Carbonation behavior and mechanical performance of low-carbon recycled concrete under different CO2 pressures

The construction industry faces challenges from high cement-related carbon emissions and construction waste. To address carbon reduction, sequestration, and waste utilization, accelerated carbonation tests were conducted on low-carbon recycled concrete (LCRC) to evaluate the effects of recycled aggregate (RA) replacement ratio of natural aggregate (NA), cement replacement ratio by ground granulated blast-furnace slag (GGBS), and external CO 2 pressure on carbonation depth and compressive strength. Microstructural analyses (SEM, EDX, MIP, and TGA-DSC) were conducted to analyze the microstructure, elemental distribution, pore structure, and compositional changes in LCRC before and after carbonation. Results showed that the LCRC carbonation depth increased and the compressive strength decreased with the increase of the RA and cement replacement ratios. The elevated external CO 2 pressure significantly accelerated carbonation, enhancing both carbonation depth and compressive strength. Under supercritical condition (7.5 MPa), the carbonation depth of the LCRC increased by an average of 164 % compared to samples at 0.1 MPa, while the average compressive strength was 47 % higher than that of the uncarbonated samples. Carbonation converts flocculent C-S-H gel into CaCO 3 , refining the pore structure and reducing porosity by 28–34 %. Based on the TGA result difference inversion, a method for determining Ca(OH) 2 and C-S-H contents in LCRC was proposed, revealing pre-carbonation contents of 17–37 % and 63–83 %, respectively. The maximum carbon uptake capacity reached 96.09 kg CO 2 /m 3 of LCRC, highlighting its potential to support a low-carbon circular concrete industry. • Systematic LCRC carbonation tests were conducted from atmospheric to supercritical conditions. • The average compressive strength of LCRC increased by 47 % under supercritical conditions. • A model was proposed to determine C-S-H and Ca(OH) 2 contents in LCRC. • LCRC can sequester up to 64.24 kg CO 2 /m 3 after supercritical carbonation treatment. • The accelerated carbonation mechanism of LCRC under different CO 2 pressures were revealed.