Effect of various parameters on carbonation treatment of recycled concrete aggregate using the design of experiment method

Recycled concrete aggregate (RCA) comprises old mortar adhered having several interfacial transition zones that induces inferior properties in the concrete. This study used pressurized carbonization to strengthen the performance of RCA, and the effects of aggregate size (5–10 and 10–20 mm), carbonization pressure (0.5, 1, and 2 Bar) and time (2 and 4 h) on the performance of RCA and prepared concrete were investigated. The physical properties of RCA, including water absorption, loose and bulk density, and aggregate crushing value were determined. Meanwhile, the workability and mechanical properties of concrete prepared by 100% RCA were investigated. The Design of the Experiment method, a statistical approach, was applied to identify the optimized combination parameters to obtain better carbonation and thus higher strength of concrete mixed with RCA. For RCA, the best optimum carbonation regime was 1 Bar and 2 h (pressure and duration), and further increased carbonization pressure and time was detrimental to further development of RCA performance. When a carbonization pressure of 1 Bar and a carbonization time of 2 h were applied, water absorption and aggregate crushing value values of carbonized RCAs were decreased by 48.8%–49.3% and 9.7%–12.8%, respectively than uncarbonized RCA. The compressive strengths of concrete prepared by 100% carbonized RCA with a size of 5–10 and 10–20 mm were increased by 27.63% and 27.5% and split tensile strengths were increased by 11.7% and 10.6%, respectively compared to the concrete prepared by uncarbonized RCA. The results of microstructure analysis also showed that using the appropriate carbonization regime was beneficial to improve the denseness of the RCA by generating carbonate. Therefore, by adjusting the RCA and carbonized parameters, it can effectively strengthen the performance of the concrete prepared by RCA, so as to provide technical guidance for actual engineering.