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 alpha-calcium sulfate hemihydrate (alpha-HH) whiskers, with calcium sulfate dihydrate (DH) as the precursor, in ethylene glycol (EG)-water solutions at 95.0 degrees C and atmospheric pressure. The as-synthesized alpha-HH whiskers exhibit a width of 3-5 mu m and a length of 600-700 mu m, showing highly promising applications as reinforcing agents. An EG volume percentage of 22.5 vol % is necessary to drive the conversion of DH to alpha-HH at 95.0 degrees 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-alpha-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 alpha-HH nucleation, the surface property of precursor DH via Na+ doping, as well as surface precipitate of the solid solution eugsterite (Na4Ca(SO4)(3)center dot 2H(2)O). In shape control, an increased Na+ concentration can elongate the alpha-HH but simultaneously raise the possibility of whisker agglomeration. The bridging action of Na+ among the crystal interfaces associated with the alpha-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 alpha-HH whiskers, which should lead to a deeper understanding of whisker formation and advance their scale applications.