| Flue gas desulfurization (FGD) gypsum is a by-product, which is produced bywet gas desulphurization in coal-fired power stations. In recent years, with promotionand implement of wet gas desulphurization technology, large quantity of FGDgypsum has been produced. However, owing to impurities existed in FGD gypsum（such as, sodium, magnesium and chloride）, which can be detrimental to thegypsum products, the effective utilization of FGD gypsum hasn’t been carried out. Inview of these problems, the physicochemical characteristics and influence ofimpurities on properties of FGD gypsum were studied. Pure gypsum and gypsumcontaining different impurities were prepared by simulating FGD process atlaboratory. The effect of impurities on structure, thermal dehydration kinetics andhydration and hardening performance of synthetic gypsum were investigated. Theimpact of results on application of FGD gypsum was also briefly discussedThe soluble ions mainly monitored are sodium (Na+), magnesium (Mg2+) andchloride (Cl-) in FGD gypsum. Some sodium ions exist in the gypsum lattice and theother resides in fluid inclusions. Most magnesium ions may exist in the gypsum lattice.Most chloride ions reside in fluid inclusions. FGD gypsum exhibits smooth surfaceand coarse surface. The dehydration reaction of FGD gypsum occurs in two steps,which follow Avrami-Erofeev equation. Interaction of sodium and chloride isbelieved to promote the hemidyate to dihydrate, shorten the setting time and reducecompressive strength. Magnesium ion content from0.24wt%to0.67wt%has no effecton the setting time. In the presence of1.1wt%magnesium ions, the setting time ofgypsum plaster has been retarded.Effect of impurities (sodium, magnesium and chloride) on the structure andmorphology of synthetic gypsum was studied by XRD and SEM. The results showthat impurities have no effect on the lattice parameter of synthetic gypsum. Puresynthetic gypsum exhibits two different morphological features: compact, smoothsurface and coarse surface made of small gypsum crystal units. Synthetic Gypsumcontaining sodium and magnesium ions exhibits smooth and compact surface.Synthetic gypsum containing chloride shows coarse surface, and chloride ions couldmodified small gypsum crystal units from disordered to certain arrangement. Chlorideexists mainly in the fluid inclusions. Some sodium ions exist in the fluid inclusions. Some sodium and most magnesium ions may reside in interstitial positions amongstructural water molecules by dipole-ion interaction with water.Effect of impurities (sodium, magnesium and chloride) on thermal dehydrationkinetics of synthetic gypsum was studied by TG-DSC. The results show that theactivation energy of gypsum containing impurities is lower than that of pure gypsum.The activation energy of gypsum containing chloride ions is lower than that of sodiumand magnesium ions. Lower activation energy is responsible for lower dehydrationtemperature. It can be noted that with dehydration temperature increasing, impuritiesexisted in fluid inclusions or interstitial positions among structural water are released,and these impurities are adsorbed at crystal surface of the highest energy. It leads tothe deformation and distortion of gypsum lattice. Consequently synthetic gypsumcontaining impurities exhibits lower activation energy.Effect of impurities (sodium, magnesium and chloride) on the hydration andhardening performance of synthetic gypsum was studied by conductivity, ICC andSEM. The results show that with an increase of sodium ions concentration, the settingtime is retarded and the compressive strength is reduced, but there is a suddendecrease in sodium ions up to0.5wt%. It is believed that sodium ions acceleratehydration by increasing rate of hemihydrate dissolution. In the presence of sodiumions, gypsum plaster which normally crystallizes into long interlocking needle shapedcrystals has been modified to lath and twin shaped crystals of variable sizes. Thesetwin and lamellar-like crystals are related to poor interlocking structure. Magnesiumions up to0.5wt%have no effect on the initial setting time, and reduce the finalsetting time. In the presence of0.9wt%magnesium ions, the setting time of gypsumplaster has been retarded.There is many pots at the surfaces of gypsum. It is inferedthat magnesium ions are adsorbed at the crystal surfaces of growing gypsum, whichresults in retardation of hemihydrate hydration. In the presence of magnesium ions,the hyration products exhibit interlocking structures. In the presence of0.5wt%chloride, the setting time is retarded, and the compressive strength is reducedconsiderably. The formation of poor lath and tabular shaped crystals are responsiblefor fall in strength. |
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