Research On The Hydration Machanism And Application Of Anhydrous Calcium Sulphate

With the development of the industry of gypsum all over the world,the application of gypsum is rising. In the exploitation of gypsum resource, natural gypsum is the most important part. However, it is not renewable resource.As comsumption of gypsum is increasing and effect on the unbalance of the resource distribution is getting worse, the further research on the potential hydrous activation of the anhydrous calcium sulphate(AH ), the mineral amount of which takes 2/3 of the total mineral amount of the calcium sulphate,takes on especially important.In the article,a large amount of experiences on AH have been done. Through analysis of XRD, concentration of Ca2+, hydrous rate of AH, the machanism on hydration of AH is given.First, Hydration of AH is a spontaneous and slow process, when AH is added in dissoluble sulfate, there is a moving balance: CaSO₄ (s)(?)Ca²⁺(aq)+SO₄^2-(aq) CaSO₄·2H₂O, the Chemical balance is a competitive reaction, change of SO₄^2- will bring about the following effect: it makes the balance move toward CaSO₄·2H₂O, and makes dissolution of CaSO₄ reach balance easily. If concentration of SO₄^2- increases, it will restrain dissolution of CaSO₄, and has an adverse affect on hydration of AH. However, hydrous rate of AH will increase when concentration of dissoluble sulfate rises. These results indicate that sulphate does not have an effect on over-saturation of AH.Second, through the research on hydration of AH in the following sulphate(Na₂SO₄,K₂SO₄,Rb₂SO₄,Cs₂SO₄,(NH₄)₂SO₄), reaction speed and offspring of AH with the previous sulphate, results indicate that double-salt theory on hydration of AH is not accurate, through the experiments on hydration of AH in the following sulphate (Li₂SO₄,MgSO₄,Al₂(SO₄)₃,MnSO₄,CuSO₄,COSO₄,CdSO₄,ZnSO₄,NiSO₄,FeSO₄), dissoluble sulfate hydrate origining from hydration process of AH affects it's hydrous rate, hydrous rate of AH does not connect with Z/R of cation in stimulation, theory that Z/R affect hydrous rate of AH does not exist.Third, hydration of AH in dissoluble sulfate complies with dissolve-nucleate -grow theory, concentration of Ca²⁺ from dissolving of CaSO₄ is higher than one of CaSO₄·2H₂O, calcium sulphate dihydrite (DH) gets a motivity of crystallization. dissoluble sulfate takes the following function: double-salt or dissoluble sulfate hydrate from hydration of AH in it becomes heterogeneous particle and a kind of nucleation catalyzer, and reduces surface barrier when DH becomes crystal because the crystal of DH precedently appears in uneven place. Results indicate hydration of AH does not conform with double-salt theory, but it conform with this mechanism: dissolve-nucleate-grow, formation of DH is an uneven nucleation and crystalizaion process.Finally, experiment results indicate that K₂SO₄ is the most widely and best stimulation effect in familiar dissoluble sulfate. In the end, the addition of water and K₂SO₄ are researched in practically application.