Cement Production and Manufacturing Process | Portland Cement Industry

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 construction material.

What is Portland cement?

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.

Constituents of Portland cement

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 Portland cement.

1. Wet method
2. Dry method

Wet method

Raw materials are mixed in water to obtain a slurry of uniform composition which is then dried and ignited.

Dry method

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.

Moisture removal

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

Clinker composition can be written as (C_xS_y.C_mA_n.A_sP_q). For quality control of cement, tests are done in random manner to check by using following ratios.

Moduli

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 manufacturing process.

1. Due to releasing of dust in the mining operation, air is polluted.
2. Also mining operation causes to deforestation if limestone is located in a forest.
3. Sound pollution - due to noise and vibration of machines at the factory and during the blasting of ore in quarries.
4. Emission of sulfur dioxide into the atmosphere due to combustion of coal
5. 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