Influence of binder-to-sand ratio on mechanical and durability properties of ultra-high-performance concrete reinforced with novel polyoxymethylene fiber

Ultra-high performance concrete (UHPC) faces cost and shrinkage challenges due to high binder contents. This study systematically investigates the influence of varying binder-to-sand (B/S) ratios (0.8, 0.9, 1.0, and 1.2) coupled with novel polyoxymethylene fibers (POMFs, 0 and 2% dosages) on UHPC properties. A fixed water-to-binder ratio (0.17) and 50% ground granulated blast-furnace slag replacement were used. Freshness, mechanical properties, durability, and microstructural properties were evaluated. Results indicate that the B/S ratio critically governs UHPC performance. Contrary to the assumption that higher binder content is always superior, an optimal balance emerged at a B/S ratio of 0.9. This ratio yielded the highest 28-day compressive and splitting tensile strengths, the lowest water absorption, and exceptional sulfate resistance, with no strength degradation after 180 days. A low B/S ratio (0.8) caused insufficient aggregate coating, increasing porosity, and reducing strength. Conversely, an excessive B/S ratio (1.2) induced severe autogenous shrinkage and microcracking, impairing durability and strength. While a 2% POMF addition provided limited gains in compressive strength, it significantly improved splitting tensile strength and shifted the failure mode from brittle to ductile. Scanning electron microscopy confirmed that POMF’s hydrophilic nature ensures excellent interfacial bonding with the matrix. Ultimately, a B/S ratio of 0.9 provides the optimal eco-efficient mix design, balancing high performance with material economy for UHPC engineering applications.