# Effects of Freeze-thaw Cycles on the Mechanical Properties and Microstructure of a Dispersed Soil
## 3 Result and Discussion
### Influence of Freeze-Thaw Cycles on the Microstructure of the Soil Samples  
Figure 14 displays the SEM images of the soil samples after undergoing various numbers of freeze-thaw cycles. The unfrozen-thawed soil particles (Figure 14a) have well-defined bound pores, primarily linked through point-to-point or point-to-face interactions. However, significant changes in particle morphology and pore characteristics are observed after the freeze-thaw cycles, as depicted in Figure 14b-h. After experiencing freeze-thaw cycles, newly developed fissures (highlighted by dashed green lines) emerge within the soil matrix, resulting in a more porous arrangement. Throughout the freeze-thaw phenomenon, microcracks initially form on the exterior of larger particles and progressively amalgamate to create more extensive fissures. These cracks continue to propagate with subsequent freeze-thaw cycles, ultimately resulting in the fragmentation of soil particles (highlighted in the red box). With an escalation in the frequency of freeze-thaw cycles, the gaps within the soil extend over greater distances, leading to a more porous and loose soil structure.  
Figure 14: Scanning electron microscopic photographs of the tested soil samples after different numbers of freeze–thaw cycles (F–T n represents the number of freeze–thaw cycles and (**a–h**) represent microstructural images after 0 to 15 cycles of freeze-thaw).  
Additionally, the fracture width tends to increase, which is accompanied by a higher incidence of particle breakage, particularly for larger particles. Consequently, the proportion of smaller particles in the soil rises over subsequent freeze-thaw cycles. Furthermore, salts crystallize from the pore solution, as illustrated by the yellow circle in Figure 14b,f.  
The average diameter of soil particles can reflect their size. The fluctuation in the average particle size of the examined soil samples, with an escalation in the frequency of freeze-thaw cycles, was determined by analyzing the magnified SEM images (800 times magnification) using the IPP 6.0 software. As shown in Figure 15a, it is evident that the average particle size of the soil samples tends to decrease as the number of freeze-thaw cycles increases. The most significant changes occur within the range of 0-9 freeze-thaw cycles, decreasing from 3.9 \(\upmu\)m to 3.21 \(\upmu\)m and stabilizing after that. This trend aligns with the results obtained from particle size analysis tests. This phenomenon can be attributed to the rapid destruction of larger soil particles into smaller ones during the initial nine freeze-thaw cycles, resulting in a weaker inter-particle association and increased distance between soil particles. Surface porosity is an indicator that reflects the pore content of soil to some extent. A higher value indicates a more significant proportion of pores in the soil and a looser soil structure. The variation in surface porosity (N) of the tested soil samples with an increase in the number of freeze-thaw cycles was calculated using the IPP 6.0 software, and the results are presented in Figure 15b.  
It can be observed that the surface porosity (N) of the soil samples exhibits an increasing trend with an escalation in the frequency of freeze-thaw cycles. During the 0-9 freeze-thaw cycles, the value increases from 14.47% to 30.5% and gradually stabilizes. While the surface porosity of the soil samples, determined through the SEM analysis, does not encompass internal pores within soil particles, it does offer valuable insights into the changes in pore content of the soil samples as the number of freeze-thaw cycles rises.