What are Portland cement and its composition? Describe the manufacturing process of cement with diagram. Manufacturing of Portland cement: Dry and Wet Process, Describe the wet process of manufacturing of Portland cement, with the chemical reaction involved in it. Discuss the role of gypsum in Portland cement.
Ans. Manufacturing of Portland cement: There are two methods:
Dry process: In dry process the raw materials are separately crushed to 50 min or smaller pieces and then dried and stored separately. Crushing is done in gyratory crushers and drying is done by means of rotary dries.
Wet process: It is quite common and is universally employed for manufacture of Portland cement.
1. Crushing: Raw materials are grinded in ball mills and passes to tube mill conveyed through means of screw driver. This slurry contains about 38 to 40 percent water. This slurry is then pumped into the rotary kiln when it is burnt.
2. Burning: Actual chemical changes take place in the rotary kiln. The rotary is long steel cylinder with length 30-60 meters and diameter 2-4 meter lined with refractory bricks. The kiln is slightly inclined downwards towards the exit end. It can be rotated as desired speed as it is mounted as on rollers. The material is introduced in the rotary kiln from the upper end. As the kiln rotates, the material passes slowly from the upper to lower and at a lower rate controlled by slope and speed of the rotation of the kiln. Heating of kiln is done by burning of pulverized coal which is introduced through special burners. Temperature maintained is 1400-1500 degree centigrade. As the material passes through the kiln its temperature is raised to the point of clicking temperature where the actual chemical reactions takes place. In fact, there are different zones in the rotary kiln viz.
(a) Drying zone: This is upper one fourth part of rotary kiln, here the temperature is moderate (100-500 degree centigrade). This zone is known as drying zone, because in this zone water is driven out from the slurry by means of hot gases.
(b) Calcinations zone: Its temperature is about 1000 degree centigrade and it is the middle portions of the kiln. Here organic matter burns away. Calcium carbonate decomposes into calcium oxide and carbon dioxide which escapes.
CaCO3 = CaO + Co2
(c) Burning zone: It is the hottest and bottom portion of the rotary kiln. In this part temperature is about 1400 degree centigrade-1500 degree centigrade. In this zone mixture melts and forms little rounded pasty masses about the size of peas which are called clinks. The clinkers produced is greenish or black in colour, the clinks formed are falls out from this lower end of the kiln. The main reactions taking places are following:
CaCO3 = CaO + CO2
2CaO + SiO2 = 2CaO.SiO2
3CaO + SiO2 = 3CaOSiO2
3CaO + Al2O3 = 3CaO.Al2O3
4CaO + Fe2O3 + Al2O3 = 4CaO.Fe2O3.Al2O3
(d) Grinding: After cooling the clinks are carried to ball mills for grinding, the clinkers are mixed with 2-3% gypsum during grinding.
Role of Gypsum in Portland cement: Gypsum is added during grinding of cement. Gypsum is used as a retarder, for setting otherwise cement when mixed with water will set quickly during grinding. Gypsum is good fire-resistant material having good insulating properties and thus retards fine damage to wooden and steel framing.