Abstract: A large volume, thermally insulated tank is formed by constructing the tank sidewall in superimposed sections. Each section is constructed without the use of external support forms by laying a first outer course of thermal insulating material and then forming a first inner course of refractory material spaced therefrom. The cavity between these two courses is then filled with cementitious material, and the process is repeated until a tank wall of the desired height is completed. The resultant tank wall will have an outer layer of light, thermal insulating material, an inner layer of refractory material and a core of cementitious material joining the two layers.