Sodium Cation-Mediated Crystallization of α‑Hemihydrate Whiskers from Gypsum in Ethylene Glycol–Water Solutions

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.