A non-combustible flooring system for construction comprising: a metal frame ; a horizontal shear diaphragm fastened to the metal frame , the horizontal shear diaphragm comprising a reinforced, lightweight, dimensionally stable cementitious panel ; the cementitious panel having a density of 65 to 90 pounds per cubic foot (1041.20 to 1441.66 kg/m 3 ) and capable of resisting shear loads when fastened to the metal frame and comprising a continuous phase resulting from the curing of an aqueous mixture comprising, on a dry basis, 35 to 68 wt. % of a blend of reactive powders, 23 to 49 wt. % uniformly distributed lightweight filler particles, and 5 to 17 wt. % uniformly distributed alkali-resistant glass fibers, and optional superplasticizer; the continuous phase being uniformly reinforced with the glass fibers and uniformly containing the lightweight filler particles, the lightweight filler particles comprising ceramic microspheres and the lightweight filler particles having a particle specific gravity of from 0.02 to 1.00 and an average particle size of about 10 to 500 microns (micrometers), and wherein the blend of continuous phase results from the curing of an aqueous mixture of reactive powders comprising comprises, on a dry basis, 45 to 75 wt. % calcium sulfate alpha hemihydrate, 20 to 40 wt. % Portland cement, 0.2 to 3.5 wt. % lime, and 5 to 25 wt. % of an active pozzolan comprising silica fume, wherein the flooring system has a horizontal design shear capacity of the floor shear diaphragm of 300 to 1000 pounds per linear foot (446.46 to 1488.19 kg/m), wherein the cementitious panel has a thickness of about 1⁄4 to 11⁄2 inches (0.635 to 3.81 cm). a metal frame ; a horizontal shear diaphragm fastened to the metal frame , the horizontal shear diaphragm comprising a reinforced, lightweight, dimensionally stable cementitious panel ; the cementitious panel having a density of 65 to 90 pounds per cubic foot (1041.20 to 1441.66 kg/m 3 ) and capable of resisting shear loads when fastened to the metal frame and comprising a continuous phase resulting from the curing of an aqueous mixture comprising, on a dry basis, 35 to 68 wt. % of a blend of reactive powders, 23 to 49 wt. % uniformly distributed lightweight filler particles, and 5 to 17 wt. % uniformly distributed alkali-resistant glass fibers, and optional superplasticizer; 35 to 68 wt. % of a blend of reactive powders, 23 to 49 wt. % uniformly distributed lightweight filler particles, and 5 to 17 wt. % uniformly distributed alkali-resistant glass fibers, and optional superplasticizer; the continuous phase being uniformly reinforced with the glass fibers and uniformly containing the lightweight filler particles, the lightweight filler particles comprising ceramic microspheres and the lightweight filler particles having a particle specific gravity of from 0.02 to 1.00 and an average particle size of about 10 to 500 microns (micrometers), and wherein the blend of continuous phase results from the curing of an aqueous mixture of reactive powders comprising comprises, on a dry basis, 45 to 75 wt. % calcium sulfate alpha hemihydrate, 20 to 40 wt. % Portland cement, 0.2 to 3.5 wt. % lime, and 5 to 25 wt. % of an active pozzolan comprising silica fume, 45 to 75 wt. % calcium sulfate alpha hemihydrate, 20 to 40 wt. % Portland cement, 0.2 to 3.5 wt. % lime, and 5 to 25 wt. % of an active pozzolan comprising silica fume, wherein the flooring system has a horizontal design shear capacity of the floor shear diaphragm of 300 to 1000 pounds per linear foot (446.46 to 1488.19 kg/m), wherein the cementitious panel has a thickness of about 1⁄4 to 11⁄2 inches (0.635 to 3.81 cm). <EOS>