# Effects of Freeze-thaw Cycles on the Mechanical Properties and Microstructure of a Dispersed Soil
## 3 Result and Discussion
### Mechanisms of Changes in the Microstructure of the Soil Samples  
Based on the particle size analysis test and the SEM experiment (the data summary table is available in Supplementary Materials as Table S1), it is evident that the content of sand particles in the soil samples gradually decreases, while the contents of clay particles and silt increase with an increasing number of freeze-thaw cycles. Throughout a freeze-thaw cycle, soil particles undergo simultaneous fragmentation of coarse particles and aggregation of fine particles. The complete process of soil particle transformation is illustrated in Figure 16. As the number of freeze-thaw cycles increases, internal stress is generated within soil particles, resulting in the fragmentation of coarse particles. During freezing, the liquid-free water in the pores transforms into solid ice, forming cracks on the surface of soil particles. The expansion of ice crystals and their wedging into the cracks  
Figure 15: Effect of freeze–thaw cycles on microstructure: (**a**) the mean diameter of the soil samples and (**b**) the surface porosity of the soil samples.  
cause a volume expansion of approximately 9% [37], thus exerting pressure on soil particles and leading to the development and extension of microcracks, ultimately forming larger cracks. Consequently, the water film thickness within the gaps between soil particles increases, leading to further fragmentation and disintegration of coarse particles into smaller ones. There is consensus among researchers regarding these findings [20; 38].  
The aggregation of fine particles occurs due to a bound water film adsorbed around clay particles and the action of the double electron layer, which promotes the aggregation of fine particles. Additionally, the negatively charged surface of clay particles attracts polar water molecules, resulting in the formation of a bound water film and the development of attractive forces between soil particles. Freeze-thaw cycles disrupt the lattice of minor minerals, alter the charge characteristics of soil particles, and enhance molecular forces between fine soil particles, leading to their aggregation. Moreover, during the freezing process, dissolved substances migrate along unfrozen water, resulting in their absorption by the surface of clay mineral particles. This contributes to the binding of clay particles into powder particles and further promotes particle aggregation [38]. Furthermore, the dissociation and crystallization of polymers during ice's volume expansion generate new particles [39]. With the aggregation of fine particles, the porosity within the soil increases gradually, as evidenced mainly by the increase in surface porosity observed in this study.  
When subjected to shear deformation, the unfrozen soil samples need to break up intact soil particles, resulting in a higher shear strength. In contrast, when subjected to shear stress, the frozen-thawed soil samples primarily overcome the bonding forces between fine particle aggregates. However, due to the disruption of colloidal connections between fine particle aggregates caused by the freeze-thaw cycles, the inter-aggregate connections weaken, thus significantly reducing strength. Simultaneously, with the gradual escalation in the number of freeze-thaw cycles, the soil structure forms irreversible through cracks that progressively widen, posing a severe threat to the stability of the soil structure and further reducing its strength. Consequently, the mechanical and physical properties of the soil are affected, potentially causing instability and damage to engineering projects within the study area.