The effects of carbonation conditions on the physical and microstructural properties of recycled concrete coarse aggregates

•Inefficiency of carbonation under wet atmosphere (i.e., RH > 95%).•Considerable refinement of RCA pore structure as a result of carbonation at RH 40–70%.•Significant improvement of RCA resistance to freeze-thaw cycles due to carbonation.•Transformation of the interfacial transition zone as a result of carbonation. In this research, the effects of carbonation conditions on the physical, mechanical, microstructural and durability properties of recycled concrete aggregate (RCA) are investigated. The results suggest that regardless of the carbonation conditions, carbonation of RCA under wet moisture condition or moist carbonation atmosphere results in no meaningful drop in water absorption or porosity. However, carbonation of RCA (preconditioned at ≤95% relative humidity) in the absence of external source of water vapor results in significant drops in water absorption (up to 27%), porosity (27%), micro-Deval wear factor (40% regression) and freeze–thaw resistance (77% regression). Carbonation of RCA under optimal conditions was found to densify the exterior surface of hydrated cement within the RCA grains through progressive deposition of carbonates on the exterior of RCA grains. Carbonation also transforms the transition zone between the old cement and aggregates and results in formation of a dark rim around alite and belite grains indicating reactivation of the residual unhydrated cement fraction.