Question

Raw materials used for the manufacture of Portland cement are:
A. gypsum and limestone
B. limestone and clay
C. alumina, clay and gypsum
D. gypsum and clay

Answer 1

Hint: As we know that cement is one of the most important building materials. It is basically a mixture of aluminates and calcium silicates, that set into a hard mass when mixed with water.

Complete Solution :
- The main raw materials used for the manufacture of cement are clay and limestone. It is found that limestone supplies lime, whereas clay supplies alumina, silica and iron oxide.
- Cement has 50-60 % calcium oxide, 1-2 % iron oxide, 20-25 % silicon oxide.
- The clay and limestone are mixed in the ratio of 3:1. The mixture is heated in a rotary kiln at about 1770-1870K.
- We can see the reaction when limestone decomposes to give carbon dioxide and alumina:
\[CaC{{O}_{3}}\to CaO+C{{O}_{2}}\]

- We can see that further lime is found to react with silica, alumina and form a mixture of calcium aluminate and calcium silicate:
\[\begin{align}
 & 2CaO+Si{{O}_{2}}\to 2CaO.Si{{O}_{2}} \\
 & 4CaO+A{{l}_{2}}{{O}_{3}}+F{{e}_{2}}{{O}_{3}}\to 4CaO.A{{l}_{2}}{{O}_{3}}.F{{e}_{2}}{{O}_{3}} \\
\end{align}\]
So, the correct answer is “Option B”.

Note: - It is found that when cement is to be used, it is mixed with water, which is found to be an exothermic reaction. During this, cement reacts slowly with water to form a mass that sets into a hard mass slowly, which has chains –Si-O-Al and Si-O-Si.