Doping behavior and occurrence state of Na impurity in α-calcium sulfate hemihydrate prepared in Na2SO4 solution

alpha-calcium sulfate hemihydrate (alpha-CSH) is a green building material. The synthesis of alpha-CSH from industrial phosphogypsum (PG) in Na2SO4 solution is a promising technique, but Na impurity may be introduced into alpha-CSH crystal. In this study, the doping behavior of Na into alpha-CSH was investigated, and the occurrence state of Na in alpha-CSH was clarified. The results show that Na impurity was doped into alpha-CSH in the form of Na2Ca5(SO4)2 & sdot;3H2O, and the doping content of Na2O in alpha-CSH increases with the Na2SO4 concentration and reaction time. Density functional theory (DFT) calculation shows Na impurity is likely to be doped into alpha-CSH crystal through interstitial filling. Unfortunately, the doping of Na into alpha-CSH could cause a visible frost phenomenon for the hardened plaster, which has a negative impact on the practical utilization of alpha-CSH.