In the present work, attempts were made to investigate synthetic conditions, crystallographic data and characteristics of new compound on CaO-Al₂O₃-CaSO₃·nH₂O system formed by aqueous solution reaction of Ca (OH) ₂-AlCl₃-Na₂SO₃ system.
The new compound was formed by dropping AlCl₃-Na₂SO₃-KOH mixed solution (pH 12.5) into Ca (OH) ₂ solution under the condition of compounding mole ratio CaO-Al₂O₃-CaSO₃·nH₂O=4 : 1 : 1. In X-ray diffraction patterns of the compound, appeared many unknown peaks without corresponding to those of various calcium aluminate hydrates, CaSO₃ 1/2 H₂O, etc.. From chemical analysis, the compound was made clear to exist as solitary compound of composition 3CaO.Al₂O₃-CaSO₃-12H₂O.
Moreover, new compound belonged to hexagonal system (rhombohedral lattice) from X-ray diffraction data. The unit cell dimensions were : a_H=5.769A, c_H=25.467A (hexagonal axes); a_R= 9.119A, α=36.88° (rhombohedral axes). The crystal shape was very thin hexagonal plate-like crystal with particle size of 1-3μm.
These crystallographic data considerably agreed with those of low-sulphate form monosulfate, 3Ca·Al₂O₃·CaSO₄·12 H₂O as already known and the both structures had very close similarities. Therefore the form of crystallization water in layer structured [Ca₄Al₂ (OH) ₁₂] ²⁺ · [SO₃·6 H₂O] ^2- existed as two different kinds of OH^- groups and H₂O, the latter lied perpendicular to c axis. These crystallization waters dehydrated to 1H₂O at 100°C, 5 H₂O at 100-180°C and 12OH^-groups above 180°C with heating. New compound, 3CaO·Al₂O₃·CaSO₃·12H₂O could be expected as structural material and inorganic filler.

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In the present paper, the critical stress intensity factors of hardened gypsums, Kw, were measured by use of the notch beam test in various environmental conditions. As the notch cutting at the specimen's surface of hardened gypsum was apt to work up easily, the reappearance of the data of K_ic was very successful. Generally the value of K_ic measured in water has been smaller than that of under.drying condition by half. Further, the crack velocities of hardened gypsums, V, were measured by use of the double torsion method in various environmental conditions, and the data were plotted on a log-log paper making use of the experimental equation, V = AKIⁿ (A, n : constant). The values of n obtained from the data which were determined in water were smaller than those of under other environmental conditions. The stress corrosion caused by water for the specimen of hardened gypsum has been suggested by these results.

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Steel-plate-laminated gypsum planks, glass-cloth-laminated ones and glass-roving-reinforced ones were manufactured for laboratory experiments. The properties of those planks were examined by bending test.
The steel-plate-laminated planks had much higher ultimate bending strength than the plywood in case or the materials with the same thickness. Both the bulk density and the strength comparable to those of the plywood were not obtained in gypsum planks, however, the glass-roving-reinforced planks possessed almost the same ultimate flexural strength and toughness as the plywood.

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