Huge amount of flue gas desulphurization (FGD) gypsum not only occupies the farmland but also causes severe pollution to the surrounding environment. The most effective way to achieve a high-value utilization of FGD gypsum is to prepare short columnar alpha-calcium sulfate hemihydrate (alpha-HH) since short columnar crystals show better mechanical strength than needle-like ones. Here, malic acid, a prolific, inexpensive and environment-friendly modifier was explored for the first time to effectively tune the crystal morphology of a-HH prepared from FGD gypsum in glycerol-water-NaCl solutions. When the concentration of malic acid reached 18.54 x 10(-4) mol/kg, alpha-HH crystals with an average aspect (length-to-diameter) ratio of 1.9 (compared to 29.4 in the absence of malic acid) were prepared. The selective complexation of malic acid with Ca active sites on different alpha-HH crystal planes played a dominant role in the alpha-HH crystal morphology transformation, which was then explained by the surface broken bonds theory for the first time. The broken bond number per active Ca atom (N-get (Ca)) and broken bond density of Ca atoms (D-b(Ca)) on the (2 0 4) end plane were larger than those on the (0 2 0) or (2 0 0) side planes. Therefore, the (2 0 4) end plane was more reactive with organics, resulting in the preferential adsorption of malic acid on the end planes, which reduced the specific surface energy of (2 0 4) and led to an increased exposure of this plane and a decreased exposure of (0 2 0) or (2 0 0) side planes in the final alpha-HH crystals. Consequently, using malic acid as modifier, the alpha-HH crystal gradually transformed from a needle-like shape to a short columnar one. This work provided important insights into and perspectives for the selection of crystal modifiers and explanation of the mechanism during the preparation of calcium-containing crystals with controllable morphology. (C) 2018 Elsevier Inc. All rights reserved.