Effect Mechanism Of Fluorine And Phosphorus On Hydration Process Of Cement Clinker

Large quantities of industrial by-products are produced every year by chemical and agricultural process industries. These materials such as phosphorous slag(PS) and phosphor gypsum(PG) are latent active material and its utilization as addition is a important approach to cement development. In China, a lot of phosphorous slag and phosphor gypsum has not been used widely and effectively. The main reason is that the fluorine and phosphorus in phosphorous slag and obviously lengthens the setting time and decreases the early strength of cement. Consequently, it can affect the engineering construction progress. Studys show that addition of activation, such as calcined gypsum, calcined alumstone and sodium sulphate, can resolve these questions and improve the properties of cement. In order to improve these faults of PS and PG, effect of fluorine and phosphorus in phosphorous slag and phosphogypsum on hydration process of C3S and C3A was explored. Base on this results, chemical methods was used in this study. Alkali activators such as calcined lime, sodium hydrate and sodium chloride were used to accelerate hydration speed of PS and PG cement.Both of tricalcium silicate(C3S) and tricalcium aluminate(C3A) were prepared by burning. Phosphate slag and phosphor gypsum soak solution instead of water were used. By means of hydration heat rates, chemical bonding water, XRD and SEM, the effect of the phosphorus and fluoride on the hydration process of cement minerals were explored. Studies have shown that the soluble phosphorus and fluoride can obviously put off the hydration of cement minerals. Compared with the hydration process in the water solution, the process that C3S in the phosphate slag and phosphor gypsum soak solution releases less heat quantity, and the induction period prolongs, the first exothermic peak of immersing is markedly lower, consequently delays the appearance of the second exothermic peak. The characteristic, which is reflected in the whole exothermic curve, takes on a phenomenon that the exothermic peak is low, and the appearance of the peak is delayed. Furthermore, insoluble calcium phosphate has a certain function of delaying cement single mineral hydration, and insoluble calcium fluoride almost takes little effect on the hydration rate of C3S and C3A. Because of the delayed effect of phosphate and fluoride in the hydration process, there exist a few pores in the micro-structure of early hardened paste, whereas they could also reduce the chance of cracks.When various forms of phosphate and fluoride are added in the Portland 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.According to characteristics of phosphorus slag, chemical methods was used to stimulate the activity of phosphorus slag. The results show that NaOH(or NaaCO3), together with the compound of CO(NH2)2or NaCl, can accelerate the hydration of phosphorous slag cement. However, when NaOH combined with CO(NH2)2or NaCl, the affect of the combination is more obvious. The stimulation function of activator to phosphorus slag, mainly occurs in the early-age hydration of phosphorus slag, but the late-age of which fluctuates slowly. CaO can heighten the strength of3days and28days of phosphorous slag cement. When compared with phosphorous slag cement mixed with no CaO, the strength of3days can increase0.5MPa (2.0%/growth rate), meanwhile, the strength of28days can increase4.7MPa(12.2%/growth rate).In this study, a series of research work was carried on phosphor gypsum. The strength and setting time of cement in which PG was used as retarder were determined. Compared with natural gypsum with the same SO3wt%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 slag or fly ash is 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, the quality of PG can be improved by sifting, calcining or liming. With2wt%lime, PG can be used as retarder of cement.