This work reports a facile sodium cation (Na+)-mediated approach to synthesize α-calcium sulfate hemihydrate (α-HH) whiskers, with calcium sulfate dihydrate (DH) as the precursor, in ethylene glycol (EG)–water solutions at 95.0 °C and atmospheric pressure. The as-synthesized α-HH whiskers exhibit a width of 3–5 μm and a length of 600–700 μm, showing highly promising applications as reinforcing agents. An EG volume percentage of 22.5 vol % is necessary to drive the conversion of DH to α-HH at 95.0 °C, while the Na+ plays a crucial role in regulating both the conversion kinetics and the whisker quality. The conversion rate presents a volcano-like variation versus Na+ concentration (0.05–0.30 M) with the peak one achieved at 0.10 M Na+. Simultaneous boosting and retarding effects of the metallic cation on the DH-α-HH transformation were reported for the first time in alcohol–water systems. The tuning role of Na+ arises from its effect on the supersaturation for α-HH nucleation, the surface property of precursor DH via Na+ doping, as well as surface precipitate of the solid solution eugsterite (Na4Ca­(SO4)3·2H2O). In shape control, an increased Na+ concentration can elongate the α-HH but simultaneously raise the possibility of whisker agglomeration. The bridging action of Na+ among the crystal interfaces associated with the α-HH nucleation accounts for the morphology evolution of whiskers, and the optimal Na+ concentration is 0.075 M with an average aspect ratio of 105. This work provides a systematical investigation on the cation-mediated crystallization of α-HH whiskers, which should lead to a deeper understanding of whisker formation and advance their scale applications.

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Alcohol–water solution-mediated transformation of gypsum into α-hemihydrate (α-HH) whiskers provides a green alternative for the high-value-added recycling of flue gas desulfurization (FGD) gypsum. However, the role of non-lattice cations during the transformation is still unclear. We report an evolution from “boosting–retarding” to “boosting–retarding–boosting” and finally to “boosting only” effect of non-lattice Na + functioned by the concentration of ethylene glycol (EG) in water solutions. The driving force increased almost linearly upon the introduction of Na + through the formation of ion pairs, and a higher slope was obtained at a higher EG concentration. Adsorption of Na + ions and solidification of eugsterite on gypsum surfaces blocked the nucleation sites of α-HH. The retarding effect first rapidly increased and gradually approached a limit, following a parabolic trend after Na + ions were introduced. Pentasalt, with a structure similar to that of α-HH, precipitated on the gypsum surface at higher c (Na + ). The interaction of the driving force and the structural evolution of calcium sulfate ionic clusters accounts for the evolution of transformation kinetics. The retardation zone was compressed with the increase in EG volume ratios, and a monotonic boosting effect upon Na + was observed at a 35.0 vol % of EG. Nucleation kinetics dominates the aspect ratio of α-HH whiskers. This study may provide a significant guidance for the utilization of FGD gypsum.