Portland cement clinker - overview

Portland cement clinker is a dark grey nodular material made by heating ground limestone and clay at a temperature of about 1400 C-1500 C. The nodules are ground up to a fine powder to produce cement, with a small amount of gypsum added to control the setting properties.

This page gives a thumbnail sketch. For more information see the following pages, or better still, the Understanding Cement e-book - links at the bottom of this page.


sem image of nodule
Polished section of nodule (scanning electron microscope image). Most of the nodule is alite (light grey) - some clusters of belite are visible (arrowed). Aluminate and ferrite are present but not visible at this relatively low magnification.




Nodules range in size from 1mm to 25mm or more and are composed mainly of calcium silicates, typically 70%-80%. The strength of concrete is mainly due to the reaction of these calcium silicates with water.

Portland cement clinker contains four main minerals:

Alite: approximately tricalcium silicate (typically about 65% of the total)

Belite: approximately dicalcium silicate (typically about 15% of the total)

Aluminate: very approximately tricalcium aluminate (typically about 7% of the total)

Ferrite: very approximately tetracalcium aluminoferrite (typically about 8% of the total)

The balance is made up of alkali sulfates and minor impurities. The typical mineral contents shown are subject to wide variation.


optical microscope image of minerals
Optical microscope image (polished section) of nodule. Brown crystals are alite, blue crystals are belite, bright interstitial material is mainly ferrite, with small dark inclusions of aluminate. Grey material is the epoxy resin used to make the specimen. NB: Alite is not actually brown and belite is not actually blue - they appear brown and blue here because the polished section has been etched to show the crystals more clearly.

A clinker chemical analysis is normally given in oxide form - a typical example might be:


Example of a typical clinker analysis (oxide weight%).

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

SO3

LOI

IR

Total

21.5

5.2

2.8

66.6

1.0

0.6

0.2

1.0

1.5

0.5

98.9

Free lime = 1.0% CaO

Balance is typically due to small amounts of oxides of titanium, manganese, phosphorus and chromium.


From the chemical analysis, the quantity of each of the four main minerals can be calculated using the 'Bogue' calculation (click on the link below for more information.)

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The "Understanding Cement" book or e-book contains even more information!
Compared with this web site, the book has, for example, about two-and-a-half times as much on ASR, one-and-a-half times as much on sulfate attack and nearly three times as much on carbonation. It has sections on alkali-carbonate reaction, frost (freeze-thaw) damage, steel corrosion, leaching and efflorescence on masonry. It also has about four-and-a-half times as much on cement hydration (comparisons based on word count).

NEW! Now available as a printed book
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Click here

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You are in: Portland cement clinker - overview
The following pages have more details on clinker composition, reactions in the kiln and on cement milling:

Cement notation / Clinker compositional parameters / Bogue calculation /
Combinability / Reactions in the kiln / Cement milling

Check the Article Directory for more more articles on this or related topics


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There's much more on cement in the Understanding Cement book/e-book: image of book
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