Synergistic Enhancement of Recycled Sand Concrete by Slurry-Coating Mixing and Nano-SiO2: Mechanical Properties, Durability, and ITZ Microstructure

To improve the performance of recycled sand concrete (RSC), this study investigated the synergistic effects of the slurry-coating mixing method and Nano-SiO2 (NS). The strength, durability, and microstructure of concrete with different recycled sand replacement ratios (RSR), mixing methods, and NS dosages were systematically examined. The results indicated that the strength and durability of concrete generally decreased with increasing RSR. Compared with the conventional mixing method, the slurry-coating mixing method increased the 7-day compressive strength by 8.9–13.9% and the 28-day compressive strength by 4.1–9.3%. Similarly, the addition of NS substantially improved both the mechanical strength and durability of concrete. The maximum increases reached 24.4% for 7-day compressive strength, 9.7% for 28-day compressive strength, and 18.2% for 28-day splitting tensile strength. Meanwhile, the electric flux, the mass loss after freeze–thaw cycles, and the relative dynamic modulus loss were noticeably reduced when either the slurry-coating mixing method or NS was applied. When both modification methods were combined, a pronounced synergistic enhancement effect was observed. The 7-day compressive strength increased by up to 43.2%, while the 28-day compressive strength increased by approximately 21%. In addition, the electric flux value decreased by up to 29.2%, and the freeze–thaw resistance was significantly improved. Microstructural observations revealed that the slurry-coating mixing method promoted the formation of a dense slurry layer on the aggregate surface, while NS enhanced hydration through its filling effect, pozzolanic reactivity, and nucleation effect. The combined action effectively densified the interfacial transition zone (ITZ) and optimized the pore structure, thereby improving the mechanical properties and durability of RSC. These results demonstrate that the synergistic modification of slurry-coating mixing and NS is an effective strategy for enhancing the overall performance of RSC. This study provides a promising approach for improving the performance of RSC and promoting sustainable construction materials.