Formation and Transformation of Five Different Phases in the CaSO4−H2O System: Crystal Structure of the Subhydrate β-CaSO4·0.5H2O and Solu

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• 作者：Axel N&#xf8 ; rlund Christensen ; Maja Olesen ; Yngve Cerenius ; Torben R. Jensen
• 刊名：Chemistry of Materials
• 出版年：2008
• 出版时间：March 25, 2008
• 年：2008
• 卷：20
• 期：6
• 页码：2124 - 2132
• 全文大小：925K
• 年卷期：v.20,no.6(March 25, 2008)
• ISSN：1520-5002

文摘

At least five crystalline phases can be found in the CaSO₄−H₂O system, which are gypsum CaSO₄·2H₂O, the subhydrates α- and β-CaSO₄·0.5H₂O, and the soluble and insoluble anhydrite CaSO₄. The formation of these five phases in the CaSO₄−H₂O system and their transformations were investigated by in situ time-resolved synchrotron radiation powder X-ray diffraction (SR-PXD) in this study. Furthermore, revised structural models for β-CaSO₄·0.5H₂O and soluble anhydrite CaSO₄ are presented. The hydration of α-CaSO₄·0.5H₂O was studied at 25 °C and showed that the reaction with H₂O started immediately after mixing the two reactants and that the formation of CaSO₄·2H₂O was coupled to the depletion of α-CaSO₄·0.5H₂O. The thermal decomposition of CaSO₄·2H₂O was investigated in the temperature range of 25−500 °C and showed the formation of α-CaSO₄·0.5H₂O followed by the formation of soluble anhydrite AIII-CaSO₄, which was gradually converted to insoluble anhydrite AII-CaSO₄. The thermal decomposition of α-CaSO₄·0.5D₂O was investigated in the temperature range of 25−500 °C and showed successive phase transformations to β-CaSO₄·0.5D₂O, soluble anhydrite AIII-CaSO₄, and insoluble anhydrite AII-CaSO₄. The two polymorphs of anhydrite coexist in the investigated temperature range of 200−500 °C. The hydrothermal decomposition of CaSO₄·2H₂O was investigated in the temperature range of 25−200 °C using a 1 M HNO₃ or a 1 M LiCl solution, and in both experiments, CaSO₄·2H₂O was converted to α-CaSO₄·0.5H₂O and further to insoluble anhydrite AII-CaSO₄. A structural model for β-CaSO₄·0.5H₂O is proposed on the basis of SR-PXD data and a trigonal unit cell (in hexagonal setting) a = 6.931 45(3), c = 12.736 17(4) Å, Z = 6, and space group P3₁. A structural model for soluble anhydrite AIII-CaSO₄ is also proposed on the basis of powder neutron diffraction data, and a hexagonal unit cell parameters are a = 6.9687(1), c = 6.3004(1) Å, Z = 3, and space group P6₂22.