PURPOSE: To obtain lightweight cellular concrete of high quality composed of highly crystalline tobermorite in a short time.

CONSTITUTION: This method for producing lightweight cellular concrete is to mix water with a raw material, consisting essentially of a crystalline siliceous and a calcareous raw materials and further containing an alumina and an alkaline components and cure the resultant mixture in an autoclave. In the method, 1-10 pts.wt. bauxite as the alumina component and 0.3-2 pts.wt. alkaline hydroxide as the alkaline component are added to 100 pts.wt. crystalline siliceous and calcareous raw materials. Thereby, the alkaline component is capable of accelerating the dissolution of the crystalline siliceous raw material to provide highly crystalline tobermorite in a short time. As a result, the quality can be improved.