Concrete, from the Latin word concretus meaning "having grown together," generally consists of Portland cement, water, and a relatively unreactive filler called aggregate. The filler is usually a conglomerate of gravel, sand, and blast-furnace stony matter known as slag.
Portland cement consists of finely pulverized matter produced by burning mixtures of lime, silica, alumina, and iron oxide at about 2,642°F (1,450°C). Chemically, Portland cement is a mixture of calcium aluminum silicates, typically including tricalcium silicate (3CaO SiO2), dicalcium silicate (2CaO SiO2), and tricalcium aluminate (3CaO Al2O3); it may also contain tetracalcium aluminoferrate (4CaO Al2O3 Fe2O3). Small amounts of sulfur, potassium, sodium, and magnesia may also be present. The properties of the Portland cement may be varied by changing the relative proportions of the ingredients, and by grinding the cement to different degrees of fineness.
When Portland cement is mixed with water, the various ingredients begin to react chemically with the water. For a short time, the resultant mix can be poured or formed, but as the chemical reactions continue, the mix begins to stiffen, or set. Even after the mix has finished setting, it continues to combine chemically with the water, acquiring rigidity and strength. This process is called hardening.
Ordinarily, an excess of water is added to the concrete mix to decrease its viscosity so that it can be poured and shaped. When the chemical reactions have more or less finished taking place, the excess water, which is only held by secondary chemical bonds, either escapes, leaving behind voids, or remains trapped in tiny capillaries.
It is important to recognize that setting and hardening result from chemical reactions between the Portland cement and the water. They do not occur as the result of the mixture drying out. In the absence of water, the reactions stop. Likewise, hardening does not require air to take place, and will take place even under water.
The strength of concrete is determined by the extent to which the chemical reactions have taken place by the filler size and distribution, the void volume, and the amount of water used.
Concrete is usually much stronger in compression than in tension.
Concrete may be modified with plastic (polymeric) materials to improve its properties. Reinforced concrete is made by placing steel mesh or bars into the form or mold and pouring concrete around them. The steel adds strength.
See also Bond energy.