Editing Portland cement (section)

==Composition==
[[ASTM International|ASTM]] C219<ref>{{cite web|url=https://www.astm.org/standards/c219|title=ASTM C219-25, Standard Terminology Relating to Hydraulic and Other Inorganic Cements|publisher=ASTM International |date=2025-01-08}}</ref> defines portland cement as:

{{blockquote|a hydraulic cement produced by pulverizing clinker, consisting essentially of crystalline hydraulic calcium silicates, and usually containing one or more of the following: water, calcium sulfate, up to 5 % limestone, and processing additions}} The [[European Committee for Standardization|European Standard]] EN 197-1 uses the following definition:

{{blockquote|Portland cement clinker is a [[hydraulic lime|hydraulic]] material which shall consist of at least two-thirds by mass of [[calcium silicate]]s, [[alite|(3&nbsp;CaO·SiO<sub>2</sub>]], and [[belite|2&nbsp;CaO·SiO<sub>2</sub>)]], the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The ratio of [[calcium oxide|CaO]] to [[silicon dioxide|SiO<sub>2</sub>]] shall not be less than 2.0. The magnesium oxide content ([[magnesium oxide|MgO]]) shall not exceed 5.0% by mass.}}

(The last two requirements were already set out in the [[German Standard]], issued in 1909).

Clinkers make up more than 90% of the cement, along with a limited amount of [[calcium sulphate]] (CaSO<sub>4</sub>, which controls the set time), and up to 5% minor constituents (fillers) as allowed by various standards. Clinkers are nodules (diameters, {{convert|0.2|–|1.0|in|mm|abbr=off||disp=sqbr}}) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The key chemical reaction distinguishing portland cement from other hydraulic limes occurs at these high temperatures (>{{convert|1300|C|F}}) as [[belite]] (Ca<sub>2</sub>SiO<sub>4</sub>) combines with [[calcium oxide]] (CaO) to form [[alite]] (Ca<sub>3</sub>SiO<sub>5</sub>).<ref>{{cite web|url=http://www.cementkilns.co.uk/ckr_therm.html|title=Cement Kilns: Clinker Thermochemistry|author=Dylan Moore|work=cementkilns.co.uk|url-status=live|archive-url=https://web.archive.org/web/20140306044913/http://www.cementkilns.co.uk/ckr_therm.html|archive-date=6 March 2014|df=dmy-all}}</ref>