Study On Modified Phosphogypsum Used As Cement Setting Time Agent

Phosphogypsum(PG) is a by-product of phosphoric acid and some compound washing product industry. The main composition of PG is CaSO₄·2H₂O, whose content is higher than the average level of natural gypsum. This paper compared the effect of PG and NG on the property of cement, firstly. The results showed that the cement mixed with PG has longer setting time than the one mixed with NG. The main is PG has soluble phosphates. According to the principles and methods of material science, it is proposed to combine the PG with lime and fly ash, to make the PG replace natural gypsum(NG). Comparing to the traditional treatment of washing and mixing with slaked lime, this method(modified PG, MPG for short)is more effective.Choosing PG and NG as guide sample of cement retardation, searched the best batch formula of the clinker and MPG, and get the setting time and strength of the cement, which use the MPG as retardation. Tested the content of phosphate and the soluble phosphate and fluorine, by using fluorine ion electrode and ICP(Induction Coupling Plasma-pump), and researched the level of action of fly ash in MPG, by the Scanning Electronic Method(SEM). Study the influence of different contents of phosphate and lime on hydration of Portland cement, using a non contact electrical resistivity meter.The results showed that: the PG content of phosphate and the soluble phosphate and fluorine, and the phosphate and the soluble phosphate make the setting time longer, while the fluorine has little influence on it. So most PG can not be used as cement retardation, and should be modified. The best batch formula of MPG is "Lime: fly ash: PG=5: 30: 65". Using this MPG, the cement has almost the same setting time as the NG, and its strength is higher than the latter, also the usage of clinker decrease.The level of action of fly ash in MPG showed that: this level can be improved by increasing the curing time, or milling fly ash, or adding activator. And then the strength of the MPG increases, but it has little influence on the physical performance of cement. The electrical resistivity experiments showed that: the bulk resistivity curve of the paste with phosphate is delayed in early period, and resistivity declines as the contents of the phosphate increase. The lime has little effect on the hydration of cement, but has an effect to react with the phosphate and offset the retardation of phosphate. The cement added with lime and phosphate has almost the same rule of resistivity change as the cement without lime and phosphate.