Abstract: The present invention is of a cementitious composition containing OPC, calcined gypsum, a source of amorphous silica and a source of amorphous alumina. The ratio of calcium sulfate hemihydrate to OPC is 0.7:1.0 to 1.4:1.0, the ratio of amorphous silica and amorphous alumina to OPC is 0.26:1.0 to 0.4:1.0, and the ratio of amorphous alumina to amorphous silica is 0.3:1.0 to 1.5:1.0. The cementitious composition, by itself and mixed with aggregates, is fast-setting and exhibits good early compressive strength and very high compressive strength after hydration. Despite the high content of calcium sulfate relative to prior art OPC formulations, the cementitious composition according to the present invention is essentially waterproof and exhibits excellent strength characteristics, even after 2 years under water. The use of calcined gypsum in place of alumina cement or even OPC is of great economic advantage, and in addition, provides the cementitious composition with quick-setting characteristics.