α-calcium Sulfate Hemihvdrate Prepared From FGD Gypsum In Ca（NO₃）₂ Aqueous Solution And The Role Of K₂SO₄ In The Crystallization

a-calcium sulfate hemihydrate (a-HH) is an important class of cementitious materials, and widely used in constrution, medicine, mold and other fields for its superior performance. Salt solution method is the most promising technology, in which a-HH is prepared in mild conditions. This work presents a new method of a-HH preparation in Ca(NO3)2 solution under atmosphere, using Flue gas desulfurization (FGD) gypsum as raw material. Compared to the CaCl2 solution which is commonly used as the crystallization medium, the Ca(NO3)2 solution benefits for reducing the possibility of facility corrosion and increasing the product quality.In the Ca(NO3)2 solution, K2SO4 was added to accelerate the dehydration kinetics, optimize the habits and mechanical property of the products. As the concentration of K2SO4 increases from 0 mM to 87.50 mM, the transformation is accelerated significantly, and the time for the total transformation decreased from 11 h to 3 h. The mechanism study indicates that the synergy of K+ and SO42- account for the accelerating effect, and SO42- play a more significant role. The size of crystals varies with K2SO4 concentrations. The length of a-HH decreased from 354.28 μm to 55.11 μm, and the width decreased from 48.88 μm to 10.81 μm when the concentration of K2SO4 adjusted from 0 mM to 87.50 mM. Corresponding aspect ratio decreased from 7.25 to 5.10. Mechanical strengthes of a-HH synthesized under different K2SO4 concentrations are measured. As the concentration adjusted from 0 mM to 87.50 mM, the 2 h bending strength decreased from 5.62 MPa to 2.47 MPa, and 2 h compressive strength decreased from 11.20 MPa to 4.10 MPa. At the same time, the dry bending strength decreased from 9.63 MPa to 5.72 MPa, and the dry compressive strength decreased from 22.90 MPa to 16.83 MPa. The size of hydration product increased and the morphology changed from plate, sheet to needle-like. The interlocking between crystals is not compact and the pore diameter increased, so the structure of the cement destroyed and the strength decreased.The work presents a new crystallization medium of Ca(NO3)2 solution for a-HH preparation from FGD gypsum under atmospheric pressure at 95℃. Further addition of K2SO4 plays a vial role in modulating the transformation kinetics, the crystal morphology and mechanical property of a-HH.