Editing Portland cement (section)

==Manufacturing==
[[File:Father and son loading and unloading cement in Goma, Congo.jpg|thumb|A father and a son in [[Goma]], Congo, in 2017. ]]
Portland cement clinker is made by heating, in a [[cement kiln]], a mixture of raw materials to a [[calcining]] temperature of above {{convert|600|C|F}} and then a fusion temperature, which is about {{convert|1450|C|F}} for modern cements, to [[sintering|sinter]] the materials into clinker.

The materials in cement clinker are alite, belite, [[tricalcium aluminate]] and tetracalcium alumino ferrite. The aluminium, iron and magnesium oxides are present as a [[Flux (metallurgy)|flux]] allowing the calcium silicates to form at a lower temperature,<ref>McArthur, Hugh, and Duncan Spalding. ''Engineering materials science: properties, uses, degradation and remediation''. Chichester, U.K.: Horwood Pub., 2004. 217. Print.</ref> and contribute little to the strength. For special cements, such as low heat (LH) and sulphate resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3&nbsp;CaO·Al<sub>2</sub>O<sub>3</sub>) formed.

The major raw material for the clinker-making is usually [[limestone]] ([[Calcium carbonate|CaCO<sub>3</sub>]]) mixed with a second material containing clay as source of alumino-silicate. Normally, an impure limestone which contains clay or SiO<sub>2</sub> is used. The CaCO<sub>3</sub> content of these limestones can be as low as 80%. Secondary raw materials (materials in the raw mix other than limestone) depend on the purity of the limestone. Some of the materials used are [[clay]], [[shale]], [[sand]], [[iron ore]], [[bauxite]], [[fly ash]], and [[slag]]. When a cement kiln is fired by coal, the ash of the coal acts as a secondary raw material.

===Cement grinding===
[[File:LDCementFM10MW.jpg|thumb|A 10 MW cement mill, producing cement at 270 tonnes per hour.]]
{{Main|Cement mill}}
To achieve the desired setting qualities in the finished product, a quantity (2–8%, but typically 5%) of calcium sulphate (usually [[gypsum]] or [[anhydrite]]) is added to the clinker, and the mixture is finely ground to form the finished cement powder. This is achieved in a [[cement mill]]. The grinding process is controlled to obtain a powder with a broad [[particle size distribution|particle size range]], in which typically 15% by mass consists of particles below 5 μm diameter, and 5% of particles above 45 μm. The measure of fineness usually used is the '[[air permeability specific surface|specific surface area]]', which is the total particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the cement on addition of water is [[Proportionality (mathematics)#Direct proportionality|directly proportional]] to the specific surface area. Typical values are 320–380&nbsp;m<sup>2</sup>·kg<sup>−1</sup> for general purpose cements, and 450–650&nbsp;m<sup>2</sup>·kg<sup>−1</sup> for 'rapid hardening' cements. The cement is conveyed by belt or powder pump to a silo for storage. Cement plants normally have sufficient silo space for one to 20 weeks of production, depending upon local demand cycles. The cement is delivered to end users either in bags, or as bulk powder blown from a pressure vehicle into the customer's silo. In industrial countries, 80% or more of cement is delivered in bulk.

{|class=wikitable
|+ Typical constituents of portland clinker plus gypsum<br><small> showing [[cement chemist notation]] (CCN)</small>
! Clinker
! CCN
! Mass
|-
|[[alite|Tricalcium silicate]] (CaO)<sub>3</sub> · SiO<sub>2</sub>||C<sub>3</sub>S|| 25–50%
|-
|[[belite|Dicalcium silicate]] (CaO)<sub>2</sub> · SiO<sub>2</sub>||C<sub>2</sub>S|| 20–45%
|-
|[[Tricalcium aluminate]] (CaO)<sub>3</sub> · Al<sub>2</sub>O<sub>3</sub>||C<sub>3</sub>A|| 5–12%
|-
|[[Calcium Aluminoferrite|Tetracalcium aluminoferrite]] (CaO)<sub>4</sub> · Al<sub>2</sub>O<sub>3</sub> · Fe<sub>2</sub>O<sub>3</sub>||C<sub>4</sub>AF|| 6–12%
|-
|[[Gypsum]] CaSO<sub>4</sub> · 2 H<sub>2</sub>O||CS̅H<sub>2</sub>|| 2–10%
|}

{|class=wikitable
|+ Typical constituents of portland cement<br><small>showing cement chemist notation</small>
! Cement
! CCN
! Mass
|-
|Calcium oxide, CaO||C|| 61–67%
|-
|Silicon dioxide, SiO<sub>2</sub>||S|| 19–23%
|-
|Aluminium oxide, Al<sub>2</sub>O<sub>3</sub>||A|| 2.5–6%
|-
|Ferric oxide, Fe<sub>2</sub>O<sub>3</sub>||F|| 0–6%
|-
|Sulphur (VI) oxide, SO<sub>3</sub>||S̅|| 1.5–4.5%
|}