## 4 Conclusions  
In this paper, the impact of RHA on the mechanical characteristics and freeze-thaw resistance of recycled concrete was investigated, and the mechanism of the freeze-thaw damage deterioration of recycled concrete by RHA was revealed by combining SEM experiments. The main conclusions are as follows:  
Figure 10: (**a**) Schematic diagram of the component phases of RCA; (**b**) Schematic of the deterioration mechanism of RHA on recycled aggregate concrete freeze–thaw damage.  
1. The gradual increase in the RHA replacement ratio increased the cementitious material's total specific surface area and porosity, which will increase the mixtures' friction resistance and discourage the flow, which in turn leads to the decrease in the concrete slump and the reduced workability.
2. With the continuous hydration reaction process, the compressive strength of recycled concrete increased with the growth of the curing age, and the growth rate shows a trend of first being rapid, then slowing down and finally gradually stabilizing. In addition, the optimum compressive strength of recycled concrete is observed with a 20% RHA replacement ratio.
3. With the gradual increase in the number of freeze-thaw cycles, the mass loss of recycled concrete showed a tendency to first decrease and then increase, while the relative dynamic elastic modulus showed a continuous decreasing trend.
4. Compared with ordinary concrete (without RHA), the RHA particle's high specific surface area will promote the cement hydration and the formation of C-S-H gel by providing nucleation positions before the freeze-thaw cycle. During the process of the freeze-thaw cycle, the existence of the recycled aggregate attached mortar's porous structure will cause a large amount of water absorption. With the continuous accumulation of expansion pressure, the internal pores and microcracks will gradually expand and extend, which will provide a channel for water migration, thus resulting in recycled concrete undergoing more severe freeze-thaw damage, and the degree of freeze-thaw damage deterioration grows as the RHA replacement ratio increases.
5. In addition, more exploration and experimentation in other durability aspects are needed to expand the application of RHA in recycled aggregate concrete.  
Conceptualization, W.Z. and H.L.; methodology, W.Z. and H.L.; software, W.Z.; validation, W.Z.; investigation, W.Z.; resources, W.Z.; writing--original draft preparation, W.Z.; writing--review and editing, C.L; visualization, W.Z.; supervision, W.Z.; project administration, W.Z. and C.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Grant NO. 51878546, Grant NO. 52178251), the Science Foundation Project for Outstanding Youth of Shaanxi Province (Grant NO. 2020 JC-46), the Key Research and Development Projects of Shaanxi Province (Grant NO. 2020 SF-367) and the Xi'an Science and Technology Plan Key Industrial Chain Core Technology Project (Grant NO. 2022)H-ZCZC-0026).  
Not applicable.  
The authors thank the editor and reviewers very much for their comments and helpful suggestions.  
The authors declare that they have no conflict of interest to report regarding the present study.