Editing Cement (section)

===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>