Role of temperature in the spontaneous precipitation of calcium sulfate dihydrate

The spontaneous precipitation of calcium sulfate in supersaturated solution s over the temperature range between 25.0 and 80.0 degrees C was investigat ed by monitoring the solution specific conductivity during desupersaturatio n. From measurements of the induction times preceding the onset of precipit ation the surface energy of the forming solid, identifield as gypsum, was f ound between ca. 12 and 25 mJ m(-2) for the temperature range between 80.0 and 25.0 degrees C, respectively. Kinetics analysis showed that over 50 deg rees C it is possible that anhydrous calcium sulfate is forming as a transi ent phase converting into the more stable calcium sulfate dihydrate. The li near dependence of the rates of precipitation on the relative solution supe rsaturation suggested a mechanism according to which the growth units are i ntegrated into the active sites of the supercritical nuclei by surface diff usion. According to the morphological examination of the crystals it is pos sible that crystal growth occurs by the advancement of steps.