## 3 Results and Discussions
### Mechanistic Analysis of the Influence of RHA on the Microstructure of Recycled Aggregate Concrete  
Figure 8 shows the influence of different RHA replacement ratios on paste samples' microstructure and hydration products before the freeze-thaw cycles. As shown in Figure 8a, the 0% RHA paste is characterized by significant amounts of rod-shaped CH crystals and numerous pores in the matrix, and the surface microstructure is relatively loose. This is probably because the insufficient hydration of cement particles leads to a decrease in the degree of paste compactness, which agrees with previous research results [45; 46]. The 10% RHA paste is characterized by a dense paste without obvious pores in the matrix, and cracks of significant width were found in the interfacial transition zone. The 20% RHA paste is characterized by dense paste at different locations in the matrix, small microcracks between the interfacial transition zone of the aggregate and mortar and no significant CH crystals in the paste. This may be due to the smaller particle size of RHA particles, which can promote cement hydration due to their high specific surface, resulting in a pozzolanic reaction between the CH generated by the cement hydration and the amorphous silica of the RHA to reach a threshold value, leading to the optimum compacting effect of the generated C-S-H gels on the paste pores and enhancing the bonding of the interfacial transition zone, reducing the formation of cracks, which will contribute to the mechanical characteristics of concrete enhancement [47; 48]. As shown in Figure 8d, the 30% RHA paste is characterized by large pores and a large number of randomly distributed needle-shaped AFTs in the matrix. This may be due to the lower cement content, which leads to a reduced CH content generated by cement hydration, which in turn leads to a weaker degree of pozzolanic reaction, which is not conducive to pore compactness. In conclusion, the compactness degree of the paste samples showed an increase followed by a decrease with an increasing RHA replacement ratio, which corresponds to its compressive strength magnitude. The microscopic morphology of paste samples with different RHA replacement ratios after undergoing freeze-thaw cycles is presented in Figure 9. As shown in Figure 9a, the 0% RHA paste shows a relatively dense microstructure with a small number of pores. It can be seen from Figure 9b that a penetrating crack was found on the surface of the 10% RHA paste sample, which may be caused by human factors during the sample preparation process [40; 49]. Figure 9c shows that 20% RHA paste is characterized by a noticeable  
Figure 8: Microscopic morphology of the paste samples with different RHA replacement ratios before the freeze–thaw cycles. increase in the number of pores as well as a large number of staggered cracks in the cement paste matrix, which may be because, although the low content of RHA can improve the paste compactness and the bonding of the interfacial transition zone, it cannot compensate for the lack of its porous properties, which will cause a reduction in the freeze-thaw resistance of the concrete. As shown in Figure 9d, the surface of the 30% RHA paste sample showed significantly porous characteristics, and the mortar matrix appeared brittle, indicating that the addition of a large number of RHA would further deteriorate the compactness degree of the cement paste. This is probably because of the recycled aggregate attached mortar's porous structure, which increases the water absorption of the concrete. In addition, a reduction in cement content leads to a weakening of the paste densification. This leads to a more significant ic pressure generated inside the concrete subjected to the freeze-thaw cycles, an increase in the number of pores and cracks in the paste due to the expansion pressure and further damage to the internal structure, which is macroscopically manifested by the increase in mass loss and the decrease in dynamic elastic modulus.