The main constituent of Portland cement is limestone (65%), which is broken up and ground with quartz sand and clay or just clay. The sulphur content must be below 3%.
In the wet process, water is added during grinding so that it becomes slurry. A dry process has now almost completely replaced the wet process. The dry process is considerably less energy intensive. The mixture is calcined in kilns at 1400-1500 °C and sintered to small pellets called cement clinker.
Vertical shaft kilns or rotating kilns can be used, but the rotating kiln is dominant in the industry. Rotating kilns, at their most efficient, yield 300 to 3000 tons a day; shaft kilns produce 1 to 200 tons a day. Modern shaft kilns have a higher efficiency and certain functional advantages, such as low energy consumption (Spence, 1980).
After firing, the mass is ground again and usually a little finely ground glass or gypsum is added to regulate setting. Pure Portland cement is seldom used today - it is usually mixed with lime or pozzolana.
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