Burning the prepared mix in a kiln. It is the third stage of the process that is of most interest to us here. So far we have ascertained that the composition of ordinary portland cement is of a natural origin but it does contain heavy metals of which a high percentage are lost to the atmosphere on ignition. We now proceed to look into the energy content of production. Burning The earliest kilns in which cement was burned in batches were bottle kilns, followed by chamber kilns and then continuous shaft kilns.
Shaft kilns are still in use in some countries, but the most modern way is by rotary kiln.
These kilns are up to 200 metres in length and 7 metres in diameter. They are steel and lined with a refactory material. The kilns rotate slowly on an axis inclined a few degrees to the horizontal. The previously blended material is fed into the kiln at the upper end and moves slowly along the kiln by gravity towards the firing end. The firing temperature ranges between 1,300 to 1,500 degrees centigrade according to the raw materials being burned.