The preparation by hydrothermal synthesis of a high efficiency, calcium silicate based thermal insulation is described using raw material mixes of silica, bentonite, and magnesium acetate or acetic acid. The materials were reacted in a high pressure autoclave at 200-250? C resulting in a product consisting of cenospheres of needle-like xonotlite crystals and some tobermorite. Properties including density, strength, microstructure, thermal stability, and water absorption and water repellence are presented. BCT/357 The authors are in the Research Institute of Engineering Technology of CNPC, 40 Jintang Road, Tanggu, Tianjin, 300451. China. Manuscript received 8 June 1998; in final form 16 January 1999.