Upcycling of industrial by-product gypsum for the synthesis of short columnar α-hemihydrate gypsum using erythritol

Alcohol–water systems, extensively used for converting waste gypsum into alpha-calcium sulfate hemihydrate (α-HH), typically take over 2 h to yield high-quality α-HH. This work developed an innovative erythritol–water medium, supplemented with trace mixed organic acids, which expedited the conversion of industrial by-product gypsum into high-quality α-HH within just 1 h, outpacing the majority of documented alcohol–water systems. The resultant α-HH exhibited a large-size and short columnar form with an aspect ratio of 2.32, marking a significant advancement for the industrial-scale production of α-HH. Importantly, α-HH synthesized using erythritol showcased higher crystallinity compared to commonly employed medium such as glycerol and ethylene glycol, due to the fact that the hydroxyl groups of erythritol adhered to facets of α-HH, occupying the vacancies at Ca sites and thereby inhibiting the active growth sites. Further investigations revealed that erythritol fostered favorable thermodynamic conditions for the dissolution of dihydrate gypsum, and established a homogeneous supersaturated surrounding for the nucleation and growth of α-HH, thereby resulting in the formation of α-HH with reduced aspect ratio and improved dispersibility. This study provides a cost-effective alternative to the common glycerol–water system and chloride salt solution for waste gypsum conversion to α-HH.