The effect of the addition of chloride and nitrate salts of Zn, Cu, Pb and Cr on the hydration reaction of a mixture of 3CaO. Al2O3 (C3A) and gypsum with molar ratio 1: 3 was studied. Different ratios of each salt were used namely; 0.5, 2 and 4 % by weight of the solid mixture. Hydration reaction was carried out at 35°C for various time intervals; 1/2 hour, 3, 6 and 24 hours, 3 and 7 days. Hydration kinetics of the system 3CaO. Al2O3 – CaSO4. 2H2O in absence and presence of different salts was studied via the determination of the combined water contents.