# Research Progress on the Influence of Thermo-Chemical Effects on the Swelling Pressure of Bentonite
## 3 Influence of Thermo-Chemical Effect on the Swelling Pressure
### Effect of the Pore Salt Solution Concentration on the Swelling Pressure  
Studies have shown that the increase of the pore solution concentration leads to a decrease in bentonite expansibility [3; 36; 37]. Different types of solutions have different effects on the swelling pressure of bentonite. The concrete material in an HLW repository is gradually degraded during the interaction with pore water producing an alkaline solution, while montmorillonite, the main mineral component of bentonite, is dissolved gradually by the alkaline solution [38; 39]. As shown in Figure 5, GMZ01 bentonite with a montmorillonite content up to 75.4% was tested for its constant-volume swelling pressure in the presence of different saline solution concentrations and types of infiltration, while maintaining the temperature at 20 \(\pm\) 1 \({}^{\circ}\)C [26]. According to the classical double- layer theory, increasing the solution concentration will inhibit the swelling of the double layer [40], and the thickness of the diffusion double layer is inversely proportional to the square root of Na\({}^{+}\) concentration [16]. Therefore, high-concentration saline solutions will reduce the repulsive force between the particles, leading to a decrease in the swelling pressure. It is obvious that the ion types, Ca\({}^{2+}\) and Na\({}^{+}\), had similar effects on the swelling pressure at low concentrations, but when the concentrations were greater than 0.5 M, the inhibition of the swelling pressure of bentonite by Ca\({}^{2+}\) was not so obvious compared to that induced by Na\({}^{+}\). A greater base spacing of Ca-based montmorillonite under constant volume conditions was reported [41], and [PERSON] et al. [42] suggested that Ca\({}^{2+}\) replacement changed the pore fabric characteristics, leading to a greater crystalline-layer swelling deformation. However, with regard to the diffusive double-layer swelling, the Gouy-Chapman model [43; 44] suggested that the higher the electrolyte valence type and the electrolyte concentration, the smaller the double-layer thickness. Several researchers attribute the greater swelling pressure in the presence of Ca\({}^{2+}\) to the fact that its action produces larger and thicker particles [45], and the bilayer effect is not so pronounced; so, bentonite has greater mechanical strength and greater swelling pressure. As shown in Figure 6, where the effect of temperature was ignored, the constant-volume swelling pressure test was performed on GMZ Na-bentonite with 75.4% bentonite, initial water content of 11.92%, dry density of 1.7 Mg/m\({}^{3}\); the swelling pressure curve showed three phases [24]. Analogously, when the dry density was high, 1.7 Mg/m\({}^{3}\), and the initial water content was relatively low, i.e., 10.76%, the swelling process under the action of the saline solution comprised three stages: firstly, crystalline swelling, followed by the breakup of quasicrystals, and then double-layer swelling [26]. The first stage was characterized by a rapid increase in the swelling pressure, marked by increased interlayer  
Figure 4: Formation of the wedging pressure at (**a**) a nanolayer scale and (**b**) a crystallite scale [35]. cation hydration, and as the cation concentration continued to increase, water molecules within the crystals left the crystal layer, which gradually inhibited the swelling of the crystal layer [46]. The second stage was the transition from crystalline to permeable swelling, with a small decrease in the swelling pressure due to the swelling of soil particle agglomerates and the collapse of the soil skeleton, which was manifested by the splitting of thick quasicrystals into fine quasicrystals and the filling of large pores (inter-agglomerate pores). The third stage was the double-layer swelling stage, where the swelling pressure increased again, and the crystalline swelling became less pronounced; for specimens infiltrated with the salt solution or distilled water, the final swelling pressure in this stage was maintained in equilibrium [47]. [PERSON] et al.