Simultaneous effect of Na2EDTA on the phase transformation and morphology evolution during the transformation of gypsum into α-calcium sulfate hemihydrate

Abstract

Na₂EDTA 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 Na₂EDTA 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 Ca²⁺ and EDTA²⁻ in solution while further increase to 2.20 mM doesn’t continuously increase the induction time and phase transformation time. Meanwhile, the addition of Na₂EDTA 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 EDTA²⁻ on both the top and side faces of α-HH crystals where Ca²⁺ ions are distributed at different densities. This selective adsorption provides more details compared with the conventional understanding of EDTA²⁻ adsorption focusing on the top faces. Altogether, this work gives a clear view of the relationship between Na₂EDTA 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.