The main ingredients in the manufacturing of cement are lime, silica, alumina and other oxides. The ingredients present in Ordinary Portland Cement are given in Table 6.1. The contributions by different ingredients are discussed below.
Table 6.1 Ingredients of Portland cement
Lime
It is the main constituent which contributes to the strength. Its proportion has to be properly maintained so as to make the cement sound and strong. Excess of lime makes the cement unsound and causes the cement to expand and disintegrate. On the other hand, if lime is deficient, the strength of the cement is reduced and the cement sets quickly.
Silica
It contributes to the strength of cement due to the formation of dicalcium and tricalcium silicates. The excess presence of silica prolongs the setting time of cement but increases its strength.
Alumina
The quick-setting quality is imparted to the cement by alumina. Excess alumina reduces the strength of cement.
Iron Oxide
It provides colour, hardness and strength to the cement.
Magnesium Oxide
The correct quantity of magnesium oxide imparts hardness and colour to the cement. Soundness of cement is seriously affected, if it is present in excess quantity.
Sulphur Trioxide
It makes the cement sound when present in small quantities. Excess of sulphur trioxide causes the cement to become unsound.
Alkalis
Alkalis should be present only in small quantities. Excess quantities of alkalis cause efflorescence and straining when used in concrete or mortar.
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