What is Portland Cement and How it is Manufactured

Making cement of any kind is not a simple and straightforward process. It is a precise science that requires the mining of different minerals and then their processing in an industrial setup. The proportions in which the minerals are mixed determine the type of cement that is finally produced. The most common type of cement used today is Portland Cement.

What Is Portland Cement?

Portland cement is a finely ground powder-like substance that is used as a binding material. It is the primary material used in mortar, cement, non-specialty grout, and stucco. The ordinary Portland Cement is gray in color, but white is also available nowadays. It is produced by grinding and heating a mixture of limestone and clay or limestone and shale in a kiln.

A Few Facts About Portland Cement

  • Portland Cement was first made and used in England in the early 19th century. 
  • It was termed or coined Portland Cement because its hydration product seemed to look a lot like a building stone from the Isle of Portland in Dorset, off the British coast.
  • It was invented in 1824 by Joseph Aspdin, an English mason.
  • However, the credit for the invention of the modern Portland cement goes to his son William Aspdin.
  • The specific gravity of the particles of Portland Cement is around 3.15 approximately.

Raw Materials Used For The Production Of Portland Cement

To manufacture Portland Cement, one may use materials from any two of the following groups. The important thing to note here is that they must contain lime, alumina, and silica in the correct form and proportion.

  1. Calcareous rocks (CaCO3 > 75% like limestone, chalk, marl).
  2. Argillaceous rocks (CaCO3 < 40% like clay and shale).
  3. Argilo Calcareous rocks (40-75% CaCO3 like clayey limestone, clayey marl),

Manufacturing Portland Cement

There are two main ways in which Portland Cement is made. These are –

  1. The Dry Method
  2. The Wet Method

Dry Method For Manufacturing Portland Cement

  • Mining Of Raw Materials – The first step is mining principal raw materials, which consists of limestone and clay, along with a few other minerals.
  • Crushing Of The Mined Rock – Once the raw material is quarried, it goes through crushing. Generally, it consists of two or more stages. The first crushing will reduce the size of the rock to around 6 inches to make it more manageable. At this stage, pieces of the rock are still too large, and hence, they undergo secondary crushing. This usually consists of hammer mills where the stone is pounded into smaller and smaller sizes.
  • Mixing – The crushed rock is now mixed with other crucial ingredients like fly ash or iron ore. Together they are ground further to make for easy mixing and fed to the furnace or the oven.
  • Heating – In the oven, the mixture is heated to around 2,700℉. This process takes place in large rotating kilns that are lined with a special variety of firebrick. These kilns are cylindrical, at least 12 feet in diameter, and made of steel. When mounted, the axis of these furnaces is inclined slightly from the horizontal. This incline helps in combining the materials more efficiently.

The ground mixture of raw materials is fed from the top end of the cylindrical kiln. From the bottom, it is heated by controlled burning of gas, alternative fuels, or powdered coal.

As the heated mixture of raw materials moves through the furnace, some elements transform into gas and are driven off. Others combine and form clinkers. Once the clinker is formed, it comes out of the heating unit in the form of grey marble-sized balls.

  • Cooling The Clinker – Clinker comes out red-hot from the bottom end of the furnace. Now it needs to be cooled, and for that, coolers at different temperatures are deployed into action. The heated air captured by the coolers is transported back to the furnace, thereby saving fuel and boosting the heating efficiency.
  • Grinding And Mixing Of Clinker To Form Cement – Once the clinker has cooled down, it is ground and combined with little limestone and gypsum. This is the Portland Cement, and it is now ready to be shipped to ready-mix concrete companies. The cement that comes out is extremely fine, with a pound of it containing almost 150 billion grains.

Wet Method For Manufacturing Portland Cement

The wet and dry methods for manufacturing Portland Cement are very similar. Their difference is simply about the addition of water. In the wet manufacturing method, the raw materials are fed to the furnace after they are ground with water. In the dry process, this does not happen, and the rest all remain the same.

In the wet process, the limestone is first crushed into small pieces, and then it is mixed with clay or shale along with water to form a slurry. The slurry is a creamy liquid, and it has a water content of 35% to 50%. This mixture is fed to the furnace cylinder, and from here, it follows the same path as the dry process for making Portland Cement.

Essential Compounds In Portland Cement

When the raw materials heat together in the Kiln, they form 4 basic chemical compounds. These are –

  1. Tricalcium silicate (C3S) or 3CaO.SiO2
  2. Dicalcium silicate (C2S) or 2CaO.SiO2
  3. Tricalcium aluminate (C3A) or 3CaO.Al2O3
  4. Tetracalcium aluminoferrite (C4AF) or 4CaO.Al2O3.Fe2O3

Dry Vs. Wet Method For Producing Portland Cement

Both dry and wet methods for producing Portland Cement follow almost the same processing path. Still, they have a few key differences:

Parameter                      Dry Method     Wet Method
Consumption of Fuel Less More
Processing Time Less More
Raw Material Hardness      Hard Any Type
Quality Of Finished Product      Inferior Superior
Production Cost High Low
Overall Cost Factor Expensive Cheap

Closing Notes

Portland Cement is an essential material for any type of construction. Today, roads, buildings, homes, etc., all are built with its help. It is even used in creative work for producing sculptures and molding. No wonder its demand the world over is very high. Sadly, it is not produced everywhere, but the good news is that it is easy to transport. Therefore, nobody has to worry about a shortage and look for alternatives.