The hydration rate of C3S is accelerated in the presence of limestone probably because of the dilute effect and the multiplied nucleation sites. Jean Pera et al. reported that the total heat resulting from pure C3S hydration was 145 joules while that of the mixture (50%C3S+50%CaCO3) reached 260 joules. Ramachandran’s investigation showed that some calcium carbonate was consumed as the hydration of Portland cement proceeded. The product of the reaction of limestone and C3S is calcium carbosilicate, whose exact type cannot be identified by X-ray diffraction probably due to its amorphous property and small amount of formation.