Review on upcycling of RCA via carbonation treatment for structural concrete

The utilization of recycled concrete aggregate (RCA) in concrete conserves resources, reduces waste, and minimizes environmental impact. However, RCA suffers from high water absorption, low crushing value, and weak bonding with new concrete due to old adhered mortar, limiting its use to nonstructural applications. To enable structural use, efforts focus on either removing adhered mortar or reinforcing it. Among reinforcement methods, carbonation treatment has emerged as promising approach due to its environmental, economic, and technical advantages. This paper reviews recent advances in carbonation treatment of RCA and its integration into structural concrete. It discusses: (1) carbonation methods, including pressurized and wet carbonation, and their chemical processes; (2) changes in microstructure and physical properties of RCA after carbonation; (3) effects on workability, mechanical strength, shrinkage, and durability of RCA-based concrete; (4) comparison with other treatment methods; and (5) innovations to increase carbonation degree. This review encourages sustainable, structural use of RCA.