The Research On Preparation And Modification Of Sintering Desulfurization Gypsum As Cement Retarder

The FGD gypsum is a by-product,which is produced by limestone/lime wet flue gas desulfurization in steel enterprises for controling the content of SO2 in sintering flue gas.The FGD gypsum will lead to the second-pollution to the environment without treatment.The composition and physico-chemical properties of the FGD gypsum are similar to natural gypsum.Moreover,the natural gypsum is non-renewable resources and the procurement costs climbed quickly.So,if the FGD gypsum can instead of natural gypsum,not only can greatly reduce the enterprise cost,but also change waste into valuables.The FGD gypsum can not be used normally due to the characteristics of high moisture content and impurity species complex.Therefore,it is very important to research the using of the FGD gypsum effectively,which is great significance to the environment and economy.In this paper,the chemical composition,mineral composition,grain size and thermal properties of the FGD gypsum and natural gypsum were analyzing by XRF,XRD,DSC/TG and laser particle size analyser and other testing means.And the microscopic morphology of hydration products of the corresponding cement samples was observed by scanning electron microscope(SEM).The influence of the FGD gypsum as the retarder on the correlative physical properties of Portland cement was especially discussed.Its optimal content range and the optimum heat treatment temperature was explored.According to the composition and characteristics of the FGD gypsum,the influences of soluble salts impurities and calcium sulfite in the gypsum on the properties were studied.The results show that:(1)The natural gypsum can be replaced completely by the FGD gypsum as cement retarder,and its retarding effect is better.When mixed with 4.5% FGD gypsum of heat treatment under 380?,the cement sample initial and final setting time are maximum,287 min and 346 min respectively.The mechanical properties are the optimum as the flexural strength of 3d and 28 d are 6.2 MPa and 8.6 MPa.Their compressive strength up to 34 MPa and 48 MPa respectively.(2)Soluble salts impurities in flue gas desulfurization gypsum were removed by water.It was concluded that the impact of the method on setting time of cement sample was not obvious,but the mechanical properties were decreased.(3)Through adding oxyful into FGD gypsum can oxygenize the calcium sulfite to some extent,compared with gypsum without oxyful,the setting time of oxygenized gypsum has no obviously changes,but the mechanical properties were increased.