Cement is regarded to be the most important material in civil engineering industry. Its function is same as that of glue, It binds the sand and coarse aggregate together and to form a compact mass. It is a grayish powder (white cement is due to addition of Gypsum) when comes to contact with water gets harden. Manufacturing Of Portland cement: The most common cement used today is Portland cement which is the mixture of argillaceous i.e. clay like and calcaneous i.e. (Containing CaCO3 and other insoluble salts). Raw Materials: Chalk Lime stone Clay and other rich Aluminum and Iron Chief constituents of Portland cement
|COMPOUND/SHORTHAND NOMENCLATURE||MINERAL NAME||CHEMICAL FORMULA||TYPICAL CONC. %|
|Tri Calcium Silicate/C3S||Alite||3Ca.SiO2||60-70%|
|Di Calcium Silicate/C2S||Belite||2CaO.SiO2||10-20%|
|Tri Calcium aluminate/C3A||Aluminate||3CaO.Al2O3||5-10%|
|Tetra Calcium Alumino Ferrate/C4AF||Ferrite||4CaO.Al2O3.Fe2O3||3-8%|
Two Chief aspects in manufacturing of Portland cement: * To make a finely divided mixture of raw materials. * To heat the mixture to produce chemical composition.
To make a finely divided mixture of raw materials: First the raw materials are crushed and grinded in a chamber having steel balls (same as crushing copper ore in steel ball mill to produce finely crushed ore) to get homogenized and blended such that 80% of material passed through sieve No.200. In between two aspects there are two optional processes one is wet process and the other is dry process.
Wet Process: The material is mixed with 30-40% water by weight to make it slurry and then to second aspect.
Dry Process: Blended material is directly taken to the kiln in a dry state. Dry process is more economical then wet process and saves a lot of fuel but the process is dustier as compared to wet process.
To heat the mixture and to produce chemical composition: When the material either from dry or wet process comes in to kiln(size of horizontal kiln is about 80-150 m and 3-5 m in diameter) due to heating lime stone is broken down in to lime and calcium and carbon dioxide. Also Alumina and silica from the clay undergoes a chemical reaction in the temperature of about (2750oF when material is come from wet process) in to “Calcium aluminate”. Calcium Silicates are also formed in the kiln when material move down to kiln and the temperature is increased. Cement clinkers which are of greenish or black in color are also formed and having the diameter of about 18 mm. Cement clinkers are cooled down by decreasing temperature drastically to 150oF and stored at clinker silos. It is mixed with the gypsum to retard the setting time of cement. It is grounded in to fine powder and kept in to baggage in dry place.
Cement Function: When Cement mixes with water and aggregates a stiff and sticky paste or gel forms in which silica dioxide makes a huge network in which aggregates are trapped. it becomes harden and harden as the time elapses. This phenomenon by virtue of which the plastic cement changes into soil mass is known as setting of cement. For testing the setting time of cement we use Vicat’s Apparatus by placing a cube of cement and water, under Vicat’s apparatus and the needle is immersed in it, to get initial and final setting time of cement.
Lastly, it very important to consider about environmental pollution so cement plant designer should keep this thing in mind to control the emission of toxic substances. The priority in the cement industry is to minimize the increase in discharge particulate levels by reducing the mass load emitted from the tons, from renegade emissions, and from other sources. This could be done as in same way as in Ammonia Solvay process, by using the by-product that is using dust in kiln gases which will also improve the efficiency of it and reduce the atmospheric emission, good plant designers do Consider about it. 🙂 PhotCredit