| Phosphogypsum(PG), an industrial byproduct of phosphoric acid manufacture, has high content of CaSO4·2H2O. However, the utilization ratio of PG is very low due to the negative effect of impurities on the properties of PG plaster. The present studies on PG of its application in cement and plaster revealed that impurities in PG have harmful effects on the setting of cement and gypsum plasters. It is therefore, thought to be essential to beneficiate PG to reduce harmful impurities in it. In fact, effects of impurities on PG differ with differences in the form, type of gypsum and its applied cementing systems. For some binding materials, influences of impurities may be ignored. Pretreatments of PG should be different according the form of gypsum and its applied systems.The constitution of PG and natural gypsum(NG) was investigated by atomic adsorption spectra(AAS) in this study and by a combination of x-ray diffraction(XRD), scanning electron microscope(SEM), differential scan calorific(DSC) and tests on chemical and physical performance, the properties of raw PG were studied. The results show: PG differs significantly from natural gypsum in crystal morphology and grading due to differences in formation process between them. PG has distinct solubility compared with NG. Thermal behaviour of NG and PG was determined and transformation temperature of dihydrate to hemihydrate of PG is 20~35℃lower than that of NG and washed PG, which indicates that water soluble impurities reduce the dehydration temperature of dihydrate.The effect of PG on the performance of cement is mainly due to the influcens of impurities on the solubility and dehydration behaviour of PG. The results of test on solubility of PG showed that with increases in ambient temperature, the solubilization rate and solubility limit of PG in saturated lime solution reduce. Changes in the hydration temperature of cement leads to variation in the concentration of calcium sulfate, thus the retardation of PG was affected. Due to the lower transformation temperature of dihydrate to hemidrate, PG appears to be prone to loss its combined water at various isothermal conditions between 70 and 130℃in a first effort to simulate milling conditions. About 4% hemihydrate was present in PG at 70℃and in NG no hemihydrate was found. The content of hemihydrate was about 55% in PG and 7% in NG at 130℃. The ratios of calcium sulphate forms occur in gypsum can have a profound effect on the setting behaviour of the cement, because their respective solubilities in water are significantly different from one another. The dihydrate form is the preferred form, so the present of hemi and anhydrate form is harmful to the setting of cement. P2O5 in crystal lattices of gypsum can't be removed through pretreatment of water washed and lime neutralized, and decomposition of P2O5 in crystal lattices of gypsum because of being heated may reduce the dehydration temperature of PG, and at 90℃about 3% hemihydrate was present in washed PG and at 130℃, the content of hemihydrate was about 11%. Great care should be exercised under production conditions when using PG treated or not treated as a set retarder.Properties of hemihydrate made from PG and NG with same content of gypsum form were determined. The initial setting time of hemihydrate from NG is about 7 min, and setting of hemihydrate from PG was retarded and the strength of the hardenite was reduced. Insoluble products formed by the reaction of Ca2+ and soluble impurities covered at the surface of gypsum crystal, which leads to the coarsening of the crystal.The effect of retarding was correlated to the content of impurities. Setting time of hemihydrate made from PG was aobout twice than that of hemihydrate from NG. The fast setting of most building gypsum is unfavourable for construction. So the retarding effect of impurities can be utilized positively. Though the retarding effect of impurities was not better than the citric acid, it can still be used in some cementing systems.Impurities have hardly affected performances of anhydrate form of PG due to its slow hydration. Hydration products of anhydrate have enough time to break out the overburden layer formed by reaction of soluble P, F and Ca2+ to grow up. Compared with NG, PG has advantages to be use as raw materials for the hemihydrate and anhydrate mixing cementing systems.Impurities are thought to be harmful in almost all studies on PG, but in this paper some effects of impurities are discussed tentatively to be utilized and PG should not be necessarily pretreated in some cementing systems. Pretreatment of PG can't always effectively improve the properties of PG. In some cases effect of impurities on the properties can be ignored and even be utilized. To find out the mechanism of effect of impurities on PG according to the applied systems of gypsum is very important. |
PDF Full Text Request