Write a few lines about cement.

Explanation

It is prepared by heating a mixture of limestone and clay. The clay contains sio, along with oxides of aluminium, iron and magnesium. The Cao obtained from limestone combines with these oxides and converts into dicalcium silicate, tricalcium silicate and Ericalcium aluminate. The hard mass containing these compounds is called clinker. It is powdered, mixed with little gypsum to slow down the setting process. The powdered mass is collected in bags and sold as cement.
The average composition of cement is
CaO - 50 to 60%
`SiO_2` - 20 to 25%
`Al_2O_3` - 5 to 10 %
MgO - 2 to 3%
`Fe_2O_3` - 1 to 2 %
`SO_3` - 1 to 2%
For good quality cement the ratio of silica to alumina should be 2.5 to 4 and the ratio of lime to the total oxides of silicon `(SiO_2)`, aluminium (`Al_2O_3`) and iron (`Fe_2O_3`) should be nearly 2.
Cement is used in concrete, reinforced concrete, in plastering and in the construction of bridges, dams and buildings.