MANUFACTURING PORTLAND CEMENT

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CEMENT PRIMER

Before talking about the manufacturing of portland cement, you'll need a little primer. Concrete & cement are not the same. A lot of folks don't know the difference. Cement is a grayish power. When you mix cement with water & other materials, like sand & /or rocks, it hardens to form concrete. Concrete is what we mostly see because buildings, homes, schools, bridges, roads, curbs, city infrastructures, etc., are built using concrete (even boats!). It's pretty neat stuff--just add water & presto, instant rock. Concrete is what we see coming out of trucks during construction. Most folks never see cement unless they buy it at the hardware store in a bag.

Cement goes back as far as the Roman Empire. They made kiln dried bricks & held them together with a lime & volcanic ash mixture, which is similar to present day pozzolanic cement.

Modern cement is called Portland Cement & has its origins beginning in the early 19th century in England. Now it's one of the highest commodities used per person in the world. Just consider your home, the roads you travel on, the malls, & your work place.

Concrete is not only durable, but attractive. Driveways, sidewalks, & parking lots simply look better, last longer, take more abuse, keep their shape, & resist weed grow through & erosion. They're practically inert & endure solvent spills without (literally) going to pieces. Real estate "curb appeal" & prestige is always enhanced by concrete driveways and sidewalks. These concrete structures represent long-term investments because replacement or re-topping costs are practically non-existent.

Cement is a basic industry--it uses basic (naturally occurring) materials. Most of cement comes from common limestone. A lot of limestone was made millions of years ago from the bones of dead animals & fish. Bones are composed of calcium, & limestone is simply calcium carbonate. Did you know that a lot of common antacids you take for heart burn are flavored limestone? No, don't take cement for heart burn!

Other ingredients are iron ore (rust), sand, clay, gypsum (to control how quickly cement gets hard). From these natural ingredients, we get silica & alumina which add to cement's fantastic strength & bonding abilities.

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MANUFACTURING

In a nutshell, cement is made by heating limestone. The process, of course, is much more complex. There are different types of cements for different jobs & quality standards demand precision control.

A more technical description about the world of cement chemistry is linked here.

Limestone is generally dried & ground to a fine powder, like a talcum power. It is so fine that it's almost as fluid as water at times. Sand & clay are added during or before the grinding process. Older processes use water while grinding, to produce material referred to as slurry. Modern plants keep this in the dry powdery state & call it raw meal.

The raw meal is heated (in different fashions depending on the type of plant) to around 2,700 degrees F, in a kiln. The meal essentially melts & forms a small dark gray rock called a clinker.

Clinker is cooled & ground again, adding gypsum, where the fine powder called cement is formed. Cement is put in bags, in trucks, in rail cars, on barges, & ships to reach every point on the planet. A lot was even flown in at Chernobyl to seal up the reactor & protect, well, the world, from radiation.

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ENVIRONMENT

Because cement is strong & stable, the EPA uses cement to stabilize a lot of sites nation wide, to prevent them from harming the environment. Cement is essentially inert when it hardens, so structures constructed from cement have no harmful effect on our environment. Cement certainly plays a major part in the prevention of soil erosion. It can be thought of as just another very hard rock!

Since cement is made from natural materials, the raw materials present little threat to the environment. Most visible emissions are simply dust. Many years ago, when the EPA began to regulate dust emissions, mid-west farmers protested the new laws! Cement dust has similar properties to lime (since it is mostly limestone!) & the dust was producing better crops. Just like most of us put lime on our lawns, the farmers were getting free lime. Some one driving down a dirt road might generate more dust in an hour than a cement plant can generate in a year.

Cement kilns are also a good place to dispose of unsightly tires. With flame temperatures estimated to be at least 3,500 degrees, cement kilns safely & completely burn tires using waste as a fuel & preserving our natural resources. The alternatives are that the tires pile up unsightly alongside the roads as a breeding ground for often dangerous mosquitoes.

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SAFETY

Since cement is similar to lime, you should follow the same common sense rules. Lime is caustic & if you expose your skin to it for any length of time, it will cause the skin to get raw & burn. Always read & follow the instructions on cement before you handle it, as with any material you use that could affect your well being.

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PROCESSING

There are, of course, many types of cement plants. This just describes one representative plant. It is not meant to represent any one manufacturer or location and is only one type of process in the production of cement. Cement plants can be "wet" or "dry." Since a dry process is the more modern, I will describe a typical dry process plant that incorporates advanced equipment and energy conservation techniques.

A typical type of cement plant could be described as a dry process plant with a dual reinforced suspension preheater (RSP) and a single, 280' long, dry kiln, rated at 4,650 tons/day. Some plants make more and some less.

Limestone could be barged from a quarry & unloaded using, say, 2 gantry cranes. It's then stock piled and reclaimed at a fixed rate & fed into 2 DRYERS. The dryers use waste heat generated by other parts of the process to dry the materials to less than 1% moisture.

