Chemistry of Cement

Cement shows hydraulic binding properties under water. It is made from calcining and sintering of finely ground and well blended mixture of oxides of Calcium, Aluminium, Silica and Iron. Naturally occuring  Limestone is the main source of Calcium Carbonate ( Ca CO3), which when heated to 650 to 900 deg. C , decomposes and lets off C02 gas and converts itself to Calcium Oxide( CaO).This process is known as calcination.Clay, Marl, Lime Marl are good sources for Aluminium Oxide (Al2O3), Silicon Dioxide (SiO2).Iron Oxide can be obtained from Laterite or Roasted Pyrites or clay rich in Fe2O3.

For convenience, in the process technology of cement, these oxides are represented as follows:

The final composition of cement , then, is presented as:

It simply means , formation of cemplex compounds of cement by first calcining of CaCO3 and then sintering to 1350 to 1500 deg. C to enable CaO to combine with SiO2 , Al2O3 and Fe2O3.

At the end of the reactions, it must be ensured that free lime (CaO) presence is minimum ( < 1%)

The semi liquid sintered material is cooled in air to morm cement clinker which is a stable product and can be stored for a long period.

The clinker is finely ground along with 3 to 5 percent gypsum to form cement.

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