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Using phosphogypsum (mainly composed of dihydrate gypsum) as raw material to prepare α-hemihydrate gypsum (α-HH) is an effective way for high-value utilization of phosphogypsum solid waste. Sodium ions can effectively promote the transformation of dihydrate gypsum to α-HH and enhance the driving force of crystal transformation. However, during this process, sodium impurities may be doped into the α-HH crystals, resulting in an efflorescence phenomenon when α-HH products are used, which affects the use of the products. In this paper, the formation of sodium impurities in the process of preparing α-HH in NaCl solution was studied by the laboratory test, XRD, XPS, SEM and XRF. Possible forms and diffusion paths of sodium impurities in hemihydrate gypsum crystals are studied by density functional based tight binding methods. The results show that sodium impurities tend to exist in hemihydrate gypsum crystals by doping at interstitial sites. Moreover, there are a large number of interstitial sites in hemihydrate gypsum, which are critical for the diffusion of sodium impurities. [Display omitted] • Possible forms and diffusion paths of Na impurities in hemihydrate gypsum crystals are studied. • The interstitial site in hemihydrate gypsum crystals is critical for Na impurity doping and diffusion. • The crystal transformation of DH to α-HH in solution is the key condition for the formation of Na impurity. [ABSTRACT FROM AUTHOR]