Question

Explain the steps involved in the manufacture of cement.

Answer 1

Hint: Cement is one of the most important building materials. It is a finely ground mixture of calcium silicates and aluminates which set to a hard mass when treated with water. Cement has $50-60\mathrm{\%}$ calcium oxide, $2-3\mathrm{\%}$ magnesium oxide, $20-25\mathrm{\%}$ silicon oxide, $1-2\mathrm{\%}$ iron oxide, $5-10\mathrm{\%}$ aluminium oxide and $1-2\mathrm{\%}$ silicon oxide.

Complete step by step answer:
The raw materials for manufacture of cement are limestone and clay. The former supplies lime while the latter is the source of silica, alumina and iron oxide. The steps of manufacture of cement are as follows-
-The powdered limestone and clay are mixed in a $3:1$ ratio.
-The mixture is heated in a rotary kiln at about $1770-1870K$.
-Limestone decomposes to give lime and carbon dioxide. The reaction is-
$CaC{O}_3\to CaO+C{O}_2$
-Lime obtained, alumina and silica react together to form a mixture of calcium silicate and calcium aluminate. The reactions are-
$2CaO+Si{O}_2\to 2CaO.Si{O}_2$
$4CaO+A{l}_2{O}_3+F{e}_2{O}_3\to 4CaO.A{l}_2{O}_3.F{e}_2{O}_3$
-Due to very high temperature in the kiln, $20-30\mathrm{\%}$ mass melts and combines with solid mass to form pebbles known as cement clinkers.
-The hot clinkers are cooled and mixed with $2-3\mathrm{\%}$ gypsum. A finely powdered mixture is obtained also called Portland cement. Gypsum is added to slow down the setting of cement so that it gets sufficiently hardened.

Note:
Cement is used as an important construction material. Its uses are-
1.It is used in concrete and reinforced concrete.
2.It is used in plastering.
3.It is used in construction of buildings, bridges and dams.
4.When cement is to be used, it is mixed with water which is an exothermic reaction. The cement reacts with water to form a gelatinous mass which slowly sets into a hard mass having $-Si-O-Si-$ and $-Si-O-Al-$ chains.