Effect of EDTA on the morphologies and mechanical properties of hydrothermally prepared calcium sulfate hemihydrate crystals

For the efficient utilization of industrial waste gypsum resources, their conversion into α-type calcium sulfate hemihydrate represents a feasible approach. Notably, the post-hydration mechanical properties of α-calcium sulfate hemihydrate considerably depend on its morphology and size. This study attempted to control the morphology of α-calcium sulfate hemihydrate using ethylenediaminetetraacetic acid disodium salt (EDTA-2Na). Experimental and simulation analyses (DFT and MD) were conducted to elucidate the action mechanism of EDTA on the crystal morphology of calcium sulfate hemihydrate. Furthermore, its effects on the mechanical properties of calcium sulfate hemihydrate after hydration were explored. It was found that the length to diameter ratio of calcium sulphate hemihydrate decreased from 5.47 to 1.9 with increasing addition and the strength of the hydrated blocks increased from 32.1 MPa to 47.95 MPa. The results revealed that EDTA effectively transformed the morphology of α-calcium sulfate hemihydrate from acicular whiskers to short columnar crystals, and the EDTA molecules chemisorbed onto the surfaces of α-calcium sulfate hemihydrate, stabilizing their adsorption and thereby regulating the crystal morphology. The findings of this study can aid in the efficient utilization of industrial waste gypsum resources and in mitigating pollution caused by them.