CEMENT

Cement 

Cement is a fine gray powder of calcium silicate and calcium aluminate that is prepared by calcium carbonate (CaCO3) and special type of clay (AhO3SiO2). It sets into hard rock like form that binds other materials together during construction of infrastructures. 

Manufacture of cement 

The raw materials that are used for the manufacture of cement are:

1. Lime stone (CaCO3)

2. Special type of clay (AhO3_SiO2).

3. Gypsum (CaSO4.2H2O)

The manufacture of cement from above mentioned raw materials involves following steps:

1. Crushing and grinding

The big lumps of limestone and the special type of clay are first crushed and grounded into fine powder separately.

2. Slurry formation

The powder of limestone and clay is mixed in 2:1 ratio along with water to make semi liquid mixture known as cement slurry.

3. Heating of slurry

The slurry thus formed is fed into the rotary kiln where it is heated at the temperature of about 1400 C to 1600 C.In the kiln limestone decomposes into calcium oxide and carbon dioxide. The calcium oxide so formed combines with alumina (AhO3) and silica (SiO2) to form calcium aluminate and calcium silicate respectively. As a result, small pea sized red balls of cements are formed which are known as cement clinkers. 

Chemical composition of cement clinkers are calcium silicate and calcium aluminate. 

4. Mixing with gypsum

To slow down the setting time of cement and improve its quality, the cement clinkers are mixed with 2-3% of hydrated calcium sulphate known as gypsum.

5. Final grinding

The mixture is again ground into fine powder. This fine powder is finally called cement, which is packed into airtight bags and stored in dry place to protect it from moisture.

Uses:

• Cement is used to make cement mortar, which is a mixture of cement, sand and water, which is used for building and plastering of walls.

• It is used to make concrete, which is a mixture of cement, sand, water, and gravel, which is used for roofing, flooring and making pillars of buildings.

• It is used to make reinforced cement concrete (RCC) which is a mixture of cement, sand, water and gravel along with iron framework inside which is used for making pillars, roofs, floors of building and bridges.