Concrete is a mixture of portland cement, water, and aggregates. Aggregates are materials such as sand, gravel, crushed rock, and blast furnace slag (waste). The cement and water form a paste that binds the aggregates into a rocklike mass as the paste hardens. Builders generally use both a fine aggregate such as sand, and a coarse aggregate such as crushed rock, to make concrete. The aggregates must be free from silt, mud, clay, dust, and other materials that might weaken the concrete. The water used to make concrete should also be free from dirt and other impurities.
Builders may add materials called admixtures to concrete to give it special properties. Very fine materials such as fly ash, a product of coal-burning power plants, make fresh concrete more plastic (easily shaped). Other admixtures include various fats, sugars, and minerals. These are used to speed up or slow down the hardening of the concrete or to give it color or increased durability and weather resistance.
Before concrete is mixed, workers measure the proper amounts of the materials. The strength and durability of concrete depend chiefly on the amount of water used. If too much water is added, the cement paste will be too weak to hold the aggregates together firmly when it hardens. The less water used, within reasonable limits, the stronger the concrete will be.
Concrete can be mixed either by hand or by machine. Machine mixing makes more uniform batches. Proper mixing coats every particle of aggregate and fills all the spaces between them with cement paste. For most home repairs, concrete can be hand mixed.
The methods for mixing concrete by machines vary. The concrete may be mixed by machines at the place where the concrete will be used. Ready-mix companies make huge batches of concrete at mixing plants, and haul it to the work site in trucks. Some firms use mixing machines mounted on trucks. These machines mix the concrete as the truck carries it to the building site.
Homeowners can buy prepared mixtures of cement and aggregates for small repair jobs. Only water has to be added to such mixtures.
Workers place the freshly mixed, wet concrete into forms made of wood, plywood, or steel. The forms hold the concrete in shape until it hardens. The concrete may be dumped directly into the forms, or poured down chutes. Workers use wheelbarrows, two-wheeled carts called buggies, small rail cars, trucks, or buckets lifted by cranes. The concrete may also be pumped through steel pipes.
After the concrete is placed, it must be worked into the corners and sides of the forms with wooden spades and puddling sticks. The concrete should also be tamped, or packed down, to prevent open spaces called honeycombs. Sometimes workers stick vibrators into the concrete or fasten them to the forms in order to help settle the concrete.
Concrete placed for floors, sidewalks, and driveways should be leveled off with a straight-edged board. Next, it should stand until the film of moisture on its surface has disappeared. Then, the concrete should be smoothed off with a wooden trowel called a wood float. The float produces a rough surface that prevents slipping or skidding after the concrete hardens. A smoother surface can be made by using a steel trowel after the wood float. Motorized rotary steel floats are often used.
Curing makes concrete harden properly. After the concrete becomes firm enough to resist marring, it should be sprinkled with water, then covered with wet canvas, wet burlap, or wet sand. This cover keeps the concrete from drying too rapidly. A chemical reaction between portland cement and water makes concrete harden. For this reason, the longer concrete remains moist, the stronger it becomes. In hot weather, concrete should be kept moist at least three days. Cold weather slows the rate at which concrete hardens. Hardening concrete must be protected by canvas or straw when the temperature drops near freezing.
Concrete shrinks as it hardens. This results from the loss of moisture as the concrete dries, or from the cooling of the concrete. The chemical reaction of water and portland cement produces heat. When large amounts of concrete are used, as in dams, this heat must be drained away to make the concrete harden properly. This is usually done by running cold water through pipes stuck into the concrete. Cement companies have developed a special portland cement that produces less heat than other cements.
Portland cement contains about 60 per cent lime, 25 per cent silica, and 5 per cent alumina. Iron oxide and gypsum make up the rest of the materials. The gypsum regulates the setting, or hardening, time of cement. The lime comes from materials such as limestone, oyster shells, and a type of clay called marl. Shale, clay, silica sand, slate, and blast-furnace slag provide the silica and alumina. Iron oxide is supplied by iron ore, pyrite, and other materials.
Most cement plants are located near limestone quarries. They may also be near deposits of clay and other raw materials. Ships, trains, trucks, and conveyer belts haul the limestone and other raw materials to the plants. In the plants, the materials go through a chemical process that consists of three basic steps: (1) crushing and grinding, (2) burning, and (3) finish grinding.
Crushing and grinding
The quarried limestone is dumped into primary crushers that can handle pieces as large as an upright piano. This first crushing smashes the rock into pieces about the size of a softball. Secondary crushers, or hammer mills, then break the rock into pieces about 3/4 inch (19 millimeters) wide.
Next, the crushed rock and other raw materials are mixed in the right proportions to make portland cement. This mixture is then ground in rotating ball mills and tube mills. These mills contain thousands of steel balls that grind the mixture into fine particles. The materials can be ground by either a wet or dry method. In the wet process, water is added during the grinding until a soupy mixture called a slurry forms.
After the raw materials have been ground, they are fed into a kiln, a huge cylindrical furnace made of steel and lined with firebricks. A cement kiln rotates about one turn a minute, and is the largest piece of moving machinery used in any industry. It may be over 25 feet (8 meters) in diameter and 750 feet (229 meters) in length. The kiln is mounted with one end higher than the other. The ground, raw materials are fed into the higher end and slide slowly toward the lower end as the kiln revolves. It takes about four hours for the materials to travel through the kiln. Oil, gas, or powdered coal is burned at the lower end. This produces a flame that heats the materials to 2600 to 3000 0F. (1430 to 1600 0C). The heat changes the materials into a substance called clinker, in pieces about the size of marbles.
Large fans cool the clinker after it leaves the kiln. The clinker may be stockpiled for future use, or it may be reground at once in ball or tube mills. A small amount of gypsum is added to the clinker before the regrinding. This final grinding produces powdery portland cement that is finer than flour. The cement is stored in silos until it is shipped.
Cement plants ship cement either in bulk (unpackaged) or packed in strong paper sacks. Unpackaged cement is shipped by railroad, truck, or barge. Packaged cement is shipped in sacks containing 94 pounds (43 kilograms), or 1 cubic foot (0.03 cubic meter), of cement to the sack.