Cementing the process
CEMENT HAS, over the years, proven itself to be an invaluable building material. From the Egyptians’ ancient cement made from calcined gypsum and the Greeks and Romans’ early version formed by adding sand to the heated limestone, manufacturing cement has certainly undergone a great deal of technical developments, specifically for cost- cutting purposes and being more environmentally friendly.
HOW CEMENT IS MANUFACTURED
The dry process basically begins with quarrying of raw materials, which are divided into two kinds: calciumrich or calcareous such as chalk and limestone; and silica-rich, which is typically clay. These are acquired either by blasting or scraping, and are then transported for crushing.
Crushing is technically done through a milling process, during which the chemical composition is verified by letting the crushed materials pass through an on-line analyzer. Minerals obtained from other industries’ by-products — like iron ore or fly ash — are also added during this stage to ensure the correct pile composition.
From pre-blended piles, fine powder is then produced through further grinding. This powdered substance, called raw meal, is carried into a continuous blending storage silo where aeration is used to achieve a homogenous product.
This raw mix is fed into a pre-heater, where calcination partially occurs, and later directly to a rotary kiln. In the wet process, though, the raw materials are ground with water first before feeding them into the kiln.
The cement kiln, lined with a special firebrick and is fired with pulverized coal, heats all the ingredients to about 1450 degrees Celsius. It is also mounted with its axis inclined slightly by three degrees from the horizontal to allow for passage of the finely ground raw material, at this point known as slurry, over a period of almost half an hour.
As the slurry moves through the kiln, its chemical components are broken and are driven off in the form of gases. The raw material then forms compounds that produce cementitious properties, which after the burning section of the kiln, turn into a solid substance called clinker.
The clinker — whose appearance resembles that of gray marbles — is discharged red- hot into the clinker cooler, which brings down its temperature to 100 degrees Celsius. In order to save fuel and increase so- called burning efficiency, the heated air from the coolers is returned to the kiln to be used again as combustion air for its firing system. The cooled clinker is then transported to the clinker storage silo via conveyors.
The last stage is when cement is finally produced. The cooled clinker is ground and is added with 3% to 5% gypsum to regulate the early setting characteristics of cement. Further additions, like limestone, may also be added in required proportions besides gypsum. These incorporations give the cement specific properties.
Lastly, the finished cement is stored in cement silos and is packed into bags or shipped in bulks to construction sites. –