The present study investigated the effects of Mg2+, Al3+, and Fe3+ ions, as additives, on crystallization of type alpha hemi-hydrated calcium sulfate (alpha-HH) under simulated conditions of hemi-hydrate wet-process phosphoric acid production. Results showed that induction time of alpha-HH without additives reduces with increasing supersaturated ratios, but increases in the presence of Mg2+, Al3+, and Fe3+ ions. Under the same concentration of impurity ions, the order of extending induction time of alpha-HH is as follows: Fe3+ > Mg2+ > Al3+, and surface energy and critical nucleus radius of alpha-HH increase in the presence of Mg2+, Al3+, and Fe3+ ions. Accordingly, nucleation rate and growth efficiency of alpha-HH crystals reduce. Length of alpha-HH crystals decreases and its diameter thicken in the presence of Mg2+, Al3+, and Fe3+ ions. The crystals of alpha-HH easily grow in the direction of [0 1 0], and feature long-needles. On the other hand, Mg2+, Al3+, and Fe3+ ions are mainly adsorbed on the exposed surface {1 - 1 0} of alpha-HH crystals. Thus, crystal shapes of alpha-HH eventually show wedge or short columns. (C) 2018 Elsevier B.V. All rights reserved.