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The controllable transformation of flue gas desulfurization (FGD) gypsum to calcium sulfate hemihydrate (HH) whiskers under facile, readily monitored, and cost-effective hydrothermal conditions could play a vital role in the preparation of high quality HH whiskers and improve our understanding of the transformation process. This work assessed the conversion of FGD gypsum to HH whiskers in 5×10–4 mol/L H2SO4 and 1.5 wt% CuCl2 at 120 °C while determining the effects of temperature as well as H2SO4 and CuCl2 concentrations on transformation kinetics. The preparation of HH whiskers was found to involve a solution-mediated transformation from the dihydrate (DH) to the α-HH. This transition was determined to proceed via a dissolution crystallization mechanism, the rate of which was controlled by nucleation and growth of the HH whiskers. An autocatalytic kinetic model was established based on variations in the HH whiskers mole fraction over time, and this model accurately fit the experimental data with R2 = 0.990. Increasing the temperature or H2SO4 concentration accelerated the transformation by modifying the super-saturation and water activity in the reaction solution, while increasing the CuCl2 concentration had the opposite effect. The hydrothermal conditions had an important effect on the transformation from FGD gypsum to HH whiskers.

Effect of calcium sulfate variety on the microstructure and properties of cement-based grouting materials made by sulfoaluminate cement clinker, calcium sulfate resource, and quicklime was investigated in this paper. Isothermal calorimetric, XRD, DTA-TG, and SEM-EDS were used to characterize the reaction process of cement-based grouting materials. The mechanical property and expansion of the harden body were also tested. The results showed that (1) the formation of ettringite can be completed during the first 2 h when gypsum was selected, causing nonexpansion of the harden body, and ettringite tends to be precipitated near the clinker grain, which has a negative influence on the “skeleton effect” of ettringite, resulting in lower strength of the hardened body. (2) Ettringite can be continually and slowly precipitated when an anhydride was chosen, and the hardened body shows significant expansion characteristic. Besides, ettringite in anhydride paste tends to be precipitated in the form of complex interleaving, which can generate a positive influence on mechanical property of the hardened paste.

Utilizing the purified flue gas desulfurization (FGD) gypsum as raw material and AlCl3 as crystal modifier to prepare calcium sulfate whiskers (CSWs) by hydrothermal method, the effects of the AlCl3 concentration and the pH of reaction slurry on the crystal morphology of CSW were discussed. The results show that the AlCl3 concentration and the pH of reaction slurry had a significant effect on the crystal morphology of CSWs. CSWs grown in the AlCl3-H2O slurry had diameters ranging from 2 to 6 um and aspect ratio above 80 μm, and their phase structure was calcium sulfate hemihydrate (HH).