Enhancement of recycled concrete aggregate through slag-coated carbonation

Abstract

This study aims to enhance the utilization rate of recycled concrete aggregate (RCA) in structural concrete through the application of a novel carbonation treatment method to improve the microstructure of old adhered mortar in RCA and enhance its compatibility with the new concrete. Carbonation of RCA is a commonly used method to densify the microstructure of old adhered mortar, but it exhibits limitations due to the low available calcium content in RCA for reacting with CO₂. To address this, a novel approach by coating the RCA with blast furnace slag before subjecting it to carbonation was proposed. This involves two key benefits: (1) it introduces external calcium, thereby increasing the CaCO₃ content after carbonation and densifying the microstructure, and (2) it provides silica, facilitating a pozzolanic reaction that enhances bonding with new concrete. The effectiveness of pressurized and wet carbonation methods was evaluated and compared in the research. Following carbonation of the slag-coated RCA, the microstructure densification and improvement in interfacial properties between RCA and new cement paste were assessed through water absorption and slant shear test, respectively. The underlying mechanism was investigated by TGA, XRD, and SEM-EDS. The findings indicate that slag coating significantly enhances microstructure densification, reducing water absorption by up to 40% and increasing bond strength by up to 65% after carbonation. Pressurized carbonation enhances CO₂ penetration and dissolution in RCA, increasing CaCO₃ production and improving the microstructure. It also produces more needle-like aragonite, strengthening the bond between RCA and new concrete.