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Natural fibrous gypsum is a kind of high-quality gypsum resources, whose high-value added utilization pathway is to be transformed to α -calcium sulfate hemihydrate (α -HH), a cementitious material that can react with water to form pastes with mechanical strength. In this work, α -HH was prepared from natural fibrous gypsum in KCl mediated glycerol-water solution under mild condition. KCl played a simultaneous function on regulating the phase-transition kinetics, morphology and comprehensive strength of α -HH. Addition of low concentration of KCl prominently retarded the nucleation and growth rate of α -HH and decreased its aspect ratio due to the formation of syngenite on the side faces. While further increase of KCl concentration accelerated the nucleation and growth rate and decreased the crystal sizes. In the presence of 0.04 M KCl, the prepared α -HH presented a hexagonal prism shape with a length / width of 257.69–579.71 μm / 13.13–36.61 μm, and its dry comprehensive strength had reached 38.1 MPa that was comparable to those prepared in commercial, chloride and nitrate system. Altogether, the foreign ion-free glycerol-water solution contributed to transforming natural fibrous gypsum to α -HH with high purity and high mechanical strength. • Transformation of natural fibrous gypsum to α -HH in glycerol-water solution. • Regulation of KCl on phase-transition kinetics, morphology and mechanical strength. • Hexagonal prism shape α -HH with high aspect ratio presents high mechanical strength. • α -HH paste with high mechanical strength has a compact microstructure. [ABSTRACT FROM AUTHOR]