Cement Making Process
The raw materials used for the manufacture of Portland cement are limestone CaCO3 and clay which are finely powdered and then mixed in the ratio 3 : 1 by mass. The mixture is again ground to a fine powder and water is added. The finely ground powder called slurry is heated to 1450-1500°C in a rotary kiln.
At around 1000°C, the limestone CaCO3 converts into quick lime and CO2. as shown in the below reactions:
When it is further heated at around 1450-1500, the Quick lime reacts with various minerals of clay such as provides SiO2 , Al2O3 and Fe2O3 as given in below reactions:
Thus, the output of the heating is a hard substance in the form of a mixture of dicalcium silicate, tricalcium silicate and tricalcium aluminate called clinker. The clinker is cooled and a small amount of gypsum (2 – 5%) is added to it. The mixture of clinker and gypsum is then ground to a fine powder which is called ordinary Portland cement. It is stored in tall structures called silos. The cement is then packed in water-proof bags and sold in markets. Thus, Ordinary Portland cement is comprised of Lime (CaO) (around 60 – 70%), Silica (SiO2)(20 – 25%), Alumina (Al2O3)(5 – 10%) and Ferric oxide (Fe2O3) (2 – 3 %)
Virtually all the cement produced globally is mixed with sand, aggregates and water, and used to make concrete and mortars. Concrete is second only to water as the most consumed substance on earth, with nearly three tons used annually for each person on the planet.