## 5 SEM Micro-Mechanism Analysis  
[PERSON] et al. [30] conducted electron microscopy tests and found that both single aggregate modification with water glass and composite modification with water glass and silane could improve the state of the interfacial transition zone, and the composite modification was better than the single modification. The unfrozen and thawed RAC group (ordinary recycled concrete) was selected as the control group; the RAC group with 250 water freeze-thaw cycles, the RAC group with 250 chloride freeze-thaw cycles, and the RAC group with 250 sulfate freeze-thaw cycles were selected as the test groups for the scanning electron microscope test; and the internal damage of the RAC group was observed in the freeze-thaw cycles under different media. The microscopic condition is shown below. Figure 11 shows the microscopic morphology of the interior of the specimen when it is not frozen or thawed. As can be seen from Figure 11a, the surface of the specimen is relatively dense, and there is a penetrating crack in the interface transition zone, which provides a channel for moisture and other salts and corrosive substances to enter the concrete. As can be seen from Figure 11b, the interfacial transition zone is relatively smooth, the cracks are relatively smooth and flat, and there exist cement particles on the surface that have not yet been fully hydrated, as well as C-S-H gel produced by the hydration reaction and Ca(OH)\({}_{2}\) crystals, etc., which are roughly uniformly distributed in the vicinity of the interfacial transition zone and are tightly connected, forming a mesh to fill the weak region of the interfacial transition zone, which makes the internal structure of recycled concrete denser in comparison with the other. Figure 12 shows the microscopic morphology of the interior of the specimen at 250 freezing and thawing cycles in clear water, and it can be seen from Figure 12a that the cracks in the interfacial transition zone have expanded, and some new small holes and cracks have appeared on the surface. As can be seen in Figure 12b, the microcracks and holes increased significantly, the cracks were rougher and uneven, the gaps between the C-S-H gels became larger as well, the Ca(OH)\({}_{2}\) crystals were arranged haphazardly, and the densities in the interfacial transition zone decreased. The reason for these phenomena is that water molecules penetrate the concrete interior through the pores and cracks, and through the freezing and expansion effect, the pores and cracks inside the concrete continue to expand, and new pores and cracks are generated, which leads to the loosening of the internal structure and causes damage. Figure 11: Microscopic morphology of specimens in the RAC group without freeze–thaw; (a) 1000X and (b) 5000X. Figure 13 shows the internal microscopic morphology of the specimen after 250 cycles of freeze-thaw cycles with chloride salts. As seen in Figure 13a, the surface appears to be more uneven, and the cracks and pores in the transition zone of the interface are extended significantly and increased in number. Figure 13b shows that the damage of the cracks in the interface transition zone is very serious, and the gaps between the needle-like calcite are larger, with almost no tight connection, and the arrangement is more chaotic and disorganized; additionally, it can be found that there is a lot of crystalline material near the interface transition zone, which indicates that the concrete specimen in the chlorine salt solution in the freeze-thaw cycle can not only withstand freezing and expansion, but it can also withstand the crystallization pressure due to the difference in concentration. Together, these two factors accelerated the destruction of the concrete. Figure 14 shows the microscopic morphology of the specimen at 250 sulfate freezing and thawing cycles. Figure 14a shows that the interface transition zone is rough and uneven, and there are a lot of cracks and holes of different sizes, but it is slightly better than the case of chloride salt.