Abstract: The high water absorption rate and porosity of recycled aggregates (RA) due to their loose and porous nature and the existence of the interfacial transition zone (ITZ) have limited the wide application of recycled aggregate concrete (RAC). The use of CO₂ and nano-SiO₂ for the synergistic strengthening of RA is an effective solution. This study conducted a synergistic strengthening treatment of RA using CO₂ carbonation and nano-SiO₂ impregnation and prepared RAC. The effects of the synergistic strengthening treatment on the physical properties of RA, the mechanical properties of RAC, and its resistance to chloride ion penetration were tested. Based on the synergistic strengthening effect, the influence of secondary curing systems on the mechanical properties of RAC was studied. The test results show that the synergistic strengthening treatment can effectively enhance RA, reducing its water absorption rate and crushing value by 18.2% and 34.5% respectively, and increasing its apparent density by 6.9%. The compressive strength and splitting tensile strength of RAC prepared based on the strengthened RA increased by 16.7% and 15.4% respectively, and the maximum reduction in the chloride ion diffusion coefficient was 24.7%. The "CO₂-CH solution" cyclic curing was found to be the most effective in enhancing the mechanical strength of RAC. SEM microstructure analysis indicates that the synergistic strengthening treatment improves the mechanical and chloride ion penetration resistance of RAC through the combined effects of "pore filling effect" and "interfacial transition zone strengthening effect".