Performance Evolution of Mass Concrete Under Multi-Factor Coupling Effects: Influence of Manufactured Sand, Water–Binder Ratio, and Fly Ash

This study evaluates the feasibility of utilizing manufactured sand as a full or partial replacement for river sand in mass concrete production, motivated by the growing scarcity of natural river sand and stringent environmental regulations on mining. The influence of the manufactured sand replacement level, water-to-cement ratio, and fly ash content on key properties including workability, mechanical strength, early-age shrinkage, and thermal stress was systematically investigated. The results demonstrate that, while the incorporation of manufactured sand marginally impairs workability, it contributes to an improved particle size distribution of the fine aggregate. At 100% replacement, the 56-day compressive, flexural, and tensile strengths, as well as the elastic modulus of manufactured sand concrete, exceed those of river sand concrete, accompanied by a notable reduction in early-age shrinkage. A decrease in the water–binder ratio enhances mechanical performance but concurrently elevates the risk of cracking due to the increased autogenous shrinkage and adiabatic temperature rise associated with a higher cement content. The addition of an optimal amount of fly ash (e.g., 25%) effectively improves both workability and mechanical properties while substantially mitigating hydration heat, thereby reducing temperature differentials and the associated cracking risks. Microscopic analysis reveals that unhydrated particles, including fly ash and quartz, may act as initial defects within the microstructure. Overall, the replacement of river sand with manufactured sand in mass concrete is technically feasible, and an appropriate mix design optimization can achieve a desirable balance between performance and crack resistance.