Simultaneous effect of Na 2 EDTA on the phase transformation and morphology evolution during the transformation of gypsum into α -calcium sulfate hemihydrate

Na 2 EDTA is an effective morphology regulator in preparing α -calcium sulfate hemihydrate ( α -HH) with high mechanical strength, but its simultaneous effect on the phase transformation process and morphology evolution of α -HH is still unclear. This study finds that the addition of Na 2 EDTA from 0.00 to 1.40 mM dramatically prolongs the induction time and phase transformation time from 2 to 240 min and 118 to 480 min respectively, due to the complexation of Ca 2+ and EDTA 2− in solution while further increase to 2.20 mM doesn’t continuously increase the induction time and phase transformation time. Meanwhile, the addition of Na 2 EDTA generates the morphology evolution of α -HH from fibrous to short columnar with an average aspect ratio from 31.5 to 1.7, resulting from the selective adsorption of EDTA 2− on both the top and side faces of α -HH crystals where Ca 2+ ions are distributed at different densities. This selective adsorption provides more details compared with the conventional understanding of EDTA 2− adsorption focusing on the top faces. Altogether, this work gives a clear view of the relationship between Na 2 EDTA concentration and phase transformation and morphology evolution of α -HH, which contributes to providing theoretical guidance on preparing α -HH crystals with an appropriate phase transformation rate and ideal morphology.