Lime (CaO): The main ingredient of cement is lime. Its ratio ranges from 60 to 67 percent. Excessive lime makes the cement poor and causes the cement to spall and crack and also inhibits the setting properties. On the other hand, if lime is deficient, it reduces the strength of cement.
Silica (SiO2): It constitutes 17 to 25 percent of ordinary Portland cement. It imparts strength to cement due to the formation of dicalcium and tricalcium silicates. An excess of silica increases the strength of the cement, but also prolongs the setting time.
Alumina (Al2O3): It acts as a flux and lowers the clinkering temperature. It imparts quick setting property to cement. If excessive, it weakens the strength of the cement.
Calcium sulfate (CaSO4): This component occurs in the form of gypsum. It is usually added to cement in very small amounts (2 wt percent) towards the final stage of cement preparation to shorten the setting time of the cement.
Iron oxide (Fe2O3): It is responsible for giving the characteristic gray color to cement. Its percentage varies from 0.5 to 6 percent.
Magnesia (MgO): Magnesia varies from 0.1 to 45%. An excess of magnesia reduces the sound of cement. It gives hardness and color to cement.
Sulphur: It varies from 1 to 2.5 percent. If it is excessive, it makes the cement irregular.
Alkalis: Most of the alkalis present in the raw material are carried away by the flue gases during heating. If they contain high cement content, they result in alkali-aggregate reaction, efflorescence and staining when used in masonry work.
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