Influences of citric acid on the metastability of α-calcium sulfate hemihydrate in CaCl2 solution

Alpha-calcium sulfate hemihydrate (α-HH), an important class of cementitious material, has found its promising way of preparation from dihydrate (DH) in salt solution. However α-HH prefers to hydration to DH or dehydration to anhydrite (AH) due to its metastability. Thus it is crucial to expand the metastable lifetime (MLT) of α-HH for the advantage of preparation and separation in industry. In the concentrated 3.54 M CaCl2 solution, α-HH transforms to DH and exhibits an increase in MLT from 0.5 to 48 h along with temperature from 25 to 65 °C, and then the transformation bends to AH and shows a monotonous decrease in MLT to 6 h at 95 °C. A 0.1–1.0 mM of citric acid allows a redouble expansion of the MLT, especially at temperature from 55 to 75 °C. The chemical adsorption of citric groups on the crystal surfaces hinders the nucleation of new phase and increase the energy barrier for α-HH phase transition, resulting in an expansion of MLT. This work presents a simple, highly efficient approach to expand the MLT of α-HH and will benefit for the control of product purity.