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.

 

  • It is invented by the Joseph Aspdin in 1824. But the credit for the invention of the modern Portland cement goes to his son William Aspdin in 1840s.

Raw Materials for PC

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)

 

For the manufacturing of PC, materials from any two of these groups may be used but they must contain in proper form and proportions of lime, silica and alumina.

 

Chemical Composition of Portland Cements

The raw material that is used in the manufacturing of Portland cement mainly consists of lime, silica, alumina and iron oxide.

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
Also Read: 16 Types of Cement used in Construction Work

A Typical Chemical analysis of PC is 

 

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
During the heating of raw material in the kiln, the oxides interact with each other and forms more complex compounds. The Portland cement consists of 4 basic chemical compounds. These four compounds are given in the table below:

 

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)

 

What is Portland Cement and How it is Manufactured?

 

  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.
 The various production steps of the Portland cement are

 

  • 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.
For Better Explanation Watch the Video Given Below:

 

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