The kiln is mounted with one end higher than the other. The ground, raw materials are fed into the higher end and slide slowly toward the lower end as the kiln revolves. It takes about four hours for the materials to travel through the kiln. Oil, gas, or powdered coal is burned at the lower end. The heat changes the materials into a substance called clinker, in pieces about the size of marbles.
Large fans cool the clinker after it leaves the kiln.
The clinker may be stockpiled for future use, or it may be reground at once in ball or tube mills. A small amount of gypsum is added to the clinker before the regrinding. This final grinding produces powdery portland cement that is finer than flour. The cement is stored in silos until it is shipped. Unpackaged cement is shipped by railroad, truck, or barge.