Structural and Chemical Stability of TiO2-Doped Basalt Fibers in Alkaline and Seawater Conditions

Alkali resistance is a critical factor for the long-term performance of glass fibers in cementitious composites. While zirconium oxide doping has proven effective in enhancing the durability of basalt fibers, its high cost and limited solubility motivate the search for viable alternatives. This study presents the first systematic investigation of titanium dioxide (TiO2) doping in basalt-based glasses across a wide compositional range (0–8 mol%). X-ray fluorescence and diffraction analyses confirm complete dissolution of TiO2 within the amorphous silicate network, with no phase segregation. At low concentrations (≤3 mol%), Ti4+ acts as a network modifier in octahedral coordination ([TiO6]), reducing melt viscosity and lowering processing temperatures. As TiO2 content increases, titanium in-corporates into tetrahedral sites ([TiO4]), competing with Fe3+ for network-forming positions and displacing it into octahedral coordination, as revealed by Mössbauer spectroscopy. This structural redistribution promotes phase separation and triggers the crystallization of pseudobrukite (Fe2TiO5) at elevated temperatures. The formation of a protective Ti(OH)4 surface layer upon alkali exposure enhances chemical resistance, with optimal performance observed at 4.6 mol% TiO2—reducing mass loss in NaOH and seawater by 13.3% and 25%, respectively, and improving residual tensile strength. However, higher TiO2 concentrations (≥5 mol%) lead to pseudobrukite crystallization and a narrowed fiber-forming temperature window, rendering continuous fiber drawing unfeasible. The results demonstrate that TiO2 is a promising, cost-effective dopant for basalt fibers, but its benefits are constrained by a critical solubility threshold and structural trade-offs between durability and processability.