelling or shrinkage, was inhibited by heating. During the process of suction equilibrium, the compacted bentonite showed significant anisotropy, which was positively correlated with temperature and negatively correlated with suction. The sample deformation was due to the changes in the microstructure of the inter-aggregate pores and intra-aggregate pores. The total volume of macro pores decreased obviously with increasing suction, while, the volume of micropores remained almost unchanged. Both the peak value and total volume of the macro pores were reduced by the rising temperature. Furthermore, a threshold suction of approximately 90 MPa was observed, where the temperature effect changed from inhibition of dilation to inhibition of contraction, indicating the suction-dependent temperature effect.