## 3 Results and Discussion
### Microstructure and Mineralogical Analyses  
#### 3.3.1 Scanning Electron Microscope (SEM) with Particle and Crack Analysis System (PCAS)  
The microstructural evolution of different freeze-thaw cycle conditions was analyzed using SEM and a PCAS. SEM imaging, at 500\(\times\) magnification, distinctly revealed point-to-point, point-to-surface, and surface-to-surface contact configurations between soil particles and stabilizing agents. These contact arrangements offered key insights into the microstructural characteristics of the soil matrix. PCAS analysis applied binarization to SEM images, enabling automatic noise removal, segmentation, and identification of particles and pores. This technique facilitated the quantification of various geometric parameters, including particle content, porosity, fractal dimension, and probability distribution index. By quantifying these parameters, the pore structure characteristics of the geotechnical materials were analyzed with high accuracy. This method provides a powerful approach for the quantitative evaluation of pore features, contributing to a deeper understanding of the microstructural properties and deterioration mechanisms of carbonated solidified loess under freeze-thaw cycling conditions.  
Figure 14 illustrates the microstructural changes in carbonated solidified loess after 0 and 15 freeze-thaw cycles. This research indicates that under optimal additive conditions, the loess particles and magnesium-based carbonate products formed strong bonds through the connecting action of CL, resulting in relatively fewer pores in the soil. However, after 15 freeze-thaw cycles, the bonding force between soil particles weakened, microcracks easily developed on the surfaces of soil particles and magnesium carbonate products, and the pores enlarged. This result provides a microscopic explanation for the deterioration process of the soil structure. By analyzing the pore characteristics in the SEM images using PCAS V2.3 software, the pattern of structural changes could be visually demonstrated: as the nuber of freeze-thaw cycles increased, the number of pores rose significantly, and the porosity increased accordingly. This trend highlights the destructive effect of repeated freeze-thaw cycles on the soil's microstructure. Table 3 presents the variations in pore characteristics and related parameters of the carbonated solidified loess under 0 and 15 freeze-thaw cycles. Combined with Figure 14, it is evident that during the freeze-thaw process, the soil pores enlarged, the number of pores increased, cracks developed in soil particles, and structural damage occurred, leading to a significant decline in the soil's thermal conductivity. Further analysis revealed that the probability entropy values for both sets of images exceed 0.9, indicating minimal changes in the arrangement and orientation characteristics of microstructural units before and after freeze-thaw cycles. This consistency suggests a high degree of randomness and complexity in the soil's structure. Additionally, probability entropy values close to 1 imply that the direction of pore expansion exhibited  
Figure 13: Thermal conductivity of different freeze–thaw cycles.  
strong randomness, with low orderliness in the arrangement of soil particles, resulting in a highly complex structure. The impact of freeze-thaw cycles on the fractal dimension was relatively minor; however, after 15 freeze-thaw cycles, the fractal dimension increased by approximately 0.02. This increase reflects a reduction in the uniformity of the soil particle size distribution and a rise in heterogeneity within the soil. Such microstructural changes contributed to the progressive deterioration of the soil's macroscopic thermal conductivity.