Effect of calcium phosphate modification on the interfacial transition zone of recycled aggregate and concrete

Recycling construction solid waste is an immediate need due to the substantial amount of construction waste that occupies valuable land and poses environmental hazards. In response to challenges related to the performance and interfacial transition zone (ITZ) when using recycled aggregate (RA) in concrete, a modified formulation with low calcium phosphate (CaP) content was developed to enhance RA's performance. It was found that when the mass ratio of CaP to RA reached 0.001 g/g, the physical properties of RA were significantly improved through a precoating treatment, resulting in a 31.52% increase in the compressive strength of recycled aggregate concrete (RAC) compared to the control group. An in-depth analysis of the mechanism of action of CaP revealed that the active layer formed by CaP on the RA surface promoted the slow hydrolysis-hydration coupling reaction through the sustained release of Ca²⁺ and PO₄^3- ions. This process and the solid-phase reactions of C-S-H gel, carbonation products, and sulfate ions effectively improved the pore structure, increased the specific surface area by 47.36%, and reduced crack width by 78.95%. Furthermore, the bond strength test between modified RA and freshly hardened mortar showed a 30.29% increase in bond strength, directly demonstrating the effectiveness of CaP in enhancing the ITZ in RAC.