In order to clarify the precipitating process of calcium sulfate dihydrate from solution, the behavior of calcium ion has been investigated by use of calcium ion electrode when calcium sulfate dihydrate is precipitated in the various supersaturated solutions of dihydrate prepared by mixing calcium chloride solution with sodium sulfate solution stoichiometrically. At the same time the influence of sodium tripolyphosphate on the precipitation of the dihydrate was studied. The results obtained are as follows;
1) The precipitation of calcium sulfate dihydrate can be divided into two groups according to the initial concentrations : the one is the precipitation in solutions with the high concentrations at initial stage, and the other is the precipitation in solutions with the low concentrations at initial stage. In the case of the solutions with the initial concentrations higher than the solubility of calcium sulfate hemihydrate, the crystals of hemihydrate appear first at the ordinary temperature and then turn into calcium sulfate dihydrate. In the case of solutions with the lower concentrations at the initial stage, calcium sulfate dihydrate precipitates directly.
2) The behavior of calcium and sulfate ions in the solution before precipitation (in induction period of crystallization) was clarified.
3) Sodium tripolyphosphate does not affect at all on the formation of nuclei, but disturbs the precipitation of calcium sulfate.

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The effect of atmosphere on the dehydration of gypsum was investigated by means of differential thermal analysis and X-ray diffraction.
Two endothermic peaks were observed in the differential thermogram of gypsum under relatively high water vapor pressure. However, the two peaks shifted to lower temperature and overlapped completely with a decrease in water vapor pressure. On the other hand, only a peak was observed in that of β-hemihydrate, and the peak temperature became much lower than that of gypsum with a decrease in water vapor pressure.
In addition, it was confirmed by X-ray diffraction that β-hemihydrate was formed during the dehydration of gypsum at 93-C in air, and that β-hemihydrate was not detected at 62°C in vacuo, but β-hemihydrate was formed at lower temperature (e. g. 40°C) even in vacuo.
From above results, it was concluded that the dehydration of gypsum was essentially the consecutive reaction, CaSO₄-2H₂O→β-CaSO₄-1/2H2O→III-CaSO₄, irrespective of the atmosphere. The phenomena of two stages (in air) or one stage (in vacuo) dehydration of gypsum was due to the difference in the rates of both reactions depending on the temperature and the water vapor pressure.

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The frictional resistance of interface between gypsum and glass fiber depended on the water content of gypsum, and was greatest at the water content nH₂O/CaSO₄=0.26.
An interval of 0.4 mm between glass fiber, was necessary for the flowing of gypsum slurry between glass fibers without separation of gypsum from water.
The glass fibers coated with calcium sulfite were better than other surface treated glass fibers for GRG.

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