Effect of maleic acid and pH on the preparation of α-calcium sulfate hemihydrate from phosphogypsum in Mg(NO3)2 solution

Phosphogypsum (PG) is a by-product of the wet-process phosphoric acid process, which causes serious environmental issues. In this work, PG was taken as raw material to prepare α-calcium sulfate hemihydrate (α-CSH) at atmospheric pressure using Mg(NO3)2 solution, which is an environmentally friendly chlorine-free salt solution. The crystallization water content and mechanical strength of α-CSH were detected, meanwhile the relationships between structure, morphology and properties were discussed deeply by DSC, XRD, SEM, FTIR and XPS characterization techniques. The results showed that with the increase of maleic acid (MA) concentration and slurry pH, the dehydration rate of PG reduced, causing the changing of α-CSH crystal from elongated prism to short column. MA is adsorbed on the surface of α-CSH by the complexation of carboxyl (COO) and Ca atom, adjusting the growth of crystal. Well crystallized α-CSH can be successfully prepared by the hydrothermal method in 25 wt% Mg(NO3)2 solution, with a reaction time of 6.0 h at 95 °C, and the average aspect ratio is 1.24. The corresponding sample paste has the 3d bending/compressive strength of 8.9/36.8 MPa, displaying better properties than commercial product. This chlorine-free solution method provides an alternative technology for preparing α-CSH with controllable morphology and properties.