Morphologies and mechanical properties of basalt fibre processed at elevated temperature

• Morphologies and properties of basalt fibre processed at different temperatures were studied. • Reduction on the strength of fibre upon thermal treatment occurred in three steps. • Thermal treatment led to the formation of crystal on fibre surface. • Crystallization on fibre began with the spontaneous formation of spinel phase. • Spinel crystal was the nucleation site for the formation of pyroxene structure. In this study, the morphologies and properties of basalt fibre (BF) treated at different temperatures in air atmosphere were studied. The results showed that the heat treatment caused the formation of crystals on fibre surface, which resulted in the alteration of BF from the paramagnetic material to the ferromagnetic one. The tensile strength of BF was reduced significantly upon thermal treatment, and such reduction occurred in three temperature ranges: i) Decomposition of organic sizing on fibre surface, which led to the exposure of more defect structures (100–400 °C), and the structural changes of BF may also occur; ii) Simultaneous relaxation of excessive enthalpy and structural anisotropy, which were responsible for the phase separation in the amorphous matrix in the fibre (500–600 °C); iii) Structural modification due to the formation of multifarious crystals in the fibre (700–1000 °C). The crystallization in BF began with spontaneous spinel phase formation, which was favored by the initial phase separation and the oxidation of Fe 2+ . The spinel crystals became nucleation sites for the formation of pyroxene structure, and this process was accompanied by the enrichment of Ca, Mg and Fe on fibre surface. Therefore, the inhibition of crystallization for BF was important to maintain the mechanical performance of filament under the continuous thermal conditions for practical applications.