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Cement Facts

History of Cement
Cement Industry in Malaysia
Manufacture of Ordinary Portland Cement
Raw Materials Preparation
Raw Materials Proportioning & Grinding
Clinker Burning
Cement Grinding
Cement Storage & Despatch
Cement Quality


Clinker Burning

The kiln is a long cylinder lined with refractory materials and supported on rollers such that it can rotate on it's own axis and erected with a slight inclination from the horizontal. The raw meal now called kiln feed enters the kiln system at the top of the pre-heater. This is essentially a counter current heat exchanger with the hot gasses rising and kiln feed falling. The heat source is located at the lower end of the kiln. The kiln is essentially made up of four sections, drying , calcining , sintering and cooling. As the kiln feed moves towards the lower end of the kiln it undergoes successive reactions as shown below:
Deg C
Reactions Thermal Change Clinker Compounds Formed
100 Evaporation of free water from raw meal Endothermic  
500+ Evolution of combined water from clay Endothermic  
800+ CaCO3 = CaO + CO2 Endothermic  
800-900 CaO + SiO2 = CaO.SiO2 Exothermic CS
900-950 5CaO + 3Al2O3 = 5CaO.3Al2O3 Exothermic C5A3
950-1200 2CaO + SiO2 = 2CaO.SiO2
2CaO + Fe2O3 = 2CaO.Fe2O3
1200-1300 3CaO + Al2O3 = 3CaO.Al2O3 Exothermic C3A, C4AF
1250-1280 Beginning of liquid formation Endothermic Molten
1260-1450 3CaO + SiO2 = 3CaO.SiO2 Endothermic C3S
1100-1300 Formation of clinker compounds "Cooling zone" C3S, C2S , C3A , C4AF
The principal chemical compounds in the clinker are tricalcium silicate , C3S (40-60%) , dicalcium silicate, C2S (16-30%), tricalcium aluminate, C3A (7-15%) and tetracalcium aluminoferrite, C4AF (7-12%). The cooling zone in the kiln is actually quite short but it is in this zone that nodulisation of the melt occurs.

The red hot clinker is then discharged into the cooler , where it is quenched cooled to around 100 degrees centigrade. The heat dissipated by the clinker is used as secondary air for the combustion in the calciner. This hot gas is also used in the dryers at the raw materials preparation stage. Rapid cooling of the clinker is essential as this hampers the formation of crystals , causing part of the liquid phase to solidify as glass. The faster the clinker cooling the smaller the crystals will be when emerging from the liquid phase. Table 7 provides a summary of the principal compounds of Portland cement and their characteristics.
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