Study On Using Modified Phosphogypsum As Cement Retarder

Phosphogypsum is a by-product of phosphate fertilizer production.Its main component is CaSO₄·2H₂O,but it also contains impurities such as phosphorus and fluorine.If it is used as cement retarder,the impurities will have adverse effects on cement properties.With the implementation of environmental protection policies and the increasing competitive pressure of cement enterprises,the cement industry has been looking for ways to reduce production costs and improve environmental benefits.Using phosphogypsu as cement retarder,is an effective way to realize the utilization of phosphogypsum resources and improve economic benefits.In this paper,raw phosphogypsum and natural gypsum were used as retarders to prepare cement,and their effects on physical properties of cement were studied.Through the preparation of cement from different sources of phosphogypsum and natural gypsum,the physical properties of cement were determined,and the influence of impurity content on the physical properties of cement was studied.The effect of phosphogypsum on early hydration of cement was studied based on X-ray diffraction analysis and scanning electron microscopy observation.Considering the local situation,the scheme of modifiying phosphogypsum with carbide slag and fly ash was put forward.By adjusting the proportion of modified phosphogypsum and preparing cement,the physical properties of cement samples were tested.The proportion of modified phosphogypsum was determined to be phosphogypsum:fly ash:carbide slag=65:30:5.The content of impurities in modified phosphogypsum was detected and the effect of impurities on the modification effect was studied.The physical properties of cement prepared with modified phosphogypsum,original phosphogypsum and natural gypsum were tested,and the hardened cement was analyzed by XRD and SEM to study the mechanism of action of modified phosphogypsum.The results showed that under the same SO₃ content,the setting time of cement prepared with phosphogypsum was much longer than that of natural gypsum,and with the increase of gypsum content,the setting time of cement was also gradually prolonged.Phosphogypsum can reduce the early strength of cement,but it had little effect on the later strength of cement,and even can slightly improve the later strength of cement.The hydration heat of cement prepared with phosphogypsum decreases and the early hydration process of cement was delayed.The modified phosphogypsum with the ratio of"phosphogypsum:fly ash:carbide slag=65:30:5"can eliminate most impurities such as total phosphorus,soluble phosphorus and soluble fluorine.The physical properties of the prepared cement were similar to those of natural gypsum,which can greatly reduce the setting time of cement and solve the problem of super retardation in the preparation of cement with phosphogypsum.