# Research Progress on the Influence of Thermo-Chemical Effects on the Swelling Pressure of Bentonite
## 4 Effect Mechanism of Swelling Properties
### Pore Distribution  
Clay represents a complex porous structural body, with clay layers and interlayer pores composing a basic unit of clay particles, such as montmorillonite lamellae and interlayer pores, as shown in Figure 9[60]. Water in clay soils includes microstructural water and macro-structural water [64]. As shown in Figure 9, water in interparticle and interlayer pores is microstructural water, i.e., adsorbed water, and water in interaggregate pores is macrostructural water. The surface of the clay particle unit carries a negative charge, and a diffuse double layer is formed between the clay particles, such as the diffused double-electric-layer water layer at the crystallite scale, shown in Figure 4b. The clay particles and the inter-clay pores form the basic clay aggregate, then the aggregates and the inter-aggregate pores develop a representative basic clay unit. Consequently, the clay block can be roughly divided into interlayer pores, intergranular pores, and inter-aggregate pores at three levels of pore space, and fractures visible to the naked eye develop from the inter-aggregate pores.  
Figure 8: Evolution of the swelling pressure under thermo-chemical effects [28].  
Investigating bentonite microstructure entails the study of the pore structure of bentonite as well as its mineral composition. Freeze-drying techniques [64; 66] and mercury intrusion pore size measurement (MIP technique) [67; 68] were used to obtain PSD distribution curves and thus distinguish between macro- and micropores. Nevertheless, the limitations of the MIP technique itself [69; 70; 71], combined with the differences in clay types, led to different pore size measures. For example, when the specimen was a mixture of bentonite and Callovo-Oxfordian (COx) claystone with low dry density, the average pore sizes of its three main pore groups were 10-22 \(\upmu\)m for macro-pores, 0.17-0.52 \(\upmu\)m for mesopores, and about 25 nm for small pores [60].  
[PERSON] et al. [72] investigated the microstructure of saturated bentonite samples with a moisture content of 0.46 in the wet-dry cycle and found that micro-pores and macro-pores corresponded to 12 nm and 6 \(\upmu\)m; they considered the size of 500 nm as a threshold to distinguish between intra-aggregate and inter-aggregate pores. [PERSON] et al. [21] performed MIP analysis of compacted MX80 Wyoming bentonite samples with 80-92% montmorillonite content and identified small and large pore groups with major pore sizes of 17 nm and 21 \(\upmu\)m, respectively; pores with a diameter of less than 100 nm were considered micro-pores [35]. In addition, scanning electron microscopy (SEM) tests can provide high-precision clay images to study the structural changes of clays more visually, and X-ray diffraction (XRD) can evaluate the changes in the composition of clay minerals to determine the causes of changes in swelling pressure [24; 73], as well as measure variations in mineral layer spacing as a support condition for changes in the swelling pressure [74]. The structure determines the properties, and the complex structure of clays dictates that variations in swelling properties are significantly influenced by changes in pore distribution and pore water status.