Cement and concrete are among the most important building materials. Cement is a fine gray powder. It is mixed with water and such materials as sand, gravel, and crushed stone to make concrete. Cement and water form a paste that binds the other materials together as the concrete hardens. People often misuse the words cement and concrete. A person may speak of “a cement sidewalk.” But the sidewalk actually is made of concrete.
Concrete is highly fire-resistant, water-resistant, and comparatively cheap and easy to make. When first mixed, concrete can be molded into almost any shape. It quickly hardens into an extremely strong material that lasts a long time and requires little care.
Nearly all the cement used today is portland cement, which is a hydraulic cement, or one that hardens under water. This cement was named portland because it has the same color as stone quarried on the Isle of Portland, a peninsula on the south coast of England.
Uses of cement and concrete
Nearly all skyscrapers and factories and many homes stand on concrete foundations. These buildings may also have concrete frames, walls, floors, and roofs. Concrete is used to build dams and bridges. Cars and trucks travel on concrete highways, and airplanes land on concrete runways. Concrete tunnels run through mountains and under rivers. Concrete pipe distributes water, carries away sewage, drains farmland, and protects underground telephone and electric-power cables.
Portland cement is used chiefly to make concrete. But it can also be mixed with soil and water to form soil-cement, which is used in road paving and dam construction and for lining reservoirs.
Kinds of concrete
There are special ways of strengthening concrete or of making concrete building materials. These include (1) reinforced concrete, (2) prestressed concrete, (3) precast concrete, and (4) concrete masonry. Types of concrete with specialized uses include (1) air-entrained concrete, (2) high-early-strength concrete, (3) lightweight concrete, and (4) polymer concrete.
Reinforced concrete
is made by casting concrete around steel rods or bars. The steel strengthens the concrete. Almost all large structures, including skyscrapers and bridges, require this extra-strong concrete.
Prestressed concrete
usually is made by casting concrete around steel cables stretched by hydraulic jacks. After the concrete hardens, the jacks are released and the cables compress the concrete. Concrete is strongest when it is compressed. Steel is strong when it is stretched, or in tension. Prestressed concrete combines the strongest qualities of the two materials. The steel cables can also be bent into an arc, so that they exert a force in any desired direction, such as upward in a bridge. Prestressed concrete beams, roofs, floors, and bridges are often cheaper for some uses than those made of reinforced concrete.
Precast concrete
is cast and hardened before being used for construction. Precasting firms make concrete sewer pipes, floor and roof units, wall panels, beams, and girders, and ship them to the building site. Sometimes, builders make such pieces at the building site and hoist them into place after they harden. Precasting makes possible the mass production of concrete building materials. Nearly all prestressed concrete is precast.
Concrete masonry
includes many shapes and sizes of precast block. It is commonly used to make masonry walls. Some masonry is decorative or resembles brick.
Air-entrained concrete
contains microscopic air bubbles. These bubbles are formed by adding soaplike resinous or fatty materials to the cement, or to the concrete when it is mixed (see Resin ). The bubbles give the water in concrete room to expand as it freezes, and provide resistance to chemicals. Thus, air-entrained concrete is a good material for roads and airport runways.
High-early-strength concrete
is chiefly used in cold weather. This concrete is made with high-early-strength portland cement and hardens much more quickly than ordinary concrete. It costs more than ordinary concrete. But it is often cheaper to use, because it cuts the amount of time the concrete must be protected in cold weather.
Lightweight concrete
weighs less than other kinds of concrete. To make it, builders may use shales, clays, pumice, or other lightweight materials instead of sand, gravel, and rock. Or they may add chemicals that foam to create air spaces in the concrete as it hardens. These spaces are larger than those in air-entrained concrete.
Polymer concrete
contains no portland cement. Instead, a polymer (synthetic resin) binds the other materials. Polymer concrete is known for its quick curing time, resistance to chemicals, and stability over a wide range of temperatures. It is often used to make precast products, including floor tiles and pipes.
How concrete is made
Materials.
Most concrete is a mixture of portland cement, water, and aggregates. Aggregates are such materials 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 clay, mud, silt, 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.
Mixing.
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.
Placing.
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. Workers cover hardening concrete with canvas or straw to hold heat in 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.
How cement is made
Raw materials.
Portland cement contains about 60 percent lime, 25 percent silica, and 5 percent 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.
Burning.
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 °F (1430 to 1600 °C). The heat changes the materials into a substance called clinker, in pieces about the size of marbles.
Finish grinding.
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.
Shipping.
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.
History
The ancient Romans developed cement and concrete similar to the kinds used today. Their cement had such great durability that some of their buildings, roads, and bridges still exist. To make cement, the Romans mixed slaked lime (lime to which water has been added) with a volcanic ash called pozzuolana. The ash produced a hydraulic cement that hardened underwater. People lost the art of making cement after the fall of the Roman Empire in the A.D. 400’s. In 1756, John Smeaton, a British engineer, again found how to make cement.
Construction of the Erie Canal created the first big demand for cement in the United States. In 1818, Canvass White, an American engineer, discovered rock in Madison County, New York, that made natural hydraulic cement with little processing. Cement made from this rock was used in building the canal.
Joseph Aspdin, a British bricklayer, invented portland cement in 1824 and gave the cement its name. Aspdin made a cement that was superior to natural cement by mixing, grinding, burning, and regrinding amounts of limestone and clay. David O. Saylor probably established the first portland cement plant in the United States at Coplay, Pennsylvania, in 1871.
At first, portland cement manufacturers developed their own formulas. In 1898, manufacturers in the United States used 91 different formulas. In 1917, the National Bureau of Standards (now the National Institute of Standards and Technology) and the American Society for Testing and Materials (now ASTM International) established a standard formula for portland cement produced in the United States. The Portland Cement Association was formed in Chicago in 1916. Its research laboratories perfected air-entrained concrete in the early 1940’s.
Joseph Monier, a French gardener, developed reinforced concrete about 1850. In 1927, Eugene Freyssinet, a French engineer, developed prestressed concrete.
The cement and concrete industry
China is by far the world’s leading producer of cement. The country produces about half of the world total. Other important cement-producing countries include India, the United States, and Vietnam. The leading states include California, Florida, Missouri, and Texas.
Manufacturers of ready-mixed concrete use a majority of the cement produced in the United States and Canada. The production of ready-mixed concrete ranks as the largest branch of the concrete industry in North America. The second largest branch is the manufacture of precast concrete for construction. Other important concrete products include bricks, blocks, and pipe.
