Mechanism of the influence of basalt fiber parameters on the mechanical properties and microstructure of high-volume red mud-cement paste.

Red mud (RM) is a highly alkaline solid waste generated by the alumina industry. The incorporation of high-volume red mud into cement-based materials often leads to mechanical degradation and microstructural instability, highlighting an urgent need for verifiable enhancement pathways and mechanistic explanations. Based on the established red mud-cement paste system, this study systematically evaluates the effects of red mud content on the mechanical properties and microstructure of the cement-based materials. Furthermore, a full-factorial design considering the “length × dosage” parameters of basalt fibers (BF) is implemented to investigate the evolution patterns of compressive and flexural strengths at 3, 7, and 28 days, as well as their correlations with structural characteristics. The results indicate that without fiber reinforcement, 30% red mud content reduces the 28-day compressive strength from 55.5 MPa to 40.2 MPa, with a slight decrease in flexural strength. Upon fiber incorporation, the effects exhibit phased evolution: At 3 days, fiber length and dosage jointly drive compressive strength improvement while having limited impact on flexural strength; At 7 days, synergistic interaction between length and dosage emerges, showing combination-dependent effects; At 28 days, fiber length dominates compressive strength development while dosage primarily governs flexural strength. Among the investigated fiber-reinforced 30% RM mixtures, the combination of 12 mm fiber length and 0.1% fiber dosage provided the best overall balance of mechanical performance. Although its 28-day compressive strength of 38.95 MPa was slightly lower than that of the fiber-free R30 specimen of 40.20 MPa, its flexural strength increased by 0.6 MPa, and the reinforced specimen exhibited fewer macroscopic cracks and better structural integrity after failure. The microscopic analysis reveals that the incorporation of red mud shifts the hydration products from predominantly crystalline phases to amorphous and layered structures. The fibers contribute through bridging and pull-out energy dissipation, while their interfacial interlocking mechanism suppresses continuous crack propagation and enables performance compensation. This study establishes a replicable evaluation framework and provides operable proportioning windows for parameter optimization and mechanistic analysis in high-volume red mud-cement paste systems.