Retardation of setting of plaster of Paris by organic acids: understanding the mechanism through molecular modeling.

To develop an understanding of the action of specific formulations, the growth of gypsum crystals under the influence of retardation agents (tartaric and citric acid) has been studied using molecular modeling. Surface energies of gypsum and plaster crystal faces were calculated using established protocols. The crystal morphology predicted for gypsum crystals in the absence of retardation agents is in excellent agreement with experiment. The simulations show that only in an alkaline environment is the crystal morphology of gypsum changed by retardation agents. The simulations provide a detailed description of retardation, for example, the specific mechanisms by which tartaric and citric acid retard setting of gypsum and how they differ. At high pH meso, D(-), and L(+) tartaric acid inhibit both the growth of gypsum and the dissolution of plaster while at low pH tartaric acid and citric acid will principally inhibit the growth of gypsum. The simulations provide a molecular rationalization for a range of experimental observations and a basis for the selection of alternate retardation agents.