Sodium doping and control during the preparation of α-hemihydrate gypsum in NaCl solution

Transformation of phosphogypsum (PG) into α-hemihydrate gypsum (α-HH) in sodium salt solution is disturbed by the doping of Na, which causes the frost of Na doped α-HH. In this study, the content evolution of Na impurities into α-HH was clarified during the hydrothermal process at 95 ℃, the frost phenomenon of hardened gypsum containing Na impurities was observed and controlled. The results show that increasing the initial NaCl concentration from 5 % to 25 % can promote the phase conversion of PG into α-HH within 30 min, but this will increase the NaO content in the product from 0.08 % to 4.37 %, which is due to the formation of the undesirable phase NaSO·5CaSO·3HO. The content of Na impurities entering α-HH presents a non-linear increase with the NaCl solution concentration and reaction time, which can be well described by Boltzmann function. An obvious frost phenomenon of the hardened plaster of Na doped α-HH can be observed after curing 3 d, and the frost is identified as NaSO. This work further demonstrates the feasibility that the addition of CaCl in NaCl solution is an effective approach to control the doping of Na into α-HH, the NaO content decreases to 0.97 % in the mixed solution of 17.5 % NaCl and 10 % CaCl, this helps to eliminate the frost phenomenon and improve the mechanical strength of α-HH.