Experimental investigation of swelling and microstructural properties of Kunigel V1 bentonite infiltrated by NaCl solutions

Understanding the effect of salinity on the swelling behavior of bentonite is crucial for evaluating the long-term performance of the buffer materials in deep geological repositories for high-level radioactive waste. In this study, the impact of NaCl solution infiltration on the swelling pressure ( p s ) of Kunigel V1 (K_V1) bentonite was assessed by conducting p s tests. X-ray diffraction tests and scanning electron microscope tests were performed to investigate the microstructural changes of K_V1 associated with NaCl solution infiltration. In addition, leachable and exchangeable cations were measured to understand the changes in the pore water chemistry of K_V1 after being infiltrated by the NaCl solutions. It was found that the p s of K_V1 at equilibrium after infiltration can be reduced by increasing the concentration ( n ) of NaCl, and that the p s evolution process can also be largely affected by n . NaCl solution infiltration under higher n conditions (e.g., 1.0–2.0 mol/L) may cause an irreversible decrease in p s , following its initial increase, whereas the infiltration under lower n conditions (e.g., 0–0.5 mol/L) brings about a temporary decrease in and subsequent recovery of p s . These macro behaviors were found to result from the microscopic changes in the montmorillonite particles and changes in the pore water chemistry surrounding them. On the one hand, higher n induces a decrease in the basal spacing of montmorillonite ( d 001 ), including the disappearance of osmotic swelling ( d 001 ≥ ∼40.0 Å) in relatively low initial dry density specimens (e.g., 1.1 and 1.3 Mg/m 3 ). On the other hand, the Na concentration in pore water also increases with an increasing n , leading to a reduction in the thickness of the diffuse double layers on the surfaces of montmorillonite particles. Combined with microscopic observations, the effects of n on the p s development were interpreted using a newly proposed conceptual model.