# Research Progress on the Influence of Thermo-Chemical Effects on the Swelling Pressure of Bentonite
## 1 Introduction  
With the rapid evolution of nuclear energy, a large amount of high-level radioactive waste (HLW) has been produced [1]. Disposing of this high-level radioactive waste safely, in an environmentally friendly way, and effectively for a long time has become a major problem that requires urgent solutions worldwide. At present, the most effective and feasible method of disposal of HLW is deep geological disposal, which requires choosing a suitable site (crust with poor water content, good stability, far from human activity zones, etc.) to set up an engineering barrier, buffering and sealing the tanks containing HLW in a deep geological repository at a depth of 500-1000 m from the surface to prevent the waste from being transported to the biosphere in case of a spill [2]. Bentonite, which contains a large amount of montmorillonite, is internationally recognized as a buffer/backfill material for the deep geological disposal of HLW due to its characteristics of low permeability, large swelling, strong adsorption, and good thermal conductivity [3, 4]. Furthermore, bentonite not only has the important characteristics of buffering and backfilling, but also has the advantages of low cost, wide distribution, easy mining, and easy processing; so, it has become the preferred buffering backfilling material around the world. In China, Gaomiaori bentonite produced in Xinghe County, Inner Mongolia, was selected preliminarily as the base material for buffering/backfill material [5, 6].  
Bentonite is in an unsaturated state after the construction of deep geological repositories. As the groundwater level rises, the compacted bentonite absorbs water (solution) undergoing an expansive deformation that can tightly enclose the waste tank and fill the gap between the buffer material and the surrounding rock mass, fully blocking the surroundingrock and the waste tank. In the meantime, a sufficiently large swelling pressure under almost constant volume conditions can improve the sealing efficiency of bentonite [7; 8; 9], which is the main governing factor for the stability of subsurface structures. However, the near-field chemical conditions are complex during the operation of an HLW repository [10], as bentonite draws water from the surrounding rock and becomes gradually saturated due to subsurface infiltration. This will result in changes in the chemical concentration of bentonite pore water. Moreover, the temperature in the repository will be changed on account of the diffuse nuclear radiation heat. In the long term, problems such as the loss of bentonite's swelling properties may arise. Therefore, the study of bentonite's swelling performance under coupled chemical and temperature gradients field conditions is a crucial issue for the safe operation of nuclear waste repositories.  
The swelling characteristics of compacted bentonite have been extensively investigated by scholars in the past decades. Focusing on this theme, this paper systematically describes the progress and limitations of the studies on bentonite swelling pressure under the effect of thermochemistry, with regard to the experimental test methods, the law and mechanism of swelling pressure variations under the influence of pore solution concentration and temperature, etc. On this basis, an outlook of future research developments to understand the influence of thermo-chemical effects on bentonite swelling pressure is provided.