Reactivity has been studied of SiO 2 fume from ferrosilicon production with Ca(OH) 2 in suspension, with stirring, under hydrothermal conditions at 190°C and 200°C from 1 to 12 hours. Reaction product after a 4-hour synthesis at 200°C is calcium silicate hydrate xonotlite C 6S 6H ★, with some C-S-H(I) phase. When glass fibers were added to the slurry, which was subsequently filtered, pressed, and dried, the product thus obtained was lightweight thermal insulation material, temperature-resistant up to 800°C and usable up to 1000°C. The product has excellent insulation properties with thermal conductivity 0.042–0.081 W/mK at bulk densities 150–500 kg/m 3.