The effect of K2SO4 activator on the hydration of chemical anhydrite obtained from burned FGD-gypsum has been studied by different experimental techniques. Results obtained show that the degree of hydration increases when the K2SO4 concentrations increase from 0.5 to 3.3 wt%. Their heat evolution rate and maximum value also increase with the increase of K2SO4 concentration. The highest values were obtained when the hydration degree was about 50%. Also a correlation between the hydration degree and the total heat evolved was obtained. The X-ray and SEM/EDX studies have shown that K2SO4 is adsorbed at the surface of CaSO4 even within 5 min of hydration and a syngenite a double salt K2SO4 . CaSO4 - H2O is formed (even in the presence of 1.0 wt% K2SO4). Also, important changes in the morphology of the dihydrate crystal are detected. Finally, in the presence of 1.0 wt% K2SO4 (water/anhydrite (WIA) ratio = 0.33), it was found that the resonance frequency, modulus of elasticity, compressive strength, and tensile strength increase with the degree of hydration whereas the total porosity decreases.