A lot of cement plants do not have to barge in all raw materials and have their quarry in nearby properties. Some quarries pre-crush their materials from big boulders to small, fist sized rocks for ease of handling. Some plants don't need dryers (if they are in dry climates) so the limestone is usually dried in their raw mill(s) instead of having separate dryer(s).

Clay & sand are sometimes introduced into the process along with the limestone. Depending on the blend, the dried product is stored in different RAW SILOS for the different blends. There are sometimes 6 main silos that make 2 smaller silos, formed by the area between the circular silos. In these small silos, iron ore can be stored.

Portions of the blended limestone & portions of the iron, are again blended together & fed into the RAW MILL system where the large rocks could be first crushed by a roll press or roll crusher. This is 2 very large cylinders that are spinning & very close together. When a lime rock comes between them, it's pressed between the rolls fracturing the rock to smaller pieces.

That crushed material is fed into a raw mill. The raw mill is a very large round pipe looking structure about the size of a large room, only round. You can stand up in it! Inside there are metal balls that are from 1 to 2 inches in diameter. The mill spins around & the balls roll around inside. When the crushed rock enters, the balls fall on it breaking it into smaller pieces until finally it's a dust-like powder. Air is being sucked through the mill all this time & when the powder is fine (small) enough, the air can carry it out of the mill. But it still may not be fine enough.

In the air stream, the powder is again separated using a separator (what else!). Material that's too heavy (or course) is sent back to the mill & the fine stuff (called raw meal) is sent to storage in the BLENDING SILOS where it's also mixed again to even out any minor materials variations.

After being blended through, perhaps, 4 silos, the material is sent to the top of a 290' PRE-CALCINER. It enters at about 200 degrees & falls through 4 stages of cyclones that not only mix the hot air from the kiln & precalciner burner but separates the hot gasses from the material. When it reaches the bottom (in about 30 seconds) the temperature is 1650 degrees where it then enters the kiln.

The KILN is a very large pipe that's 16 ft. in diameter & 280 ft long. It will turn anywhere from 1 to 3.5 rpm. As it turns, it picks up the heat from the "kiln burner" (ignited fuel) & the meal begins a chemical reaction in the heat, transforming the beige powder to a molten goop (like a volcanic lava), then, at around 2650 F, to a burned looking rock called a clinker.

Clinker is too hot to handle at around 2,500 F, so the kiln discharges into a clinker COOLER. A dozen fans blow cool outside air up through special metal grates that have holes in them, into the bed of hot clinker. The clinker is cooled from about 2,500 degrees down to only around 2-300 degrees. It's then transported into clinker silos.

At the bottom of the clinker silos there could be as many as 28 feeding devices that weigh the clinker & put it on belt(s) that lead to FINISH MILL(S). The finish mills are like the raw mill in that they have balls that tumble & crush the clinker (and gypsum) while the mill is turning. The fine gray powder is again separated in several ways & transported to storage SILOS. A typical silo can contain about 5,000 tons of cement.

Marine silos store cement to transport to any coastal locations in barges (if the facility is on the water) . Land silos can fill rail cars, trucks, & also include areas to bag cement for sale in local stores under many logos & store brand names.

The primary source of FUEL is usually coal. Coal is dried then ground in a COAL MILL (did you guess?). Large balls race around pulverizing the coal until it's fine enough to be sucked up by a powerful fan. The air passes through filters that separates & collects the coal, placing it in a small storage bin. As fuel is needed in the kiln & preheater, it is blown from the bin to the burners in the system & ignited. Natural gas, oil, and tire chips also serve as supplemental fuels in some plants.

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A TYPICAL PLANT'S CAPACITIES & OTHER DATA
- Fuels burned: coal, oil, gas, tires
- Types of cement produced: I/II, masonry
- Plant cement capacity: 1,500,000 tons / year
- Plant clinker capacity: 1,415,000 t / y

CEMENT RELATED LINKS
- The early evolution of the surface of hydrating portland cement
- Holderbank, a large global cement manufacturer.
- Malaysian cement demands.
- Cement from Mexico.
- The Iowa Cement Industry.
- News clip by Heidi Wheat.
- Business history of Onoda Cement.
- The LaFarge (Alpenia, MI) Manufacturing Plant.
- A Special Cement called Flyash Cement.
- The Hydration of Cement.

If you know of a cement or concrete related topic, mail me its URL & I'll try to add it here.

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Hope you liked learning about cement! If you like, down load a MAP of the area where I live. The map file is about 19.3k zipped, expands to 111k in .BMP format, with 828x1100x2 resolution. Shows lodging & restaurants + routes to/from airport. Bring this in as an image to PaintBrush or your favorite Windows word processor & print it. Or down load a WELCOME message to the Tillman's Corner Area (hey, I have to plug the local economy!). It's 6k, zipped, pure ASCII text, with tons of phone numbers (Hotel, Car Rental, Airlines) & written instructions on how to get to the area, find food, & lodging.

THANKS: Laura, Russ, Gary, Mike, Dianne, Bob, Yvonne, & Jim!

I hope you liked my cement page. Check out my "home page"!

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