Density Functional Theory Study On The Formation Mechanism Of Sodium Impurities In Hemihydrate Gypsum

The preparation ofα-hemihydrate gypsum（α-HH）from phosphogypsum（the main component is dihydrate gypsum（DH））can realize the resource utilization of phosphogypsum solid waste and has important environmental protection value.The preparation of hemihydrate gypsum under normal pressure can be efficiently realized in sodium salt solution.Sodium ion can effectively promote the transformation of DH toα-HH,enhance the driving force of crystal transformation and adjust the quality of hemihydrate gypsum whisker.Moreover,various sodium salt additives are widely used in the preparation of hemihydrate gypsum,and sodium ions will inevitably be introduced when using these additives.However,in the process of preparing hemihydrate gypsum materials,sodium impurities may be doped into hemihydrate gypsum,which will have an important impact on the performance and application of hemihydrate gypsum products.In order to find out the existence form of sodium impurities in hemihydrate gypsum crystals,explore the formation mechanism of sodium impurities,and promote the application of various sodium salt additives in the preparation of hemihydrate gypsum,give full play to the advantages of sodium salt in adjusting crystal transformation kinetics and whisker quality in the preparation of hemihydrate gypsum to provide some theoretical reference.In this paper,the effects of different concentrations of sodium salt on the formation and content of sodium impurities in hemihydrate gypsum were studied by means of preparation test,X-ray diffraction and X-ray fluorescence spectroscopy.In addition,the structure and properties of hemihydrate gypsum crystal and surface are studied by density functional theory（DFT）,the adsorption behavior of sodium ions on the surfaces of hemihydrate gypsum（002）,（020）,（200）and（204）is investigated,and the doping and diffusion behavior of sodium impurities in hemihydrate gypsum crystal are studied.The main research results are as follows:（1）The concentration of sodium salt used in the preparation of hemihydrate gypsum has an important effect on the content of sodium impurities in the product of perylene-HH.The content of Na₂O in the reaction product increases with the increase of the concentration of sodium salt.The results of XRD and XRF show that sodium impurities enter into the lattice of Na₂Ca₅（SO₄）₆·3H₂O and Na₄Ca（SO₄）₃·2H₂O（sodium ettringite）.（2）The（002）surface,（020）surface,（200）surface and（204）surface of hemihydrate gypsum are constructed by density functional theory（DFT）.The results show that the surface energy of hemihydrate gypsum（002）is the largest and the surface is the most unstable,while the surface energy of（020）surface is the smallest and the surface stability is the best.Sodium ions can be adsorbed on the surface of hemihydrate gypsum by top,bridge,acupoint and filling adsorption.（3）The formation energy of sodium impurities is the lowest when sodium impurities are mixed into hemihydrate gypsum crystals by interstitial filling.Moreover,when the content of sodium impurities in hemihydrate gypsum crystals increases from 0.16wt.%to 1wt.%,the impurity formation energy is still much lower than that of other doping forms,indicating that sodium impurities tend to exist in hemihydrate gypsum crystals by interstitial filling.（4）There are a large number of interval in hemihydrate gypsum crystals,which are the main channels for the diffusion of sodium impurities in hemihydrate gypsum crystals.