An aqueous film forming inorganic composition which is prepared by reacting metallic silicon and an alkali metal hydroxide in an aqueous solvent in the presence of at least one compound selected from the group consisting of a fluoride and sulfurous acid, nitrous acid, phosphorous acid and a salt of the acid, characterized in that it has a pH value of 10 to 13 at 20°C and comprises colloidal particles of the product of the above reaction having various, that is, relatively large, medium and relatively small particle diameters; an inorganic foam prepared by heating the composition to 150°C or higher; and methods for preparing the composition and the foam. The aqueous film forming inorganic composition can be suitably used for preparing an inorganic foam and a thermally insulating composite material which are excellent in thermal insulation, and is not pollutant to the environment and can be recycled in the environment.
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