# Research Progress on the Influence of Thermo-Chemical Effects on the Swelling Pressure of Bentonite
## 4 Effect Mechanism of Swelling Properties
### Pore Water Status
[86] concluded that, although salt concentrations less than 1 M have little effect on the swelling of crystalline layers, the increased electrolyte concentration leads to the dehydration of montmorillonite by osmosis, which is detrimental to the occurrence of inter-montmorillonite hydration. Thus, high salt concentrations have an inhibitory effect on hydration. Figure 10: Variations of the adsorptive water content of samples at different dry densities [80]. The thickness of the double electric layer is calculated by [40]:  
\[\frac{1}{K} = \left(\frac{e^{0}DkT}{2n^{0}e^{2}v^{2}}\right)^{\frac{1}{2}} \tag{3}\]  
\(\frac{1}{K}\) is the thickness of the double layer, \(e^{0}\) is the vacuum coefficient, \(D\) is the dielectric constant, \(k\) is the Boltzmann constant, \(T\) is the temperature, \(n^{0}\) is the pore solution concentration, \(e\) is the unit charge, and \(\
u\) is the ionic valence. Significantly, because of the thermo-chemical effects on pore water, as shown in Figure 10[80], the volume of micro-pores decreased, and the volume of macro-pores increased with increasing salt concentrations, and micro-pores were more frequent at low temperatures. In addition, it was observed that more water was adsorbed at low temperatures and in high-salt-concentration environments, which seems to be contrary to the theory. By investigating the effective activation energy [87], which is known as the energy barrier for water molecule conversion, [PERSON] et al. [87] found that elevated salt concentrations accelerated the water molecule relaxation and produced a higher effective activation energy compared to samples saturated with distilled water, enhancing water molecule adsorption to some extent. In reality, such water with a short relaxation time is somewhat different from adsorbed water or exists in the form of hydrated ions surrounded by water molecules, an effect that is more pronounced in the presence of saline solutions, masking the inhibition of the double electric layer and the reduction of the micro-pore volume by the increase of the pore solution concentration; hence, the higher measured adsorbed water content [80]. Bentonite exposed to a concrete-leaching groundwater solution introduced at elevated temperatures showed a decrease in macro-pore volume and an increase in micro-pores, with the average pore size decreasing from 6 nm to 4 nm [88]. This phenomenon suggests that a high-temperature situation may also lead to an increase in micro-pores; so, the study of adsorbed water content should take into account the pore structure in the case of coupled thermo-chemical effects. In conclusion, dry and wet cycles destroy the soil pore structure and weaken the water retention capacity of bentonite; elevated temperatures reduce the water retention performance of bentonite; increased salt concentrations are detrimental to the development of micro-pores and the adsorption of adsorbed water, thereby reducing the swelling capacity of bentonite. When the thermo-chemical effects act simultaneously, combining with changes in soil structure, thermodynamic properties, and ion hydration effects, their influence on the pore water status becomes more complex. As a consequence, more in-depth studies are essential.