NMR-based analysis of pore water state of unsaturated compacted bentonite considering the saline effect

Abstract

Understanding the saline effect on the pore water state of unsaturated compacted bentonite at the microscale is critical for evaluating the performance of barrier materials in nuclear waste geological repositories. In this paper, the pore water redistribution and pore water state of unsaturated compacted bentonite considering the saline effect is investigated via nuclear magnetic resonance (NMR) technique. It is found that, with increasing pore water salinity, the pore water content decreases and pore water is distributed in smaller pores, which is consistent with the swelling deformation behavior of bentonite at the macroscale and the diffuse double layers (DDL) theory at the microscale. In addition, the saline effect on pore water distribution is weakened with the increase in matric suction due to the shrinkage deformation of bentonite during drying. Then, the saline effect on the pore water state of unsaturated compacted bentonite is discussed via the concept of effective activation energy. It is shown that the drainage of pore water and increased pore water salt concentration (or the increase of matric suction and osmotic suction) both elevate the effective activation energy of bentonite, decreasing pore water mobility. The saline effect on the permeability coefficient of bentonite is also discussed. For bentonite under larger matric suction and osmotic suction conditions, the permeability coefficient is smaller.