Modern Cement is a rather complex mineral and rather than being a combination of lime and volcanic ash, it is produced in a continuous manufacturing process where the main steps are
1. Blending Calcium (CaO), Silica (Si02), Aluminium (Al2O3) and Iron (Fe203) materials together,
2. Grinding the combined mixture into a fine powder called rawmeal in a mill, often the material is reduced to a size less than 0.1 mm,
3. Heating the rawmeal to over 1500 ℃ in a preheater and rotary kiln where it forms cement clinker,
4. Rapidly cooling the clinker to < 150 ℃ to “freeze” the clinker minerals then
5. Grinding the clinker, with Gypsum often to less than 0.05 mm to produce cement. The gypsum is an important part of cement production, as its addition controls the setting characteristics of the concrete.
The minerals produced in the rotary kiln during the clinkering process are a combination of Calcium Silicates, Calcium Aluminates and Calcium Alumino – Ferrites. During the heating process, the rawmeal partially melts (with a liquid phase typically of 25%) and the resulting clinker forms nodules as it tumbles through the kiln.
The materials utilised in the manufacture of the modern clinker are typically Limestone, Clay, Sand and Iron ore, all of which are extracted from mines or quarries. Fuel, used to heat the kilns is typically Oil, Natural Gas or Coal.
Due to the very high temperatures and the high fineness of the material required for efficient processing, the cement manufacturing process requires large amount of heat and electrical energy. Modern Cement Plants employ high efficiency Vertical Roller Mills (VRM’s) for material grinding and capture and recycle as much waste heat as possible.
Cement is obtained by grinding the clinker and gypsum in a mill. Often a high quality limestone is added to the cement, and this can improve the performance of the cement in the final concrete. Flyash (ash produced from coal fired power stations) and Steel slag are also often added to the cement in the grinding process. The Flyash and steel slag have high levels of Calcium Silicates, and in certain applications the slag and ash can improve the performance of cement in concrete.
To control emissions, modern plants install fabric dust collectors on all process exhausts to capture any dust, with the captured dust re-added to the process.