Effects of different carboxyl additives on the growth habits of hemihydrate gypsum crystals

Hemihydrate gypsum whiskers with high aspect ratios were prepared by using phosphogypsum as the raw material through the autoclaved method. The effects of additives containing different carboxyl groups, namely, sodium stearate, sodium citrate and EDTA-tetrasodium, on the average length and average aspect ratio of the hemihydrate gypsum whiskers were studied. The results show that hemihydrate gypsum whiskers with longer average length and larger average length–diameter ratio can be prepared by adding sodium stearate or EDTA-tetrasodium. The crystallisation behaviour of hemihydrate gypsum crystals with different crystal faces under different carboxyl additives was investigated through XRD, FTIR, XPS, and molecular dynamics simulation. XRD, FTIR, XPS results demonstrated that different carboxyl additives had less effect on promoting hemihydrate gypsum crystal growth along the C-axis direction at increasing number of carboxyl groups, and sodium citrate reacted with calcium ions, while sodium stearate and EDTA-tetrasodium did not react with calcium ions. Molecular dynamics simulation results manifested that the order of interaction energy between disparate carboxyl additives and hemihydrate gypsum crystal along the C-axis crystal plane was ΔE (EDTA-tetrasodium) < ΔE (sodium citrate) < ΔE (sodium stearate), indicating that the adsorption of various carboxyl additives on hemihydrate gypsum crystal along the C-axis crystal plane was related to the number of carboxyl groups.