An extended model based on the general Van Genuchten model for the “wetting” process of GMZ Na-bentonite subjected to alkali-heat treatment

During the long-term operation of a nuclear waste repository, the buffer/backfill material in the shield of the repository can eventually lose the ability to retain water owing to factors such as groundwater chemistry and heat released from nuclides. In this paper, a test platform was constructed to investigate the "wetting" behavior of GMZ Na-bentonite (a local bentonite from Gaomiaozi county, China) after being subjected to different alkali-heat treatments (AHTs). The experimental results show that (i) montmorillonite primarily decomposes and dissolves under the alkali-heat conditions, (ii) the mass moisture content and equivalent crystalline interlayer (ECI) space of montmorillonite are negatively correlated with the duration and temperature of AHT and (iii) independent of AHT, the evolutionary relationship between the mass moisture content and suction of montmorillonite exhibits a rapid decrease in the low-suction range and a gradual stabilization in the high-suction range. Significantly, an extended VG (EVG) model was developed to quantitatively characterize impact of temperature and alkaline solution on weakening the ability of bentonite to retain water. The developed model considers the contribution of the spatial perturbation factor to the saturation, so as to clearly characterize the three "wetting" states of bentonite in ascending order of the matric potential: water-linked & air-closed, water–air fully open, and water-closed & air-linked.