Study On The Preparation And Surface Modification Of Calcium Sulfate Hemihydrate Whiskers

Calcium sulfate dehydrate(CaSO4·2H2O)is the main component of white gypsum produced by wet-process phosphoric acid.High-value calcium sulfate hemihydrate xwhisker(CSHW)preparced from white gypsum is of great significance in resourcing waste gypsum and reducing envirommental pollution.In this paper,white gypsum was used as raw material to prepare CSHW by acidification method.The influence of process conditions on the preparation process was investigated and the crystal transformation process was studied.The surface modification treatment of CSHW was carried out with different modifiers to overcome its drawbacks,such as easy hydration,easy agglomeration and poor compatibility with polymer matrix.The influence of process conditions on the modification effect was investigated,and the samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FT-IR),thermogravimetric analysis(TG),X-ray photo-electron spectroscopy(XPS)and so on.The main research contents and conclusions of the thesis are as follows:(1)The preparation process of CSHW was investigated.The results showed that a product with an average aspect ratio of 48.81 could be obtained when the mixed acid solution including phosphoric acid and sulfuric acid was used as the transformation medium,the concentration of SO3 was 4.1%,the concentration of P2O5 was 28.0%,the reaction temperature was 90 ?,the liquid-solid ratio was 4.2:1 and the amount of crystal control agent MgCl2·6H2O was 1.0%.Besides,CSHW was determined as the obtained product according to the results of XRD and TG.(2)The crystal transformation process from white gypsum to CSHW was studied,and the influence of P2O5 concentration and temperature on the crystal transformation rate was also explored.The results demonstrated that the transformation rate of crystal increased with the increasing of P2O5 concentration and reaction temperature.The conversion from white gypsum to CSHW was a nucleation control process,which accorded with the accelerated reaction dispersion kinetics model.The activation energy and reaction rate decreased and increased with the prolongation of reaction time.(3)The compound modification on CSHW was earried out Taking sodium silicate as precipitant and stearic acid as organic modifier.The results showed that the whisker activation index was 0.9931,and the contact angle reached 140°.The surface properties changed from hydrophilic to hydrophobic.XRD and SEM showed no change in crystal structure and overall morphology;FT-IR and XPS analysis indicated that there were-Ca-HSiO3 and-Ca-COOR structure coating on the whisker surface.(4)The microencapsulated modification was operated by in-site polymerization taking CSHW as the core and melamine-formaldehyde resin(MF)as the shell.Two factors on the water solubility and crystal water content of CSHW were studied,such as different water to ethanol mass ratios and pH.The results showed that the solubility of modified CSHW(MF-CSHW)in water decreased from 0.93 g/100 mL H20 of CSHW to 0.41 g/100 mL H20 of MF-CSHW by 55.91%when the ratio of water and ethanol ratio was 1:9 and the pH was 5.5.And the XRD results indicated that the crystal structure of MF-CSHW obtained under the optimal conditions did not change.