## 3 Results and Discussions
### Mechanistic Analysis of the Influence of RHA on the Microstructure of Recycled Aggregate Concrete
Mechanistic Analysis of the Influence of RHA on the Mechanical Characteristics and Freeze-Thaw Resistance of Recycled Aggregate Concrete  
During the hydration process of cement paste, the RHA particle's high specific surface area will promote the hydration reaction of cement. At the same time, the amorphous silica within the RHA particles will react with the CH generated by the hydration of the cement to produce C-S-H gel. The extra C-S-H gels and the incompletely hydrated cement and RHA particles will refine the pore structure in the paste and improve the bonding of the interfacial transition zone, which leads to a densified microstructure and consequently has a positive contribution to the development of the compressive strength of the concrete. However, under the condition of a high RHA replacement rate, the reduced free water content leads to a weaker hydration reaction, less CH generation and a reduced pozzolanic reaction between CH and amorphous silica, leading to a reduction in C-S-H gel production, which is not conducive to the improved compactness of the paste pore and the interfacial transition zone, thus leading to a loose microstructure of the cement paste and a weaker bonding of the interfacial transition zone, which will have a negative impact on the load-bearing capacity of the concrete. When recycled coarse aggregate is added to concrete, two types of interfaces are formed inside the concrete: the attached mortar-old aggregate interface and the new  
Figure 9: Microscopic morphology of the paste samples with different RHA replacement ratios after the freeze–thaw cycles. mortar-attached mortar interface, as shown in Figure 10a. The recycled aggregate attached mortar's porous structure will make the concrete absorb a large amount of water during the process of the freeze-thaw cycle. The RHA particle's high specific surface area will cause water enrichment to occur on its surface and enter the interface transition zone of the new mortar-attached mortar interface of the recycled aggregate through cracks and pores. When the pore water inside the concrete is in the process of repeated freeze-thaw, it causes the continuous accumulation of the expansion pressure of the pores within the concrete, which consequently leads to the gradual transformation of the tiny pores in the interface transition zone into larger pores under the action of tensile stress and the gradual development of microcracks into macro cracks, which will provide a channel for the migration of water. The more water retained in the pores and cracks inside the concrete, the more significant the volume increase when freezing, which leads to a higher expansion pressure inside the concrete and accelerates the formation of cracks [50]. The interface transition zone, as the strength-limiting phase of concrete, makes the concrete reach an ultimate tensile strength under a slight expansion pressure, which results in the freeze-thaw damage of concrete due to stress concentration [51]. Meanwhile, as the bond is the weakest at the interface transition zone between the new mortar and the attached mortar, the transverse tensile stress continues to increase, which will cause the rapid expansion and extension of cracks in the new mortar-attached mortar interface transition zone, and the attached mortar-old aggregate interface will also generate cracks until the concrete specimen is destroyed. The effect mechanism of recycled concrete freeze-thaw damage deterioration by RHA is shown in Figure 10b. In the process of freeze-thaw cycles, with the increase in the RHA replacement ratio, the degree of cement paste compactness decreases. In addition, the introduction of a large amount of the RHA mesoporous structure causes a significant increase in the porosity and the number of cracks inside the concrete, and the increase in pore connectivity leads to the enhancement of specimen permeability, which makes the concrete more sensitive to freeze-thaw cycles.