## 4 Results and Discussion
### Microstructure Feature  
Based on SEM tests, the changes of the microstructure of expansive soil samples were obtained in two environmental conditions: high-temperature drying (50 \({}^{\circ}\)C-10%) and high-temperature humid (50 \({}^{\circ}\)C-70%), as shown in Figure 8.  
As shown in Figure 8, the microstructures of expansive soil samples under different environmental conditions are different and they changed significantly before and after dehydration. Firstly, Figure 8a shows the SEM image of the expansive soil sample under initial saturation. At this time, the overall pore structure of the expansive soil is relatively loose, and there are many loose pores and fissures among the clay particles. The large pore size is more than 25 \(\upmu\)m, and the small pore size is less than 4 \(\upmu\)m. Moreover, there are no obvious connected cracks and clay particle aggregates in the expansive soil samples under initial saturation state. The size of microstructures of initial expansive soil is similar to previous study [23].  
Figure 8b shows the SEM image of the microstructure of the expansive soil sample under the stable dehydration condition in the environment of 50 \({}^{\circ}\)C-10%. Seen from the figure that after dehydration in the high temperature drying environment, the microstructure of the expansive soil sample changes from the original loose state to the layered and  
Figure 7: The variation of expansive soil porosity with dehydration time under two kinds of environmental conditions.  
tightly stacked state, and some clay mineral particle aggregates can also be recognized from the SEM image. The shrinkage cracks are distributed between the aggregates and the surrounding layered stacked particles, and some of the cracks are cross-connected, enlarging the size of cracks. Under the high temperature drying environment, the initial saturated expansive soil sample is dehydrated rapidly and unevenly due to the difference of mineral composition and internal structure of the sample. The uneven dehydration rate among soil particles results in uneven shrinkage and cracking in soil. Moreover, the originally loose clay mineral structure becomes dense due to water loss, and the SEM image presents the shape of stacking and dense agglomerate. With the dehydrating, the shrinkage cracks between clay minerals continue to expand and gradually form partial connected cracks.  
Figure 8c shows that the microstructure of expansive soil samples becomes more compact after dehydration and stabilized under high temperature and humidity environment. Although there are some fractures with small size, there are no large granular aggregates and obvious connected cracks. Under the influence of high humidity, the initial saturated expansive soil dehydrated slowly, resulting in the slow shrinkage of the lattice structure of clay minerals. Then the internal stress of soil is adjusted to the new soil-water interaction state. In this way, the process of water dehydration is uniform, so the microstructure of soil is compact, and there are no large particle aggregates or obvious connected cracks. Compared with initial samples [23], the size of microstructures are smaller, which is close to that of treated samples [4].  
Figure 8: SEM images of expansive soil samples under different dehydration conditions. (**a**) Microstructure of expansive soil in its initial state; (**b**) Microstructure of expansive soil after dehydrated and stabilized at 50 \({}^{\circ}\)C-10%; (**c**) Microstructure of expansive soil after dehydrated and stabilized at 50 \({}^{\circ}\)C-70%.