Cement Rock is the term given to a limestone containing up to 25% clayey (argillaceous) material. By calcining and grinding this naturally occurring rock, a material known as Natural Cement or Roman Cement is produced. No additional constituents are added to the production process. Its properties are between that of a Portland cement and hydraulic lime.
The calcining temperature is too low (600 to 1220oC) for sintering and little tricalcium silicate C3S produced, therefore the strength development is slow. However, the dicalcium silicate C2S is considerable hence the initial set and hardening is rapid in comparison to Portland cement.