Large quantities of solid waste phosphogypsum (PG) not only occupy the farmland but also cause severe pollution to the surrounding environment. The most effective way to achieve a high-value utilization of PG is to prepare a-calcium sulfate hemihydrate (alpha-HH) with various morphologies. In this study, we developed a simple and Cl-free one-pot synthetic strategy to effectively transform PG into single-crystal alpha-HH regular hexagonal microplates (RHMPs) in Na2SO4 aqueous solution at 98 degrees C assisted with sodium citrate (TSC). Na2SO4 plays a role in accelerating the transformation rate, whereas TSC regulates the crystal habit of alpha-HH from rod to regular hexagonal plate. The rate of DH-alpha-HH transformation in aqueous solutions can be effectively controlled by regulating water activity (alpha(H2O)), which decreases with the increase in Na2SO4 concentration. Crystal habit control is attributed to the strong preferential adsorption of citrate ions to the (001) and (002) top facets of alpha-HH by chemical adsorption. The paste made from the obtained alpha-HH RHMPs deserves higher dry compressive/bending strength of 46.1/15.2 MPa, which is comparable with that of alpha-HH prepared by other methods. Our work provides a simple and low-corrosiveness strategy to transform PG into alpha-HH RHMPs with excellent mechanical property, thus showing great promise for a wide range of challenging reuse of solid waste in environmental protection.