Editing Cement (section)

==Chemistry==
Cement materials can be classified into two distinct categories: hydraulic cements and non-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===
[[File:LDClinkerScaled.jpg|thumb|[[Clinker (cement)|Clinker]] nodules produced by sintering at 1450&nbsp;°C]]

By far the most common type of cement is '''hydraulic cement''', which hardens by [[hydration reaction|hydration]] (when water is added) of the [[Clinker (cement)|clinker]] minerals. 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:
:C<sub>3</sub>S: [[alite]] (3CaO·SiO<sub>2</sub>);
:C<sub>2</sub>S: [[belite]] (2CaO·SiO<sub>2</sub>);
:C<sub>3</sub>A: [[tricalcium aluminate]] (3CaO·Al<sub>2</sub>O<sub>3</sub>) (historically, and still occasionally, called ''celite'');
:C<sub>4</sub>AF: [[brownmillerite]] (4CaO·Al<sub>2</sub>O<sub>3</sub>·Fe<sub>2</sub>O<sub>3</sub>).

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]] ([[Pottery#Firing|firing]]) process of clinker at high temperature in the [[Cement kiln|kiln]]. The chemistry of these reactions is not completely clear and is still the object of research.<ref>{{Cite web|url=http://cee.mit.edu/cee-in-focus/2011/spring/cement-structure|archive-url=https://web.archive.org/web/20130221105202/http://cee.mit.edu/cee-in-focus/2011/spring/cement-structure|url-status=dead|title=Cement's basic molecular structure finally decoded (MIT, 2009)|archive-date=21 February 2013}}</ref>

First, the [[limestone]] (calcium carbonate) is burned to remove its carbon, producing [[Calcium oxide|lime]] (calcium oxide) in what is known as a [[calcination]] reaction. This single chemical reaction is a major emitter of global [[Climate change#Greenhouse gases|carbon dioxide emissions]].<ref>{{cite web|title=EPA Overview of Greenhouse Gases|date=23 December 2015|url=https://www.epa.gov/ghgemissions/overview-greenhouse-gases}}</ref>
:<chem>CaCO3 -> CaO + CO2</chem>

The lime reacts with silicon dioxide to produce dicalcium silicate and tricalcium silicate.
:<chem>2CaO + SiO2 -> 2CaO.SiO2</chem>
:<chem>3CaO + SiO2 -> 3CaO.SiO2</chem>

The lime also reacts with aluminium oxide to form tricalcium aluminate.
:<chem>3CaO + Al2O3 -> 3CaO.Al2O3</chem>

In the last step, calcium oxide, aluminium oxide, and ferric oxide react together to form brownmillerite.
:<chem>4CaO + Al2O3 + Fe2O3 -> 4CaO.Al2O3.Fe2O3</chem>

===Non-hydraulic cement===
[[File:Calcium oxide powder.JPG|thumb|[[Calcium oxide]] obtained by [[thermal decomposition]] of [[calcium carbonate]] at high temperature (above 825&nbsp;°C).]]

A less common form of cement is '''non-hydraulic cement''', such as [[slaked lime]] ([[calcium oxide]] mixed with water), which hardens by [[carbonation]] in contact with [[carbon dioxide]], which is present in the air (~&nbsp;412 vol. ppm ≃ 0.04 vol.&nbsp;%). First [[calcium oxide]] (lime) is produced from [[calcium carbonate]] ([[limestone]] or [[chalk]]) by [[calcination]] at temperatures above 825&nbsp;°C (1,517&nbsp;°F) for about 10 hours at [[atmospheric pressure]]:
:<chem>CaCO3 -> CaO + CO2</chem>
The calcium oxide is then ''spent'' (slaked) by mixing it with water to make slaked lime ([[calcium hydroxide]]):
:<chem>CaO + H2O -> Ca(OH)2</chem>
Once the excess water is completely evaporated (this process is technically called ''setting''), the carbonation starts:
:<chem>Ca(OH)2 + CO2 -> CaCO3 + H2O</chem>
This reaction is slow, because the [[partial pressure]] of carbon dioxide in the air is low (~&nbsp;0.4&nbsp;millibar). The carbonation reaction requires that the dry cement be exposed to air, so the slaked lime is a non-hydraulic cement and cannot be used under water. This process is called the ''lime cycle''.