Synergistic modification of recycled aggregate using carbonation and hydrophobic Nano-Silica: Effect on interfacial performance of recycled aggregate concrete

Recycled aggregate (RA), due to the adhered old mortar on its surface, generally exhibits high water absorption and porosity, which consequently weakens the mechanical properties and durability of recycled aggregate concrete (RAC). To enhance the performance of RA and optimize the microstructure of RAC, this study proposes a carbonation–hydrophobic nano-silica synergic modification method. Initially, RA undergoes carbonation treatment to densify its microstructure. Subsequently, hydrophobic nano-silica is applied to modify the carbonated RA, forming a hydrophobic film on its surface, which significantly reduces water absorption and enhances the interfacial transition zone (ITZ) between RA and new concrete matrix. This study systematically analyzes the effects of synergic modification and singular modifications (nano-silica modification, hydrophobic nano-silica modification, and carbonation modification) on the water absorption, porosity, phase composition and surface morphology of RA. Furthermore, the influence of synergic modification on the mechanical properties, pore structure, microstructure, and phase evolution of RAC is explored. The results demonstrate that carbonation treatment and hydrophobic nano-silica modification exhibit a synergistic effect, which effectively reduces the water absorption and porosity of RA while further improving the pore structure and interfacial properties of RAC. Compared to plain RA, RAC incorporating 50 % synergic modified RA achieves a 34.9 % and 28.4 % increase in compressive strength at 7 and 28 days. This study provides a promising approach for the efficient modification of RA and offers a theoretical basis for its practical engineering applications.