Portland cement clinker

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 of Ordinary Portland Cement (OPC) clinker. For more information see the following pages - links at the bottom of this page.

sem image of clinker nodule
Polished section of clinker 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.

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

Clinker contains four main minerals:

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

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

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

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

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 clinker minerals
Optical microscope image (polished section) of clinker minerals. 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 in which the clinker was embedded to make the specimen. NB: Belite is not actually blue - it appears blue here because it has been etched to show it 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 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.)

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