Comparative effects of direct carbonation and pre-soaking limewater carbonation on recycled coarse aggregates: physic-mechanical properties and concrete performance

Amidst the escalating scarcity of resources and the burgeoning accumulation of construction waste, it is imperative to investigate the performance of carbonized recycled coarse aggregate concrete to enhance the utilization of construction solid waste and advance the adoption of environmentally sustainable low-carbon building materials. In this study, comparative effects of direct carbonation and pre-soaking limewater carbonation on the physical and mechanical properties of the recycled coarse aggregate (RCA) and recycled concrete are systematically analyzed. The carbonation mechanism is elucidated through the water absorption, crushing value, apparent density test, XRD, TG, SEM, and other microscopic characterization techniques. The results indicate that pre-soaking limewater carbonation surpasses direct carbonation, yielding more significant improvements in the water absorption (reduced by 22.9 %), crushing value (reduced by 17.7 %), and apparent density (increased by 4.1 %). These enhancements are attributed to the additional Ca²⁺ provided by limewater, which facilitates calcium carbonate precipitation to filling pores and optimizing the interface transition zone (ITZ). Regarding recycled concrete performance, the compressive, axial compressive, splitting tensile, and flexural strengths of recycled concrete prepared with pre-soaking limewater carbonized RCA are superior to those of direct carbonation group at both 7 d and 28 d ages, which is more obvious at high replacement rate. The underlying mechanism involves the carbonation products strengthening ITZ bonding and refining pore structure. The study confirms that pre-soaking limewater carbonation is an efficient technology for enhancing the properties of recycled aggregates, providing theoretical support for the development of high-performance and sustainable recycled concrete applications.