How Cement is Made
Naturally occurring limestone is the principal raw material used to make cement. Limestone in its natural form is a rock, so retrieving limestone from the quarry is the first step in making cement. The limestone quarry is usually located near the cement plant. The other major steps in cement production are as follows:
- The quarried material (large pieces of rock) is crushed in the primary crusher, reducing the rock to a maximum size of about 6 inches.
- The rock then goes to secondary crushers, where it is reduced to a maximum size of 3 inches.
- The raw materials (limestone, silica, iron and aluminum sources, and other more minor components) are blended at the right proportions and ground together.
- The ground materials are fed into the top end of a cylindrical steel rotary kiln and heated to about 2,500 oF.
- As the raw materials move through the kiln, GHG gases (mostly CO2) are released from calcination of the limestone and fuel used to heat the kiln.
- In the kiln, the calcium oxide resulting from calcination is fused with the silica to from calcium silicate, which gives cement its strength. Iron and aluminum sources are used as fluxing agents and allowing lower burning temperatures.
- After going through the kiln, a new substance called clinker is formed, which is as hard as marble balls.
- Clinker is then cooled down and finely ground with a small amount of gypsum to form cement. Gypsum is used to regulate setting, strength development, and volume change characteristics of the cement.
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