Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluidcement ( cement paste) that hardens over time. Most concretes used are lime-based concretes such as Portland cementconcrete or concretes made with other hydraulic cements, such as calcium aluminate cements. However, asphaltconcrete, which is frequently used for road surfaces, is also a type of concrete, where the cementmaterial is bitumen, and polymer concretes are sometimes used where the cementing material is a polymer.
When aggregate is mixed together with dry Portland cement and water, the mixture forms a fluid slurry that is easily poured and molded into shape. The cement reacts chemically with the water and other ingredients to form a hard matrix that binds the materials together into a durable stone - like material that has many uses. Often, additives (such as pozzolans or superplasticizers) are included in the mixture to improve the physical properties of the wet mix or the finished material. Most concrete is poured with reinforcing materials (such as rebar) embedded to provide tensile strength, yielding reinforced concrete.
Famousconcrete structures include the Hoover Dam, the Panama Canal and the Roman Pantheon. The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world 's largest unreinforced concrete dome.
