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

Aipha-calcium sulfate hemihydrate (alpha-HH), an important class of cementitious material, has found its promising way of preparation from dihydrate (DH) in salt solution. However alpha-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 alpha-HH for the advantage of preparation and separation in industry. In the concentrated 3.54 M CaCl2 solution, alpha-HH transforms to DH and exhibits an increase in MLT from 0.5 to 48 h along with temperature from 25 to 65 degrees C, and then the transformation bends to AH and shows a monotonous decreasein MLT to 6 h at 95 degrees C. A 0.1-1.0 mM of citric acid allows a redouble expansion of the MLT, especially at temperature from 55 to 75 degrees C. The chemical adsorption of citric groups on the crystal surfaces hinders the nucleation of new phase and increase the energy barrier for alpha-HH phase transition, resulting in an expansion of MLT. This work presents a simple, highly efficient approach to expand the MLT of alpha-HH and will benefit for the control of product purity. (C) 2013 Elsevier B.V. All rights reserved.