FORMATION OF CEMENT CLINKERS
The raw materials, lime and clay are mixed and subjected to following processing.
1. Physical Treatment of Raw Materials
It involves mixing and making the raw materials ready for chemical treatment. Two common processes employed for physical treatment are dry process and wet process.
(a) Dry Process: In this process, the calcareous and argillaceous materials are mixed in the ratio 3 : 1, crushed, dried, sieved and then ground to a fine powder. The mixture is now ready for the chemical treatment in the kiln.
(b) Wet Process: In this process, the rocky matter is crushed and water is sprinkled over it. The resulting materials is ground in the wet conditions to form a slurry. The slurry, which contains 35-40% of water, is stored in large tanks equipped with mechanical stirrers. It is then fed into the kiln.
2. Chemical Treatment
It involves calcination of the mixture of limestone and clay. Dry mixture from the dry process or the slurry from the wet process is fed into the rotary kiln (Fig. 55.1) for calcination.
Fig. 55.1. Rotary kiln for manufacture of portland cement.
Rotary kiln consists of a steel cylinder lined with fire bricks. It is 45-120 m long and has about 3 m internal diameter. The kiln is inclined at an angle of about 15 degrees to the horizontal and it rotates on its axis at the rate of 1/2 to 1 revolution per minute. The charge is introduced at upper end and it travels down as the kiln rotates.
The charge is heated by burning fuels like pulverized coal or, fossil fuel, or oil or natural gas. The hot fuel gases are made to enter at the lower end of the kiln with the help of a blower. As the charge moves forward, it meets higher temperatures till it reaches the lower end where the temperature is highest (around 1400°C). The charge takes 2 to 3 hours to cover the journey in the kiln. Reactions taking place in the rotary kiln in three major zones are as follows:
a) Maximum Temperature Zone (1000°C-1500°C): The main reactions between lime, alumina and silica takes place near the lower end of kiln resulting in formation of calcium silicates and aluminates as shown below.
2CaO + SiO2 → 2CaO.SiO2
3CaO.Al2O3 → 3CaO.Al2O3
3CaO + SiO2 → 3CaO.SiO2
4CaO + Al2O3 + Fe2O3 → 4CaO.Al2O3.Fe2O3
Tetracalcium alumino ferrite
a) Moderate Temperature Zone (up to 800°C): At 100°C, the removal of free moisture takes place. At 500°C, kaolin (AI2O3,2SiO2.2H2O) present in clay breaks up into amorphous silica and alumina.
(c) Average Temperature Zone (up to 800°C-1000°C):
Limestone decomposes to form lime and carbon dioxide.
CaCO3 → CaO + CO2
Moreover, due to high temperature in this zone, about 20-30% of mass melts and combines with solids mass to form pebbles (which vary in size) and are called cement clinkers.
The burnt gases (containing a good amount of dust) are made to pass through a dust chamber, where most of dust is retained and gases are allowed to escape through chimney.
The hot clinkers from the rotary kiln go to coolers which consist of series of tubes parallel to kiln in which air is forced through. As a result of this, the clinkers cool down while the air gets heated. The hot air is then used for the combustion of the fuel as an economy measure.
FORMATION AND PACKING OF CEMENT
The cooled clinkers are mixed with 3-5% of gypsum CASO4.2H2O, along with certain dispersing and water proofing agents and then ground to a fine powder. Since pure finely powered, clinkers set very rapidly, therefore, gypsum has to be added to retard the setting of cement. The powdered material is passed through fine sieves and finally it is sent to the automatic packing machines where it is packed in jute or polythene bags.
To explore the composition of raw materials used in making
- To explore samples of raw materials used for the production of cement, limestone, clay and coal.
- Explore the chemical composition of each of them using literature.
- Name the principle ingredients of cement.
- Name the ingredients supplied by limestone
- Name the ingredients supplied by clay.
- What is the function of coal in cement manufacture?
SETTING OF CEMENT
When cement is mixed with water and left as such for some time, it becomes a hard mass. This is known as setting of cement. The setting of cement is complecated process, the exact mechanism of this setting process is not known. It is believed that various aluminates and silicates present in the cement form hydrates with water which separate in the form of gel
3CaO.Al2O3 + 6H2O → 3CaO.Al2O3.6H2O
2CaO.SiO2 + xH2O → 2CaO.SiO2.xH2O
4CaO.Al2O3.Fe2O3 + 6 H2O → 3CaO.Al2O3.6H2O + Fe2O3.CaO
At the time some Ca(OH)2 and Al(OH)3 are formed as precipitates due to hydrolysis.
3CaO.SiO2 + H2O → Ca(OH)2 + 2CaO.SiO2
3CaO.Al2O3 + 6H2O → 3Ca(OH)2 + 2Al(OH)3
Gypsum combines with tricalcium aluminate to form calcium sulpho-aluminate,
3CaO.Al2O3 + 3CaSO4 + 2H2O → 3CaO.Al2O3.3CaSO4.2H2O
This reaction slows down the setting.
The gels formed start losing water partly by evaporation and partly by forming hydrates with unhydrated constituents.This results in the formation of a hard mass. Ca(OH)2 binds the particles of calcium silicate together, while Al(OH)3 fills the interstices resulting in hardening of the mass.
Setting of cement is an exothermic process. Hence, cement structures have to be cooled during setting by sprinkling water.