The investigated commercially available continuous basalt fibres fall short of selected glass fibres in their elastic and plastic response to tensile load at elevated temperatures. The onset temperature of unlimited elongation equals approximately 580 and 640, or 840 and 700_ C for the basalt or glass fibres, respectively. The best (R-glass) fibre retains its modulus up to 450_C and loses but 8 % of its room-temperature value at 600_C, while the basalt fibres reveal significant losses of modulus (10-13 %) even at 450_C. Behaviour of the basalt fibres may be affected by onset of crystallization, which was detected by X-ray diffraction after heat treatment to 750_C.
