Insights into Performance Enhancement of Recycled Sand Concrete via Water Compensation and Recycled Powder Regulation

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%.