What is Portland Cement and How it is Manufactured?

Portland Cement (PC) is a fine powder which is produced by heating limestone and clay minerals in a kiln and forms clinker. The clinker is grinded to fine powder with addition of small amount of other materials. It is the most commonly used types of cement in the world. It is used as basic ingredients of concrete, mortar, stucco etc. It is named as Portland cement because of its similarity to Portland stone which was found on the Isle of Portland in Dorset, England.

The various raw materials that is used in the manufacturing of PC are

1. Calcareous rocks (CaCO3 > 75% such as limestone, marl, chalk)

2. Argillaceous rocks (CaCO3 < 40% such as clay and shale)

3. Agillocalcareous rocks (40-75%CaCO3 such as clayey limestone, clayey marl)

 

 

Chemical Composition of Portland Cements

In the cement, the oxide content is about 90%. The oxide composition of ordinary Portland cement is given below:

Common Name	Oxide	Abbreviation	Appropriate Composition Limits (in %)
Lime	CaO	C	60-66
Silica	SiO2	S	19-25
Alumina	Al2O3	A	3-8
Iron oxide	Fe2O3	F	1-5
Magnesia	MgO	M	0-5
Alkalies
Soda	Na2O	N	0.5-1
Potassa	K2O	K	0.5-1
Sulfur Trioxide	SO3		1-3

 

S.no	Item	Percentage
1.	CaO	63.6
2.	SiO2	20.7
3.	Al2O3	6
4.	Fe2O3	2.4
5.	SO3	1.4
6.	MgO	2.4
7.	Na2O	0.1
8.	K­2O	0.7
9.	Loss on ignition	1.3
10.	Insoluble residue	0.3
11.	Free CaO	1.1

 

Name	Chemical formula	Abbreviation	Percentage
Tricalcium silicate	3CaO.SiO2	C3S	51
Dicalcium silicate	2CaO.SiO2	C2S	23
Tricalcium aluminate	3CaO.Al2O3	C3A	8
Tetracalcium aluminoferrite	4CaO.Al2O3.Fe2O3	C4AF	9

1. Tricalcium silicate

It hardens rapidly on addition of water and responsible for initial set and early strength.

 2. Dicalcium silicate

It hardens slowly. The effect of C3S on strength increases with ages beyond one week.

 3. Tricalcium aluminate

In first few weeks, it contributes in strength development. It hydrates first among all four compounds of the PC. Since it produces high heat and also reacts with soils and water that contains moderate to high sulfate concentrations, it is considered as the least desirable component of the PC.

4. Tetracalcium aluminoferrite

It hydrates very rapidly but contributes very little to the strength of the concrete. It lowers clinkering temperature i.e. temperature at which clinker is formed.

Manufacturing of Portland Cement (PC)

 

 

1. First the raw materials obtained from the source is reduced to fine particle size.
2. The raw materials are blended and mixed in such a manner that it produces uniform chemical composition containing Calcium carbonate (CaCO3), silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3) etc.
3. Now the blended raw mix is heated to a temperature at which all the moisture from it escapes out as steam or water vapor.
4. The dried raw mix is heated to 8000C. At this temperature what happens, the calcium carbonate gets dissociated into calcium oxide (known as free lime) and carbon dioxide which escapes out as gas.
5. As the temperature rises above 8000C, the formation of principal active compounds of Portland cement starts. And the completion of this process takes place at 1400-16000C. Final product that we get is PC clinker. Usually 1 ton of clinker is produced from 1.8 tons of raw material.
6. The cement clinker so produced is cooled to a temperature of 60 to 1500C and then stockpiled.
7. After that the clinker is ground to specified fineness by adding small amount of gypsum. The gypsum added controls the setting time of finished cement.
8. Finally the finished cement is stored in silos for a short period of time before it is being sent to the customers.

 

• Crushing, screening and stockpiling of raw materials.
• Proportion calculation of raw materials.
• Preparation of raw mix by blending
• Feeding of raw mix into rotary kiln.
• 100 C: Evaporation of free water.
• 150-300 degree C: Evaporation of more firmly bounded water.
• 600 degree C: MgCO3 —–> MgO + CO2
• 900 degree C: CaCO3 —–> CaO + CO2
• Reaction between lime and clay starts.
• 1300 degree C: Initiation of major compound formation.
• 1400-1600 degree C: completion temperature (clinker forms having C3A, C2S, C3S, C4AF)
• Cooling and storage of clinker.
• Clinker is ground with gypsum.
• Storing and marketing of cement.