Preparation of different phases of hemihydrate from Alamout natural gypsum and study of effective parameters on setting time

In this research the α and β phases of hemihydrate have been prepared from Alamout natural gypsum and their setting times have been measured. The IR spectroscopy has been used for characterization of α and β phases of hemihydrate from gypsum. The α and β phases of hemihydrate distinguished from each other by X-ray powder diffraction (XRD). The setting times of pure α and β phases and mixtures of them have been measured by Vicat method. The effects of various additives such as accelerators (NaCl, K2SO4, Na2SO4) have been studied on setting time. Among them, potassium sulfate had maximum accelerating effect. Optimum concentration of the additive has been determined. This amount is %3 w/w. Less and more than this amount have inverse effects on setting time.