Effects of pressurized carbonation with presoaking in calcium hydroxide solution on the fracture behaviours of recycled coarse aggregate concrete

Abstract

Carbonation is effective in enhancing recycled coarse aggregate, but the shortage of carbonizable Ca2+ in recycled coarse aggregate and the low permeability efficiency of CO2 at ambient pressure limit the carbonation efficiency and industrial application. To solve these issues, the Ca2+ content in recycled coarse aggregate and the carbonation effectiveness were increased by presoaking recycled coarse aggregate in calcium hydroxide and enhancing the carbonation pressure, respectively. The optimal carbonation condition was obtained based on the physical properties of the recycled coarse aggregate and the strength of recycled coarse aggregate concrete: the carbonation pressure was 4 bar, and the concentration of the calcium hydroxide solution was 0.04 mol/kg. The fracture behaviour and microstructure of the recycled coarse aggregate concrete under different carbonation conditions were evaluated. The recycled coarse aggregate concrete after pressurized carbonation with presoaking in calcium hydroxide solution (RCAC-C0.04-P4) has higher toughness and a more stable crack propagation process than untreated recycled coarse aggregate concrete. Pressurized carbonation with presoaking in calcium hydroxide solution significantly increased the initial cracking load, peak load, Young's modulus, initial toughness, unstable toughness, and fracture energy of the recycled coarse aggregate concrete by 67.35%, 2.83%, 12.19%, 60.77%, 2.31%, and 5.38%, respectively, and reduced the critical crack mouth (CMODc) and tip opening displacement (CTODc) by 9.13% and 10.96%. Simple pressurized carbonation made the toughness of the specimens worse than that of untreated recycled coarse aggregate concrete. Compared with untreated recycled coarse aggregate concrete, although the initial toughness, unstable toughness, and fracture energy of recycled coarse aggregate concrete after simple pressurized carbonation (RCAC-P4) were increased by 16.02%, 1.86%, and 0.28%, respectively, the Young's modulus was slightly reduced by 1.35%, and the CMODc and CTODc were increased by 5.37%. In addition, a microstructural analysis showed that the contents of Ca(OH)2 and CaCO3 of RCAC-C0.04-P4 were increased by 50.56% and 37.62% compared to untreated recycled coarse aggregate concrete, respectively, and the average indentation modulus of the old aggregate and old mortar interface of RCAC-C0.04-P4 was increased by 24.37% compared to untreated recycled coarse aggregate concrete, which confirmed that pressurized carbonation with presoaking in a calcium hydroxide solution made CaCO3 fill the cracks in the interface and improved the macroperformance. These results can promote the application of recycled coarse aggregate in engineering.