Compacted bentonite–sand mixture is proposed widely as backfill in geological repositories for disposal of radioactive waste in many countries because this material has significant swelling capacity and low water permeability. Development of the swelling pressure of backfills upon hydration is related closely to the stability of the host rock in the geological repository. No systematic experimental studies have been carried out to explore the effect of a water phase on the swelling pressure and water retention of bentonite–sand mixtures with insignificant osmotic suction. The objective of the current study was to examine experimentally the influence of a water phase involving liquid water and water vapor on swelling pressure and water retention of a bentonite–sand mixture with insignificant osmotic suction. Swelling-pressure tests with suction control and water-retention measurements under constant-volume conditions were performed on the compacted bentonite–sand mixture with a dry density of 1.80 g/cm3. Osmotic and vapor equilibrium techniques were used to make identical specimens adsorb liquid water and water vapor, respectively. The experimental results showed that the water phase had almost no effect on the swelling-pressure patterns of the unsaturated bentonite–sand mixture upon hydration over a suction range from 27 to 3 MPa. The swelling pressure increased significantly with decreasing suction from 27 to 3 MPa, regardless of the mixture adsorbing either the liquid water or water vapor. Nevertheless, the water phase had a considerable impact on both the swelling pressure and water retention of the unsaturated bentonite–sand mixture upon hydration over the same suction range. For a given value of suction in the range above, the swelling pressure and the water content of the bentonite–sand upon adsorption of liquid water were greater than those upon adsorption of water vapor. The influence of the water phase on the swelling pressure and the water retention of the bentonite–sand mixture with insignificant osmotic suction is related mainly to the hydration or swelling mechanism of Ca-rich bentonite.

Swelling deformation tests of Kunigel bentonite and its sand mixtures were performed in distilled water and NaCl solution. The salinity of NaCl solution has a significant impact on the swelling properties of bentonite, but not on its surface structure. The surface structure was characterized using the fractal dimension Ds. Based on the fractal dimension, a unique curve of the em–pe relationship (em is the void ratio of montmorillonite and pe is the effective stress) at full saturation was introduced to express the swelling deformation of bentonite–sand mixtures. In mixtures with a large bentonite content, the swelling deformation always followed the em–pe relationship. In mixtures with a small bentonite content, when the effective stress reached a threshold, the void ratio of montmorillonite em deviated from the unique em–pe curve due to the appearance of a sand skeleton. The threshold of vertical pressure for mixtures in different solutions and the maximum swelling strains were estimated using the em–pe relationship. The good agreement between estimates and experimental data suggest that the em–pe relationship might be an alternative method for predicting the swelling deformation of bentonite–sand mixtures in salt solution.