Study On The Effect Of Phosphorus And Fluoride In The Phosphogypsum On The Hydration Of Portland Cement

Phosphogypsum(PG) is a waste by-product of phosphoric acid manufacturing, essentially calcium sulfate dihydrate. Its main utilization way is used as cement retarder instead of natural gypsum. In this study, influence of phosphorus and fluoride in PG on the physical properties and hydration of Portland cement was researched.The strength and setting time of cement in which PG was used as retarder were determined. Compared with natural gypsum with the same SO3 wt% added in the cement, the results show that PG can reduce the early strength, increase the late strength and extend the setting time of cement. When various forms of phosphate and fluoride are added in the cement, it is found that the negative effect on the cement is mainly caused by the soluble ones in which HPO42- has the greatest effect, and the insoluble ones has little impact. When slag or fly ash was added to replace certain clinker, the adverse effect of PG on strength of cement can be reduced, but the setting time has also been delayed. In addition, adverse effect by the PG on the physical properties of cement can be also reduced by separately grinding materials and then mixing. Moreover, the quality of PG can be improved by sifting, calcining or liming. With 75μm sifting and 2 wt% lime, PG can be used as retarder of cement.Both of tricalcium silicate(C3S) and tricalcium aluminate(C3A) were prepared by burning. Hydration comparison test can be performed while C3S and C3A were added with the pure water and infusion of PG. By the means of heat of hydration, content of chemically combined water, XRD and SEM, effect of phosphorus and fluorine on the hydration of C3S and C3A was studied. To the C3S, the induction period of hydration was delayed and heat release was reduced with lower early hydration outputs and more late hydration outputs. To the C3A, the first phase of hydration was delayed, with the increased heat release and the more outputs of the whole hydration products. The type of hydration outputs of single mineral had not been changed by the soluble phosphate and fluoride, but the number and structure had. The retarding mechanism of effect of phosphorus and fluoride on hydration process of cement is proposed:the soluble phosphorus and fluorine reacting with calcium ions can produce insoluble calcium phosphate and calcium fluoride with large specific surface area in the alkaline environment. The outputs are easily adsorbed to the diffuse electric double layer of the single mineral, the permeability of which to the water has been reduced. So the crystallization of the C-S-H and ettringite have been delayed, result to the extended setting time and decreased early strength.