## 5 Conclusions  
In this study, three generations of RRAC were prepared from the multiple recycling of C&DW concrete with different RRCA replacement rates (25%, 75%, and 100%). The properties of fresh and harden concrete, carbonation behavior, and micro-morphologies of three generations of RRAC before and after freeze-thaw cycles were investigated. The conclusions can be drawn from this research as follows:  
* The workability and compressive strength of RRAC decreased with the increase of RRCA replacement rate and recycling number. The workability and compressive strength of RRAC were inferior to that of NAC, but could still reach the target slump value of 150 mm and strength of 40 MPa. It was feasible to prepare RRAC using multiple recycling RCA for structural use in terms of strength;
* The RRAC under different replacement rates and different recycling numbers showed diversity in carbonation resistance. The increasing of both the replacement rate and recycling number resulted in the degradation of RRAC carbonation resistance. But RAC-3-25 and RAC-2-75 displayed a comparable carbonation resistance to the RAC-1-100 one. For the multiple recycling RAC, it could achieve similar carbonation resistance as the previous generation's by reducing the substitution rate;
* The degradation of RRAC carbonation resistance was attributed to the continuous dropping in RRCA quality and weak ITZs caused by RRCA in concrete. But at a low replacement rate and low recycling number, such as RAC-1-25, the carbonation resistance of RAC was comparable or even superior to the NAC one. This resultedfrom the attached mortar on the surface of RCA which reacted with CO\({}_{2}\) to refine the pore structures of RAC;
* The RRAC specimens after freeze-thaw cycles were more susceptible to be carbonated due to the freeze-thaw damages. The spalling of cement mortar and micro-cracks as a consequence of freeze-thaw cycles aggravated the degradation of carbonation resistance of RRAC, especially at a high replacement rate (\(\geq\)75%) and recycling number (\(\geq\)two generations). This was verified by results of SEM analysis;
* The carbonation depth of RRAC after freeze-thaw cycles was much lower than the protective thickness of reinforced concrete in the design code. All three generations of RRAC could be used in micro-frozen regions for engineering applications.  
Conceptualization, H.L. and P.Z.; methodology, P.Z.; validation, H.L., M.H., and P.Z.; formal analysis, C.C.; investigation, X.W.; resources, Z.Q.; data curation, Y.D.; writing--original draft preparation, M.H.; writing--review and editing, H.L. and M.H.; visualization, M.H.; supervision, P.Z.; project administration, P.Z.; funding acquisition, H.L. and P.Z. 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 number 52078068, State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology), grant number SYSJJ2020-14, and Changzhou Science and Technology Project, grant number CJ20200079.  
Not applicable.  
The authors thank Jiangsu Lvhe Environmental Technology Co. Ltd. for providing the recycled concrete aggregates.  
The authors declare no conflict of interest.