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The mixture of silica powder and milk of lime was stirred in an autoclave and treated hydrothermally under the following conditions; molar ratio of CaO/SiO2 0.80, weight ratio of water/total-solid 12, pressure 8kg/cm2, temperature 175°C, for 4-8hrs. Thus milky slurry consisting of tobermorite was prepared.The slurry contains 8% of total solid matter and its viscosity is 1.5-2.0 poise. Crystals are tobermorite whose (002) spacings are 11.2-11.3A. Grains are spherical aggregates with 5-70μm diameter having inter-twining structures of plate-like tobermorite crystals as shown in Fig. 5, 6 and Fig. 8.The slurry is easily dehydrated and shaped in a mold under pressure and the molded material can be hardened only by drying. The properties of thus obtained light molded material are following as shown in Table 1; density 0.2, bending strength 3-4kg/cm2, thermal durability 900°C, shrinkage at 1000°C 2-3%.