This study aims to investigate the effects and underlying mechanisms of recycled sand replacement rate, additional water compensation factor, and recycled powder content on the strength, volumetric water absorption, and impermeability of recycled sand concrete. A total of 12 groups of concrete specimens with different mixtures were tested for their mechanical properties, volumetric water absorption, and chloride ion penetration. Furthermore, NMR and SEM analyses were conducted to reveal the microstructural mechanisms by which the additional water level and recycled sand content influence the mechanical performance and durability of the concrete. The results indicate that although recycled sand particles inherently contain numerous micro cracks, adhered porous cement paste, and pre-existing interfaces that enhance capillary water absorption and lead to reductions in strength and durability, these shortcomings can be mitigated by compensating for the additional water and controlling the recycled powder content. Increasing the additional water slightly reduces the strength of the recycled sand concrete. More importantly, appropriate amounts of additional water can reduce water absorption and improve the penetration resistance of recycled sand concrete. Furthermore, with an increase in recycled sand content, the strength, and impermeability of the concrete first increase and then decrease, reaching their maximum values at a recycled powder content of 4%. The water absorption of recycled sand concrete gradually increases with higher recycled powder content. Overall, recycled sand concrete can achieve satisfactory performance by optimizing the additional water amount and recycled powder content. It is recommended that the pre-saturation water compensation factor of recycled sand be maintained at 70%–80% of its 24 h saturated water absorption, and that the recycled powder content be controlled within 4%–8%.

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.