α-hemihydrate gypsum crystal concerning the anionic surfactants with vary chain structure: syntheses, morphology controlling, formation mechanisms

Abstract The control of α-hemihydrate gypsum (α-HH) crystal morphology has a decisive effect on the strength of gypsum products. This study employed natural gypsum as the precursor material and water vapor as the reaction medium. Six anionic surfactants with chain structures features were selected as crystal modifiers comprising three categories of anionic functional groups (R-SO 4 1− , R-SO 3 1− , R-COO 1− ), each with surfactants of varying carbon chain lengths. The α-hemihydrate gypsum (α-HH) was successfully synthesized under controlled hydrothermal conditions at a saturated steam pressure of 0.4 MPa. It is found that anionic surfactants with chain structures can change the α-HH crystal morphology to a short columnar shape. In terms of the impact on the aspect ratio of a-HH, the R-SO 4 1− group has the best effect, while the worst result is represented by R-COO 1− group. For crystal transformation agents containing the same functional group, the larger the molecular weight, the better effect is caused. According to solubility and MD simulations, in the process of natural gypsum reacting to form α-HH, natural gypsum does not completely dissolve but breaks into small crystals, which are then dehydrated and crystallized to form α-HH. Anionic surfactants with chain structures are adsorbed on the crystal faces perpendicular to the c-axis as the crystal size of α-HH increases, hindering the growth of α-HH crystals along the c-axis and ultimately causing the crystal morphology to become short columnar. This work provides insightful guidance from both experimental and theoretical perspectives for controlling the crystal form of α-HH and preparing high-strength gypsum.