The effect of Al3+ and Mg2+ ions, as additives, on the crystallization of gypsum was studied under simulated conditions of the phosphoric acid production. Calcium hydrogen phosphate and sulfuric acid were mixed with dilute phosphoric acid at 80degreesC, and the turbidity of the reaction mixture was measured at different time periods to calculate the induction time of gypsum crystals formation. Addition of Al3+ ions up to 2% decreased the induction time and increased the growth efficiency while addition of Mg2+ increased the induction time and decreased the growth efficiency compared with in absence of additives. Interestingly, the crystals mean and median diameters were found to increase in the presence of Al3+ and decrease in the presence of Mg2+. The surface energy increased with Al3+ and decreased with Mg2+ compared to the baseline (without additives). Gypsum morphology changed from needle-like type in absence of additives to thick-rhombic in the presence of Al3+ ions. (C) 2004 Elsevier B.V. All rights reserved.