Geochemical effects on swelling pressure of highly compacted bentonite: Experiments and model analysis

Bentonite is widely selected to to be used as buffer material for high-level nuclear waste (HLW) repositories owing to its favorite hydrogeological and geochemical properties. This is because mainly of its moisture swelling effect. Experimental and theoretical evidences indicate that the swelling characteristic is largely influenced by the porewater chemistry. A chemical swelling model for constrained condition is developed on the basis of diffuse double layer (DDL) theory and related the microscopic theory to the macroscopic swelling pressure. Experiments with purified clay fraction (< 2 mu m) of MX-80 bentonite were undertaken. The fine bentonite was compacted to a dry density of 1600 kg/m(3) with initial liquid saturation of 35.7% and then installed into a rigid container for swelling pressure experiment. The unsaturated bentonite sample was then flushed with NaCl solutions in different concentrations. With the increase of the ionic strength, the measured swelling pressure decreases. The experimental swelling pressure values agree well with the modelled results using the chemical swelling model.