Lime (CaO): This is the Major constituent of Cement. This must be within the permissible limit. Calcium oxide of lime combines with silica from clay and forms the Dicalcium & Tricalcium silicates on burning. These are mainly responsible for the Hydration, setting and Hardening of the cement. If lime is excess then this will make the cement Unsound. It expands too much on setting. Similarly if the quantity of lime is less then it will affect the strength and it sets quickly.
Silica (SiO2): This is the second most important constituent and is responsible for Strength. Excessive silica causes cement to be much stronger but will be more setting time.
Alumina (Al2O3): This is also one of the chief constituent which is responsible for Quick setting of cement. This acts as a flux. Means it lower down the temperature in which the different ingredients react to form clinker. Excess alumina causes cement highly quick setting. Less Alumina causes the setting qualities of cement.
Calcium Sulphate (CaS04): This is added to cement after burning process in the form of Gypsum (CaSo4, 2H2O). It acts as retarder. Means prolongs the initial setting time. In the absence of Gypsum the cement will be very quick setting. It will set in a few minutes after mixing with water.
Magnesia (MgO): When present in small quantity it helps to provide color to Cement, if it is more it will cause delayed setting & expansion resulting in cracking.
Alkalies: Most of the Alkalies present in raw materials will be removed in the process of manufacture and a small quantity of Alkalies will be left out in cement. If the quantity of Alkalies is more it will create Alkali- Aggregate reaction, Efflorescence, Staining etc.
The above mentioned ingredients undergo complex reactions during the manufacture of Cement especially at burning stage. The burnt product as it comes out of the cement Kiln is Called CLINKER. Various compounds identified in this clinker and their functions are of basic importance...