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Material for bonding stone or brick

A cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource.


Cements used in construction are usually inorganic, often lime or calcium silicate based, which can be characterized as non-hydraulic or hydraulic respectively, depending on the ability of the cement to set in the presence of water (see hydraulic and non-hydraulic lime plaster).

Non-hydraulic cement does not set in wet conditions or under water. Rather, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting.

Hydraulic cements (e.g., Portland cement) set and become adhesive due to a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet conditions or under water and further protects the hardened material from chemical attack. The chemical process for hydraulic cement was found by ancient Romans who used volcanic ash (pozzolana) with added lime (calcium oxide).


Cement materials can be classified into two distinct categories: non-hydraulic cements and hydraulic cements according to their respective setting and hardening mechanisms. Hydraulic cement setting and hardening involves hydration reactions and therefore requires water, while non-hydraulic cements only react with a gas and can directly set under air.

Hydraulic cement

Clinker nodules produced by sintering at 1450 °C.

By far the most common type of cement is hydraulic cement, which hardens by hydration of the clinker minerals when water is added. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main mineral phases of the clinker, abbreviated in the cement chemist notation, being:

C3S: Alite (3CaO·SiO2);

C2S: Belite (2CaO·SiO2);

C3A: Tricalcium aluminate (3CaO·Al2O3) (historically, and still occasionally, called celite);

C4AF: Brownmillerite (4CaO·Al2O3·Fe2O3).

The silicates are responsible for the cement's mechanical properties — the tricalcium aluminate and brownmillerite are essential for the formation of the liquid phase during the sintering (firing) process of clinker at high temperature in the kiln. The chemistry of these reactions is not completely clear and is still the object of research.

First, the limestone (calcium carbonate) is burned to remove its carbon, producing lime (calcium oxide) in what is known as a calcination reaction. This single chemical reaction is a major emitter of global carbon dioxide emissions.

{\displaystyle {\ce {CaCO3 -> CaO + CO2}}}{\displaystyle {\ce {CaCO3 -> CaO + CO2}}}

The lime reacts with silicon dioxide to produce dicalcium silicate and tricalcium silicate.

{\displaystyle {\ce {2CaO + SiO2 -> 2CaO.SiO2}}}{\displaystyle {\ce {2CaO + SiO2 -> 2CaO.SiO2}}}

{\displaystyle {\ce {3CaO + SiO2 -> 3CaO.SiO2}}}{\displaystyle {\ce {3CaO + SiO2 -> 3CaO.SiO2}}}

The lime also reacts with aluminum oxide to form tricalcium aluminate.

{\displaystyle {\ce {3CaO + Al2O3 -> 3CaO.Al2O3}}}{\displaystyle {\ce {3CaO + Al2O3 -> 3CaO.Al2O3}}}

In the last step, calcium oxide, aluminum oxide, and ferric oxide react together to form cement.

{\displaystyle {\ce {4CaO + Al2O3 + Fe2O3 -> 4CaO.Al2O3.Fe2O3}}}{\displaystyle {\ce {4CaO + Al2O3 + Fe2O3 -> 4CaO.Al2O3.Fe2O3}}}

Non-hydraulic cement

Calcium oxide obtained by thermal decomposition of calcium carbonate at high temperature (above 825 °C).

A less common form of cement is non-hydraulic cement, such as slaked lime (calcium oxide mixed with water), hardens by carbonation in contact with carbon dioxide, which is present in the air (~ 412 vol. ppm ≃ 0.04 vol. %). First calcium oxide (lime) is produced from calcium carbonate (limestone or chalk) by calcination at temperatures above 825 °C (1,517 °F) for about 10 hours at atmospheric pressure:

{\displaystyle {\ce {CaCO3 -> CaO + CO2}}}{\displaystyle {\ce {CaCO3 -> CaO + CO2}}}

The calcium oxide is then spent (slaked) mixing it with water to make slaked lime (calcium hydroxide):

{\displaystyle {\ce {CaO + H2O -> Ca(OH)2}}}{\displaystyle {\ce {CaO + H2O -> Ca(OH)2}}}

Once the excess water is completely evaporated (this process is technically called setting), the carbonation starts:

{\displaystyle {\ce {Ca(OH)2 + CO2 -> CaCO3 + H2O}}}{\displaystyle {\ce {Ca(OH)2 + CO2 -> CaCO3 + H2O}}}


Further Resources



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