Effect of modifiers on crystalizing habit and mechanical strength of α-hemihydrate gypsum prepared from PG by an autoclaved method

α-Hemihydrate gypsum (α-HH) was prepared by autoclaving phosphogypsum (PG) in the presence of potassium sodium tartrate, succinic acid, potassium sulfate and aluminum sulfate as modifiers. Through testing the crystal water content, XRD, XPS, SEM, and strength, and carrying out the molecular dynamic simulation, the effect and mechanism of modifiers on the crystalizing habit and mechanical strength of α-HHs was researched. The SEM pictures showed that, some α-HH crystals presented the form of fine grains, well-crystallized α-HHs were monoclinic and hexagonal and presented short column or long rod shape. The addition of modifiers obviously reduced the content of fine grains and the aspect ratio of well-crystallized crystals. The influence of modifiers on crystal morphology is related to the dehydration rate of PG, total conversion time of PG into α-HH, and selective adsorption of modifiers on crystal surface. Succinic acid and potassium sodium sulfate inhibited the dehydration of PG for 1 h, and extended the total conversion time to 5 h. The addition of potassium sulfate and aluminum sulfate did not change the induction period, only shortened the total conversion time to 3 h. The modifiers did not enter the structure of α-HH crystal, but preferentially absorbed on the plane of (1 1 1) and influenced the growth along the top plane. Considering the aspect ratio and content of well-crystallized crystals, succinic acid and potassium sodium tartrate had more significant effect than potassium sulfate and aluminum sulfate. The strengths could be improved by adding the modifiers, and the effect of succinic acid was the best. The flexural and compressive strengths of all α-HHs were greater than the values of 4.5 MPa and 30Mpa, and met the requirements of high strength gypsum of α30.