Research On The Synthesis, Characterization Of Calcium Sulfate Double Salts And The Hydration Machanism Of Anhydrous Calcium Sulphate

Natural anhydrous calcium sulfate (anhydrite) is rich in mineral resources in our country, but activity is very low. So its utilization ratio is not high. With the rapid development of national economy, increasing demand for gypsum products, the improvement of natural anhydrite's low activity has become increasingly pressing. Therefore, in order to aim at the existing problems in anhydrous calcium sulfate hydration theory (eg. the formation and decomposition of double salts, the role of z / r of the activator's cations, common ion effect etc.), reveal for the excitation mechanism of the salts to anhydrite hydration, and seek for the law of catalyzing anhydrite hydration, it is greatly theoretical significance and practical significance for promoting the development of natural mineral gypsum.The synthesis of the calcium sulfate double salt was investigated firstly and then researched thoroughly to the formulation condition, thermodynamic properties, spectroscopy characterization and solubility idiosyncrasy of the calcium sulfate double salt. The effect on hydration of the soluble sulfate to the anhydrous calcium sulphate was analyzed systematically, and the mechanism was given. The adaptability of the salts to the different natural anhydrite was also discussed. The main content and research results are as follows:1. By mean of the solution method and the high temperature synthetic method, it was investigated to the reactive law of the soluble sulfate with calcium sulfate and the synthetic condition of the double salts systematically, and optimized the parameters of the preparations. The influence of the reaction temperature, reaction time and solvent amount to the synthesis by the solution method were discussed. The result showed that although the anhydrous calcium sulfate has the complexing, in the normal temperature and the atmospheric pressure solution, only univalent sulfate (except Li) can synthesize the double salt with calcium sulfate. Increasing along with the atomic number, I-group sulfate with the anhydrous calcium sulfate is easy to synthesize. The speed of the sylvite is highest. Main effect factors are synthesis temperature and solvent volume. The transformation by the high temperature synthetic method is tardy. And it is effected by the temperature more and closely related with the reactant melting point. High temperature synthesis dynamics of calcium sulfate double salts can be described by the two-dimensional proliferation response ((1-α)ln(1-α)+α=kt) and Ginstling-Brounshtein Equation (1-2α/3-(1-α)^2/3=kt). The response is controlled by the diffusion.2. Crystal structure, spectroscopy characterization and thermal stability of the double salts were studied in this paper. And their thermodynamic, spectroscopy files were improved. The thermodynamic stability of the calcium sulfate double salts which were quenched is better, while the transformation of the hydration double salts was complicatedly. Influenced by the quantity and type of the positive ions of the double salts, the vibration spectrum of SO₄^2- changed. The type of the positive ion is more and the spectrum peak splits are more obvious.3.The solubility idiosyncrasy of the calcium sulfate double salt was researched systematically. And the process and the influencing factors of double salts' hydration were discussed. They are complicated, but have certain similarity. The main factors of the solubility transformation were the solvent quantity and the hydration time. The double salts synthesized by the solution method decompose not easily when the water volume is few, and the transformation of potassium calcium sulfate double salts when the water volume is reversible between 0.3 to 10 L/mol. The double salts decompose quickly when the water is enough and it is controlled by the diffusion. Based on that research, the dissolution kinetics and the crystallization kinetics of the double salts were investigated.And the kinetic equations have been obtained. The progressions of the solubility equations of the hydrate double salts were about 1.1, while the quenched double salts' were about 1.5-1.8. The constants of the kinetics equations were increasing as the center of the third period element. And the CaSO₄·2H₂O phase crystallized in the final stage. Its progression of crystallization kinetic equation was 1.07.4. Through the research on hydration of the anhydrous calcium sulfate in the following sulfate(Na₂SO₄,K₂SO₄,Rb₂SO₄,Cs₂SO₄,(NH₄)₂SO₄), reaction speed and offspring of the anhydrous calcium sulfate with the previous sulfate, and according to the analytic results of the double salts' synthesis and the hydrated processes, it is founded that double salt theory is not accurate. Through the experiments on hydration of the anhydrous calcium sulfate in the other sulfate, dissoluble sulfate hydrate origining from hydration process of AH affects its hydrous rate, hydrous rate of the anhydrous calcium sulfate does not connect with z/r of the cation in stimulation, and neither does the theory that z/r affect hydrous rate of the anhydrous calcium sulfate.5. Hydration of the anhydrous calcium sulfate in dissoluble sulfate complies with dissolve-nucleate-grow theory, concentration of Ca²⁺ from dissolving of CaSO₄is higher than one of CaSO₄·2H₂O,CaSO₄·2H₂O gets a motivity of crystallization. Dissoluble sulfate takes the following functions: double-salt or dissoluble sulfate hydrate from hydration of the anhydrous calcium sulfate in it becomes heterogeneous particle and a kind of nucleation catalyzer, and reducing surface barrier when CaSO₄·2H₂O becomes crystal because the crystal of CaSO₄·2H₂O presciently appears in uneven place. Results indicate that hydration of the anhydrous calcium sulfate does not conform to double-salt theory, but it conforms to this mechanism: dissolve-nucleate-grow, formation of CaSO₄·2H₂O is an uneven nucleation and crystallization process.6. It is selected from four kind of different habitats and the geologic origin natural anhydrite, for researching the compatibility of the catalysts. And it is discussed to the influence of the water and the ambient temperature to the hydration. Owing to theirs geologic origin and the geological environment differently, phase composition and foreign inclusions (e.g. alkaline metal ion) and crystal growth condition etc of different kinds of natural anhydrite are discrepant. So the different habitat anhydrite needs different catalyst. But the mechanism is consistent. The effect of FeSO₄ to the natural anhydrite with impurity is better than to analytically pure anhydrous calcium sulfate. The temperature and the water volume have the important effect to the anhydrous calcium sulfate's hydration. Ambient temperature in the production process of anhydrite cement materials and products should be within 30°C.