Synthesis and Characteristics of Fibrous II-Gypsum Anhydride

Abstract

Fibrous II-gypsum anhydride was synthesized without destroying fibrous form in dehydration (heating at 700°-800°C) of fibrous gypsum hemihydrate as the reaction product of CaSO₄-CH₃OH-H₂O system in our previous paper. Direct preparation of fibrous II-gypsum anhydride was studied by removing methanol rapidly from organogel of gypsum, which was deposited as the precipitate CaSO₄ nucleus aggregated with CH₃OH molecules by the reaction CaCl₂-H₂SO₄ system in methanol. Gelatinous conditions of the organogel and characteristics of fibrous II-gypsum anhydride were investigated by means of X-ray diffraction, thermal analysis (TG-DTA), scanning electron microscopic observation and dissolution test, The organogel was deposited by adding calcium chloride of 0.090mol dm^-3 to sulfuric acid of 0.023-1.126mol dm_-3, which were previously dissolved in 99.6% methanol. The water content in initial solutions, temperature, aging time and concentration of sulfuric acid in excess to equivalence for formation of gypsum, effected remarkably the formation area of fibrous II-gypsum anhydride in the further crystallization process. For an example, gypsum dihydrate with prismoidal or platy crystals crystallized by removing methanol from the gel which deposited in excess sulfuric acid of less than 0.05mol dm^-3 after aging for 5h at temperature below 15°C. Whereas, the fibrous II-gypsum anhydride with an aspect ratio of larger than 100 (diameter 0.15μm, length 15-30μm) was obtained under the following gelatinous conditions, water content of below 1% in the both initial solutions, temperature range of 30°-40°C, excess sulfuric acid of 0.095mol dm^-3 and aging time for 40-72h, The X-ray diffraction pattern of the fibrous II-gypsum anhydride showed strong orientation by rapid crystal growth of (020) and (002) planes. Its solubility at 20°C was 0.20g CaSO₄/100cm³ H₂O after 30 days.