Mechanical performance of basalt fiber reinforced cementitious mortars incorporating calcined red mud

The utilization of industrial by-products in cement-based materials is increasingly critical for reducing the environmental impact of Portland cement. Red mud, a by-product of the Bayer process, presents significant disposal challenges due to its large-scale generation and limited reuse potential. This study examines the combined effects of red mud calcination temperature and basalt fiber incorporation on the mechanical and microstructural performance of cement mortars. Red mud was used as a 10 and 20 wt% cement replacement in untreated form and after calcination at 600 °C and 800 °C. Basalt fibers were added at 0.15 and 0.30 vol%. Compressive strength decreased with increasing red mud content, falling by up to 23.7% at 20% uncalcined red mud replacement and by up to 39.4% at 20% replacement calcined at 800 °C relative to the control mixture, attributed to clinker dilution and limited pozzolanic activity. In contrast, basalt fibers enhanced flexural strength by up to 23% in the fiber-free series, and consistently compensated much of the tensile-related strength loss caused by red mud incorporation, through crack-bridging, while producing more moderate gains (up to 12%) in compressive strength. Microstructural analyses (X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP)) showed that 600 °C treatment refined pore structure, reducing the fraction of pores larger than 100 nm relative to the uncalcined mixture, whereas 800 °C increased macroporosity to 27.6%, the highest among all mixtures, coinciding with the lowest compressive strength (28.76 MPa). Mechanical performance was primarily governed by hydration product volume and critical pore fraction.