# Research Progress on the Influence of Thermo-Chemical Effects on the Swelling Pressure of Bentonite
## 4 Effect Mechanism of Swelling Properties
### Pore Water Status  
Pore water refers to water that is confined to soil pores or clay minerals, and the properties of pore water are changed by the interaction between soil particles and pore water. The pore water status of soils also affects their mechanical properties to some extent. Studies on soil moisture characteristics suggested that soil moisture is controlled by two mechanisms: capillary and adsorption [75; 76]. Adsorption predominates under high suction, and capillary dominates under low suction [77; 78]. [PERSON] et al. [72] tested the wet-dry cycle with different suction values in powder and compacted Ningming expansive soils. When the suction reached 24 MPa, the SWCCs curves of the two kinds of soil samples coincided. Therefore, the suction pressure of 24 MPa was defined as the threshold for capillary and adsorption of the soil samples, distinguishing capillary and adsorption effects in expansive soil with a quantitative suction magnitude. Meanwhile, some constant-volume experiments provided similar results [79]. The variation of absorbed water content in the soil with temperature is controlled by two mechanisms: pore water potential and soil structure. The pore water potential of adsorbed water is smaller than that of capillary water and decreases with the decrease in temperature [80]. According to [PERSON]'s  
Figure 9: Fabric units and pore spaces of compacted clay (modified after [2; 65]). principle [81], an increase in temperature facilitates the desorption process and inhibits the exothermic process. According to the thermodynamic theory Equation (1), both cation and surface hydration processes are exothermic because the adsorbed water is adsorbed on the crystal surface, the entropy of the system decreases, \(\Delta S_{a}<0\), and then the enthalpy of adsorption becomes \(\Delta H_{a}<0\); therefore, adsorption is exothermic [82]. The decrease in temperature enhances hydration, influencing the entropy of the hydration reaction [83]. \[\Delta G_{a}\ =\ \Delta H_{a}-T\Delta S_{a}\ =\ 0 \tag{1}\]  
The absorbed water content increases with the decrease in temperature, which can be attributed to the decrease in pore water potential and the increase in micro-pore volume, as shown in Figure 10. Depending on the Yang-Laplace equation, capillary suction can be described by Equation (2) [82]. As the temperature increases, under a given suction Pc, the water-saturated pore diameter d decreases, with the decrease of surface tension (\(\sigma\)) and contact angle (\(\theta\)) the faster the speed decreases. This results in a reduction in the amount of capillary water [84; 85]. Thus, the conversion of adsorbed water to capillary water with increased temperature and the further loss of capillary water will be detrimental to the retention of water in clays. [PERSON] et al. [82] demonstrated that the water retention capacity of three bentonite samples decreased with increasing temperature for a given suction condition. In both restricted- and free-swelling conditions, bentonite water retention capacity will decrease with the increasing temperature (20-80 \({}^{\circ}\)C), and this is especially evident when the suction is low [57]. \[P_{c}\ =\ \frac{4\sigma\cos\theta}{d} \tag{2}\]  
The adsorbed water content is simultaneously controlled by the pore solution concentration, which increases to the detriment of the adsorbed water [80]. It is clear that an increase in the pore solution concentration will inhibit the development of the bilayer thickness, according to Equation (3). However, the role of salt concentration on crystal layer swelling needs to be further investigated. As regards the changes in pore structure, the adsorbed water content is affected by the salt concentration in the same way as the micro-pore volume. [PERSON] et al.