Abstract: Magnesium hydroxide having a large specific surface area is provided. A magnesium-containing matter is chemically decomposed by a solution of acid, and a primary solution of alkali with weak alkalinity is obtained by adding a solution of alkali to the solution of acid, and impurities contained in the primary solution of alkali are eliminated, and a secondary solution of alkali with strong alkalinity is obtained by adding a solution of alkali to the primary solution of alkali, and successively a magnesium hydroxide is deposited in secondary solution of alkali.

Abstract: Chrysotile or chrysotile-containing serpentinite containing chrysotile is treated to convert the chrysotile contained therein into a non-asbestos material, so that the non-asbestos material is used as a material that can be recycled safely and is effective from the view point of environmental protection. [Means for Solving Problems] A curable composition characterized by containing at least a porous fibrous amorphous silica obtained by decomposing chrysotile or chrysotile-containing serpentinite with acid to substantially eliminate an influence of asbestos on living body, and a reinforcing fiber. The curable composition preferably contains a surfactant and a thickener and/or a filler and/or a colorant or an air-hardening material and/or water-hardening material and a thickener.