Pore structure evolution of unsaturated GMZ bentonite under thermo-hydro effects through ESEM fixed-point observation

The pore structure evolution characteristics of bentonite under thermal effects are the basis for determining its various hydraulic properties and thus are of vital importance for evaluating the long-term safety of repository operations. Therefore, the current research has been carried out to characterize the pore structure evolution of GMZ bentonite during hydration at different temperatures. ESEM fixed-point scanning was adopted at various relative humidities (RHs) and temperatures on unconfined compacted GMZ samples with an initial dry density of 1.70 g/cm3. It was found that the pore structure characteristics evolved in a significant moisture-dependent and temperature-dependent pattern. During hydration at 20 °C, large deflections of the structure are evident in the low RH, while crack formation and extension are more likely to occur in high RH. With the increase in temperature, the inter-aggregate porosity shows an increasing phenomenon, while for high moisture samples, the inter-aggregate porosity occurs first with an increase followed by a decrease and again increased phenomenon. As the temperature rises, the bentonite undergoes two processes of water loss: inter-aggregate pore water loss and intra-aggregate pore water loss, while this process also faces the effects of thermal expansion, leading to complex changes in porosity and pore size distribution under the competing effects.