Cement is a highly consumed material over the world for constructional purposes. Different types
and brands of cement products are available in the market. Sand and water are also used, where cement is used as a
A material similar to what is now called cement was first discovered by Joseph Aspidine in 1756. This was called 'Portland cement' because its similarity
to the stones found in the village of Portland.
Raw Materials Cement Production
Raw materials are taken from natural environmental as other chemical industries.
- Calcareos materials which can supply CaO. this can be limestone, chalk or marble.
- Argillaceous material providing silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3).
Although clay is generally used for this purpose. It can be replaced with materials such as slate, shale, slag from steel manufacture or even
burned paddy husk.
- Gypsum which controls the settling of cement.
The calcareous and argillaceous raw materials are dried, mixed and heated to a high temperature (around 10000C) in a furnace. The
produced granular form is called clinker. Then it is ground with 4% gypsum to obtain Portland cement. There are two main processes in manufacturing
- Wet method
- Dry method
Raw materials are mixed in water to obtain a slurry of uniform composition which is then dried and ignited.
Dried raw materials are employed.
Cement Production Process and Reactions
Each step of cement manufacturing, physical conditions and reactions occurred are explained here.
Raw material transportation
Limestone is carried to the plant from mining place usually by a train.
Some cement plants import clinker from other country or plant and add gypsum to produce cement.
Mixing and Drying of limestone and clay
Limestone is well mixed with iron containing clay in a ball mill and dried using rotary driers. The ratio between
limestone and clay is 5:1.
Heating in a rotary furnace
Then, mixture is heated in an inclined rotary furnace ( inclined angle is 15). This rotary furnace is rotated
slowly at about 0.5 turn per minute. Furnace is made of large cylindrical steel tube.It is lined with refractory
bricks of either high alumina or high magnesia.
Fuel of heating
Furnace oil is used as the main fuel. But natural gas, diesel or coal can be used as to supply the heat. Some cement
manufactures use old car tyres to get the heat.
Raw materials are supplied from top of the the furnace and they slowly moves through the furnace. Solid state reactions take
place giving granular form called clinkers.
Sold state reactions in the furnace
In the furnace following reactions take place.
All moisture removed when temperature reaches to 5000C.
Thermal decomposition of limestone
Limestone (CaCO3) decomposes to CaO and CO2 at around 9000C.
CaCO3 = CaO + CO2
Reaction of calcium oxide and clay
When temperature reaches 900-12000C, the mixture gets fused into larger lumps. At that time, calcium oxide reacts with
clay forming calcium silicate and aluminium silicate.
At the end of the furnace
Material become tough and hard ad even vitrify and the reaction gets completed. Now temperature of the furnace is 12000C.
Clinker comes out of the furnace at the bottom of the furnace.
There are three main components in clinker
- Dicalcium silicate : 2CaO.SiO2 (C2S)
- Trycalcium silicate : 3CaO.SiO2 (C3S)
- Trycalcium aluminate: 3CaO.Al2O2 (C3A)
Minor components of clinker
- 4CaO.Al2O3Fe2O3 - (C4AF)
- 5CaO.Al2O3 - (C3A)
Properly burnt cement is greenish black colour and rather glassy in part (vitreous lustre). Clinker coming out of
the furnace is very hot and have to be cooled before grinding with gypsum.
Composition of good Portland cement
|Alkali metal oxides
Clinker composition can be written as (CxSy.CmAn.AsPq). For quality control of cement,
tests are done in random manner to check by using following ratios.
Following factors are called moduli.
Weight of SiO2 / (Weight of Al2O3 + Weight of Fe2O3) = 1.7 - 4.0
Weight of Al2O3 / Weight of Fe2O3) = 1-4
Portland cement manufacturing process - Flow diagram
Importance of gypsum in cement industry
Gypsum can decrease the settlement of cement while cement gets sufficient hardness.
Uses of cement
- As a building construction material
Environmental pollution due to cement industry
There are so many environmental impacts and pullution due to cement manufacturing starting from the mining the ores and during the
- Due to releasing of dust in the mining operation, air is polluted.
- Also mining operation causes to deforestation if limestone is located in a forest.
- Sound pollution - due to noise and vibration of machines at the factory and during the blasting of ore in quarries.
- Emission of sulfur dioxide into the atmosphere due to combustion of coal
- Emission of carbon dioxide by decomposition of limestone. Carbon dioxide may cause global warming (green house effect).
What are the problems or issues in cement manufacturing process?
A large of heat is required to manufacture cement and has to burn a lot of materials to supply that heat.
Alternatives to cement
Lot of researches are being conducted to find alternatives to cement. Scientists try to find whether plastic or other sources can be added
to the cement while keeping qualities of cement.
Related Tutorials of Cement Production