New insight into influence of the morphology of α-calcium sulfate hemihydrate on the properties of high-strength gypsum blocks: Experimental and DFT simulation studies

Gypsum blocks are attractive materials, yet their application is restricted because of their poor mechanical properties. The morphology of alpha-calcium sulfate hemihydrate (alpha-HH) is one of the most important factors in determining the performance of gypsum blocks. In this study, alpha-HH crystals with different morphologies were synthesized from purified flue gas desulfurization (FGD) gypsum by a hydrothermal method and gypsum blocks were produced using the alpha-HH crystals as matrix materials. Density functional theory (DFT), scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to systematically analyze the influence mechanism of citric acid on the alpha-HH morphology and the influence mechanism of alpha-HH morphology on the properties of gypsum blocks. The results of this study demonstrated that the properties of gypsum blocks were highly responsive to changes in the morphology of alpha-HH crystals. When the citric acid concentration was 0.20 %, the prepared alpha-HH was dominated by short columns, and the average aspect ratio was close to 1. DFT calculations confirmed citric acid was preferentially adsorbed on the (002) end plane, which reduced the surface energy of this plane, inhibited its growth and led to an increased exposure of this plane; this resulted in the morphological transformation of the alpha-HH crystals from fibrous to short columnar and plate-like. The particle sizes of the short columnar alpha-HH crystals were the smallest, and the (020) crystal plane parallel to the c-axis had the largest interplanar spacing, dislocation density and microscopic stress; these characteristics were conducive to the hydration reaction of the alpha-HH crystals, resulting in a 14.29 % reduction in the initial setting time and a 6.96 % decrease in the water requirement of normal consistency of the gypsum block compared to the fibrous alpha-HH crystals. Moreover, the gypsum blocks had the highest 2 h bending strength and dry compressive strength of 5.21 MPa and 37.67 MPa, respectively; these results met the strength grade requirements of alpha 30 high-strength gypsum. This work provides theoretical guidance for the industrial production of high-strength gypsum blocks with stable properties.