<p>Sodium bicarbonate (NaHCO₃) solution can improve carbon sequestration efficiency and early mechanical properties of recycled aggregate concrete (RAC), but the effect of solution concentration on the mechanical properties of RAC is not yet clear. This paper aims to fill up this research gap by conducting cube and prism compression tests and four point bending tests on 72 specimens with different concentrations of NaHCO₃ solutions (0–10 g/L) and carbonation environments (natural environment, carbonation box, standard curing box, water environment, NaHCO₃ solution) as the main variables. The carbon fixation amount was evaluated through thermogravimetric analysis. The results showed that the presence of NaHCO₃ enhanced the mechanical properties of RAC in water environment, especially at a concentration of 10 g/L. As the concentration of NaHCO₃ increases, the carbon sequestration capacity of RAC gradually increases and is greater than that in the carbonation box. In addition, the strength index showed a trend of first decreasing and then increasing, with the maximum increase in compressive strength and flexural strength of RAC reaching 12.1% and 32.0%, respectively. A novel strength calculation method and constitutive model for NaHCO₃-carbonated RAC were developed and validated against both experimental data and existing literature. This research provides crucial insights into optimizing NaHCO₃ solution carbonation for RAC, offering a promising pathway for enhancing both mechanical performance and CO₂ sequestration in recycled concrete materials. The findings have significant implications for sustainable construction practices and carbon mitigation strategies in the concrete industry.</p>