# 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 Temperature on the Swelling Pressure  
Elevated temperatures can have an impact on bentonite and its engineering properties. For example, simulations of high-discharge waste disposal in China have shown that thermal swelling and saturation processes may affect a tank stability for long periods [52], as higher temperatures can provoke montmorillonite dehydration [53]. The effect of temperature on the swelling pressure is highly controversial. [PERSON] et al. [25] used distilled water and performed constant-volume swelling pressure tests on GMZ bentonite particle mixtures at different temperatures. It was shown that the swelling pressure changed faster and gradually increased with higher temperatures (Figure 7). A similar phenomenon was also indicated by some related studies [5; 54]. Nevertheless, other tests also showed that the swelling pressure decreased with the increasing temperature [55; 56; 57; 58].  
Figure 7: Evolution of the swelling pressure of a bentonite pellet mixture at different temperatures [25].  
In contrast, some studies have shown opposite effects of temperature on different bentonite types, such as heating leading to an increase in the swelling pressure of Na-based bentonite and a decrease in the swelling pressure of Ca-based bentonite [59]. This can be attributed to two main mechanisms: osmotic pressure and the net effect of crystalline lattice shrinkage. Lattice shrinkage caused by the dehydration of the interlayer space and the increase in osmotic pressure at the interlayer contact is different for the two clays, with lattice shrinkage dominating in Ca-based bentonite, and the increase of osmotic pressure dominating in Na-based bentonite. Different bentonite types produce different responses to temperature variation; therefore, combined with the swelling mechanism, it can be concluded that the effect of temperature on hydration greatly influences the variation of the final swelling pressure of bentonite.  
Hydration causes an increase in the interlayer pore volume of clay minerals, leading to the swelling of clay particles and, thus, to the formation of clay aggregates [60]. The bentonite swelling potential is influenced by the water content due to the coupling of micro- and macro-structures caused by hydration [61]. As shown in Figure 7, a test was conducted by injecting distilled water at the bottom of the specimen, and the results were analyzed. Firstly, the water entering from the bottom caused the soil particles to swell and densify, and thus the swelling pressure increased rapidly; however, the swollen particles prevented further water penetration, which slowed down the increase in the swelling pressure. Finally, the reduced strength of the wetted particles was counteracted by the rearrangement of the upper particles, and thus a plateau in the development of swelling pressure occurred [25]. The dual-structure model may explain the reduction of the particle strength in the wetted state [62; 63]: on the one hand, wetting increased the interlayer distance between the clay flakes leading to the swelling of the clay aggregates, and on the other hand, wetting weakened the macro-structural resistance.  
[PERSON] et al. [25] provided a comprehensive explanation of the relationship between swelling pressure and temperature, indicating that the variation of the swelling pressure with temperature depends on the competition between hydration pressure, hydraulic pressure, and osmotic pressure thermo-tropic response. The Na-based GMZ bentonite with a dry density of 1.45 mg/m\({}^{3}\) in the study exhibited an enhanced swelling pressure as it possesses a large number of pores between the particles, and the temperature-dependent increase of its swelling pressure was affected by the enhanced water pressure and osmotic pressure masking the limited decrease in hydration force. Therefore, the effect of temperature on the swelling pressure depends not only on hydration but also on the pore water pressure and electrolyte properties.