The cementitious composition of the invention is composed of magnesium oxide or semicalcinated dolomite, magnesium sulfate, an alkaline earth metal chloride, an alkali metal silicate, and an alkali metal fluorosilicate in specific proportions. Slurrying with water causes setting and hardening of the mixture whereby a water insoluble complex is formed. Filler materials may be added. The resulting products are useful building and construction materials, such as boards, panels, walls, and others.