Mechanical Performance, Thermal Resistance, and Durability of Red Mud-Based Cement Mortar Incorporating Fly Ash, Basalt Fibers, and Nano Zinc Oxide

Red mud (RM), a highly alkaline by-product of the alumina refining process, poses significant environmental challenges due to its large-scale accumulation. This study systematically investigates the mechanical performance, thermal resistance, and durability of RM-based cement mortar, evaluating the individual effects of fly ash (FA), basalt fibers (BF), and nano zinc oxide (NZO) as separate modifying constituents introduced independently into the RM matrix. Fourteen mortar mixtures were prepared with RM replacement levels of 10–35%, a constant FA content of 15%, BF dosages of 0.5–1.5%, and NZO dosages of 0.5–2.0%. Workability, 28-day compressive and flexural strengths, residual mechanical properties after exposure to 600 °C and 800 °C, sulfate resistance, and microstructural characteristics were evaluated. Increasing RM content reduced workability and mechanical strength, with compressive and flexural reductions reaching 35.5% and 29.0% at 30% RM, respectively. FA partially compensated through its ball-bearing effect and pozzolanic reactivity. The optimal 1.5% BF dosage increased the compressive and flexural strengths by 14.7% and 37.5%, respectively, and significantly enhanced the residual performance at elevated temperatures. The optimal 0.5% NZO improved the compressive and flexural strengths by 13.2% and 18.3%, respectively; however, 2.0% NZO caused complete strength loss, which may be associated with severe nanoparticle agglomeration and possible zinc-containing reaction products reported in previous studies. The formation of specific crystalline phases was not experimentally verified in the present study. Sulfate resistance deteriorated with increasing RM, whereas 1.5% BF and 1.0% NZO reduced mass loss by 58.9% and 59.7%, respectively. SEM confirmed that RM15 + FA15 exhibited the densest microstructure with minimal voids. The results demonstrate that RM can be effectively utilized as a sustainable cement replacement, with FA, BF, and NZO each independently identified as effective performance-enhancing constituents at their respective optimal dosages; their combined quaternary application remains untested and is proposed as a direction for future validation.