Synergistic multiscale reinforcement of sand concrete incorporating steel fibers and granulated blast-furnace slag

Sand concrete is increasingly recognized as a resource-efficient cementitious material; however, its widespread adoption in load-bearing applications remains limited by relatively low mechanical strength and brittle behavior. This study investigates a multiscale reinforcement strategy combining macro-scale hooked-end steel fibers and micro-scale ground granulated blast furnace slag (GBFS) incorporated as an additive granular filler, aiming to simultaneously enhance strength, toughness, and crack resistance. Twelve mixtures were designed with systematic variation of steel fiber volume fraction (0%, 0.5%, 1.0%, and 1.5%) and GBFS filler-to-cement ratio (F/C = 66% and 80%). Mechanical properties, durability indicators, and microstructural characteristics were evaluated using standardized experimental protocols. The optimal combination (Mix 10: F/C = 80% GBFS filler with 0.5% steel fibers) produced increases in compressive, splitting tensile, and flexural strengths of 58.4%, 33.6%, and 25.1%, respectively, relative to the reference mixture. For this mixture, a Synergy Index exceeding 1.0 for all mechanical properties confirms a genuinely synergistic interaction beyond simple additivity. Fiber incorporation altered the failure mode from brittle to quasi-ductile, substantially improving energy dissipation capacity. Microstructural observations confirm that GBFS densifies the matrix, refines the fiber–matrix interfacial transition zone, and enhances fiber anchorage efficiency. Higher superplasticizer dosages were required to maintain workability, and certain durability parameters exhibited sensitivity to fiber content. These findings demonstrate the promising potential of the proposed composite system for structural sand concrete applications, subject to validation across diverse material sources and exposure conditions.