Influence of dicarboxylic acid modifiers on the growth habits of α-hemihydrate gypsum crystals prepared from desulfurized gypsum and its mechanisms of action

Abstract

Dicarboxylic acid modifiers play a crucial role in the controlled synthesis of α-hemihydrate gypsum (α-HH) with an aspect ratio (AR) close to 1 from flue gas desulfurized (FGD) gypsum. However, the effect of dicarboxylic acid modifiers in the preparation of α-HH by FGD gypsum and the mechanism of action is unclear. Therefore, this paper systematically explored the impact of maleic anhydride, succinic acid, and malonic acid on the crystal growth characteristics, microstructure, and mechanical strength of α-HH under the improved vapor pressure method. The modification mechanism was clarified by XRD, FT-IR, SEM, EDS, XPS, and other test methods. The results showed that malonic acid had little effect on α-HH. Maleic anhydride and succinic acid can effectively transform the long rod-like α-HH into columnar α-HH, with ARs of 3.46 ∼ 2.31 and 2.68 ∼ 1.55, respectively. Among them, the addition of succinic acid increased the dry compressive strength by 40.1 %. The study further showed that the adsorption strengths of maleic anhydride and succinic acid on different surfaces of α-HH crystals were in the order of (111) > (010) > (110), while that of malonic acid were the opposite. This adsorption difference determines the crystal evolution of α-HH. In addition, between the two carboxyl groups of the dicarboxylic acid spaced two C atoms to play a crystal adjustment role. The double bond and ether bond enhanced the complexation ability of Ca²⁺ on the surface of α-HH crystal and promoted the modification effect. This study provides new insights into the selection of modifiers for the production of high value-added products from waste FGD gypsum.