Manufacture Cement: Dry and Wet process & Different Grades of Ordinary Portland Cement

 

Manufacturing process of Cement

Cement manufacturing process involves various raw materials and processes. Cement is a greenish grey coloured powder, made of calcined mixtures of clay and limestone. When mixed with water becomes a hard and strong building material.

It was first introduced by a British stone mason, Joseph Aspdin in 1824, who cooked cement in his kitchen. He heated a mixture of limestone and clay powder in his kitchen, and grind the mixture into powder creating cement, that hardens when mixed with water.

Cement is first used in the tunnel construction in the Thames River in 1828.

Calcareous and argillaceous raw materials are used in the manufacture of Portland cement.

The calcareous materials used are cement rock, limestone, marl, chalk and marine shell. The argillaceous materials consist of silicates of alumina in the form of clay, shale, slate and blast furnace slag.

 

There are four stages in the manufacturing process of cement:

1. Mixing of raw material

2. Burning

3. Grinding

4. Storage and packaging

1. Mixing of raw material -

Lime, Silica, Alumina, Iron oxide and small quantities of other chemicals are used majorly in manufacture of cement.

There are two methods of mixing i.e,dry process or wet process.

A. Dry process -

The both calcareous and argillaceous raw materials are firstly crushed in the gyratory crushers to get 2-5cm size pieces separately. The crushed materials are again grinded to get fine particles into ball or tube mill. Each finely grinded material is stored in hopper after screening. Now these powdered minerals are mixed in required proportion to get dry raw mix which is then stored in silos and kept ready to be sent into rotary kiln. Now the raw materials are mixed in specific proportions so that the average composition of the final product is maintained properly.

Fig: Manufacturing process of cement

B. Wet Process -

In this process first raw materials are crushed and made into powdered form and stored in silos. Then clay is washed and sticky organic matters are removed. Then powdered limestone and water washed clay are sent to flow in the channels and transfer to grinding mills where they are completely mixed and the paste is formed. Then grinding process is done in a ball or tube mill or even both. Then the slurry is led into collecting basin where composition can be adjusted. The slurry contains around 38-40% water that is stored in storage tanks and kept ready for the rotary kiln.

2. Burning of Raw Materials -

This process is carried out in rotary kiln while the raw materials are rotated at 1-2rpm at its longitudinal axis. The raw mix of dry process of corrected slurry of wet process is injected into the kiln from the upper end. Powdered coal or oil or hot gases are used to be heated up from the lower end of the kiln so that the long hot flames is produced. The lower part (clinkering zone) have temperature in between 1500-1700 degree Celsius where lime and clay are reacts to yielding calcium aluminates and calcium silicates. These aluminates and silicates of calcium fuse to gather to form small and hard stones are known as clinkers. The size of the clinker is varies from 5-10mm. The clinker coming from the burning zone are very hot. To bring down the temperature of clinkers, air is admitted in counter current direction at the base of the rotary kiln. The cooled clinkers are collected in small trolleys.

 

3. Grinding of clinkers -

The cooled clinkers are received from the cooling pans and sent into mills. The clinkers are grinded finely into powder in ball mill or tube mill. Powdered gypsum is added around 2-3% as retarding agent during final grinding. The final obtained product is cement that does not settle quickly when comes in contact with water. After the initial setting time of the cement, the cement becomes stiff and the gypsum retards the dissolution of tri-calcium aluminates by forming tri-calciumsulfoaluminate which is insoluble and prevents too early further reactions of setting and hardening.

 

4. Storage and packaging -

The grinded cement is stored in silos, from which it is marketed either in container load or 50kg bags.

Different Grades of Ordinary Portland Cement-

No.	Grade of Cement	Details
1.	33 Grade	1. The compressive strength after 28 days is 33 N/mm2. 2. Used for general construction works in normal environmental conditions. 3. Cannot be used where higher grade of concrete above M20 is required. 4. The use of this cement has progressively decreased and virtually phased out.
2.	43 Grade	1. Most widely Used cement for general construction works. 2. Minimum 28 days compressive strength 43 N/mm2. 3. Suitable for all types of applications RCC, plastering, masonry. 4. Used for construction of residential, commercial and industrial buildings, roads, bridges, flyovers, irrigation projects and other general civil construction works.
3.	53 Grade	1. Minimum 28 days compressive strength 53 N/mm2. 2. Gives 10 to 15% saving in cement consumption and 5 to 8 % saving in steel consuption provided higher grdes of concretes say M30 and above are used. 3. Useful for high rise buildings, bridges, flyovers, chimneys and pre-stressed structures where high grade concrete is required. 4. Gives better durabillity characteristices to concrete-high grade concrete can be made with lower water cement ratio.