Editing Portland cement (section)

===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%
|}