Thermochemical effect on the capillary and adsorptive water contents of unsaturated compacted bentonite

Determining the physicochemical state of water in unsaturated compacted bentonite is fundamental to the development and safety for nuclear waste barriers. Pore water in unsaturated compacted bentonite can be divided into adsorptive water and capillary water. In this study, an improved method combing nuclear magnetic resonance (NMR) and mercury intrusion porosimetry (MIP) data for distinguishing between capillary water and adsorptive water is proposed. NMR test results of unsaturated compacted bentonite under different temperature and pore water salinity conditions are analyzed using the improved method. Results show that the pore water state changes from coexistence of adsorptive water and capillary water to dominated by adsorptive water as the water content decreases. At water content of 20%, the proportion of adsorptive water can reach 90%. For saturated bentonite, an increase in salinity resulted in a reduction of adsorptive water; for bentonite with water content of about 20%, the saline effect on the transformation of pore water state is negligible. Following the temperature rises, a portion of adsorptive water is thermodynamically activated and redistributed into capillary domains. For saturated bentonite, when the temperature rises from 10 °C to 60 °C, the mass of transformed water is 10.68 g; for unsaturated bentonite at 75.5% relative humidity, the corresponding transformed mass is only 0.26 g. The results are also well explained through the evolution of pore water potential energy and microstructure.