The influence of CaF 2 on the formation and hydration properties of C 4A 3S was studied. Results showed that the product of C 4A 3S can increase by adding a suitable amount of CaF 2, but if the amount of CaF 2 is 1.0%, the product of C 4A 3S decreases. Addition of CaF 2 will result in the crystal of C 4A 3S existing in tetragonal structure mainly. At this experimental condition, a suitable amount of CaF 2 can increase the hydration activity of C 4A 3S and change the morphology of hydration product crystal.