Improving the bonding capacity of recycled concrete aggregate by creating a reactive shell with aqueous carbonation

Recycling end-of-life concrete back to construction industry in the form of aggregate has been a preferential solution to deal with the huge quantities of waste concrete. However, the recycled concrete aggregate (RCA) contains residual cement paste which adversely affects its properties and the resulting new concrete. Based on previous literature, enhancing the ITZ is one of the significant principles to improve the performance of the new concrete. Therefore, in this study, an innovative aqueous carbonation (AC) method was employed to treat modelled RCA, aiming at modifying the surface of the RCA to improve its bonding ability with new mortar and revealing the mechanism. Subsequently, AC was also applied to real RCA to validate its structural changes were compatible with the modelled RCA. Multi-scale investigations including thermogravimetric analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy line scanning, nanoindentation, solution chemistry analysis, etc. were conducted to characterize the structural changes and the reaction kinetics. The experimental results showed that the surface layers of RCA were significantly modified after AC. A shell-core structure was formed, which included, starting from the outermost region of the RCA, a calcite coating, a silica-rich layer, a partially decalcified layer, and an uncarbonated matrix. The surface modification significantly improved the frictional and tensile bonding strength of RCA with the new mortar by triggering physicochemical changes to the interface. Generally, AC can be considered as an efficient approach for improving the bonding capacity of RCA and therefore promoting the recycling of waste concrete.