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.
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 are taken from natural environmental as other chemical industries.
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.
Raw materials are mixed in water to obtain a slurry of uniform composition which is then dried and ignited.
Dried raw materials are employed.
Each step of cement manufacturing, physical conditions and reactions occurred are explained here.
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.
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.
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.
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.
In the furnace following reactions take place.
All moisture removed when temperature reaches to 5000C.
Limestone (CaCO3) decomposes to CaO and CO2 at around 9000C.
CaCO3 = CaO + CO2
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.
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.
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.
|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
Gypsum can decrease the settlement of cement while cement gets sufficient hardness.
There are so many environmental impacts and pullution due to cement manufacturing starting from the mining the ores and during the manufacturing process.
A large of heat is required to manufacture cement and has to burn a lot of materials to supply that heat.
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