Write a few lines about cement.

Explanation

Cement was discovered by Joseph Aspidin. It appears like natural limestone available at Portland in England. So it is also called as Portland cement.
It is Prepared by heating a mixture of limestone and clay. The clay contains `SiO_(2)` 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 tricalcium aluminate. The hard mass containing these compounds is called clinker. It is powdered, mixed with little gypsum to slow down the setting process and pawed in bags.
The average composition of cement is
`{:(CaO - 50 "to" 60%, SiO_(2)-20 "to" 25%),(Al_(2)O_(3) - 5 "to" 10%, MgO-2 "to" 3%),(Fe_(2)O_(3) - 1 "to" 2%, SO_(3)-1 "to" 2%):}`
For good quality cement the ratio of silica to alumina should be 2.5 and 4 and the ratio of lime to the total oxides of silicon `(SiO_(2))`, aluminium `(Al_(2)O_(3))` and iron `(Fe_(2)O_(3))` should be nearly 2.
When cement is wetted with water it sets into hard mass. This is known as setting of cement is due to hydration of the molecules of the constituents and their rearrangement.
Cement is used in concreate, reinforced concrete, in plastering and in the construction of bridges, dams and buildings.