## 3 Results and Analysis
### Micro-Structure  
Figure 8 presents a scanning electron microscope (SEM) image of sandstone samples before and after freeze-thaw cycling. Sandstone, a common sedimentary rock, consists of sand grains bound together by cement. The images clearly show cement located between the sand grains. Figure 8a shows an SEM image of an untreated sandstone sample. The image reveals strong grain-to-grain cementation and well-filled pores. As this sample has not undergone freeze-thaw cycling, the grains and cement at the imaged surface show no evidence of damage from ice crystallization stresses. The grains retain their original angularity.  
Figure 8b,d depict sandstone samples after 10 and 20 unidirectional freeze-thaw cycles, respectively. Progressive freeze-thaw cycling weakens the intergranular cement. After 10 cycles, cementation is visibly reduced. However, the grains retain significant angularity. After 20 cycles, the amount of cement coating the grains decreases further and the grains become noticeably smoother.  
Figure 8c,e display sandstone samples after 10 and 20 all-directional freeze-thaw cycles, respectively. Similarly, progressive all-directional freeze-thaw cycling gradually degrades the cement bonding between grains. This weakening results from the volumetric expansion of water during freezing within the pores, generating stresses that damage both the grains and the cement. However, after 20 cycles, compared to the all-directional freeze-thaw, the all-directional freeze-thaw sample exhibits a more pronounced reduction in grain-coating cement and increased particle smoothness. This difference arises because unidirectional freezing creates preferential pathways for water migration. As the 0 \({}^{\circ}\)C isotherm advances inward from the cooled surface, the volumetric expansion associated with ice formation drives unfrozen pore water ahead of the freezing front. This migration reduces the local water content available for ice formation near the grains later, thereby diminishing the localized stresses (e.g., crystallization pressure, frictional forces) exerted by the ice on the grain surfaces during freezing. Consequently, unidirectional freeze-thaw inflicts less damage than all-directional freeze-thaw.  
Figure 7: Specim compression failure diagram.  
Figure 8: Micro-structure. (**a**) Samples not subjected to freeze–thaw cycles (200 times); (**b**) Samples subjected to unidirectional freeze–thaw 10 times (200 times); (**c**) Samples subjected to all-directional freeze–thaw 10 times (200 times); (**d**) Samples subjected to unidirectional freeze–thaw 20 times (200 times); (**e**) Samples subjected to all-directional freeze–thaw 20 times (200 times); (**f**) 600 times electron microscope of unfrozen samples; (**g**) 600 times electron microscope of samples freeze–thawed 20 times in all directions.  
Figure 8f,g provide higher-magnification views of the intergranular cement before and after freeze-thaw cycling, highlighting the changes. These images reveal obvious fracturing within the intergranular cement after freeze-thaw cycling. The loss of cement also renders the boundaries between individual rock grains more distinct. Regardless of the freezing direction, sample deterioration initiates with the progressive degradation of the intergranular cement due to F-T cycling. This degradation facilitates the initiation of microcracks, which propagate and ultimately culminate in macroscopic sample failure.