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 book - links at the bottom of this page.
In the above image of a whole nodule in polished section, the nodule has a high alite content and so most of it consists of alite (light gray). Some clusters of belite are visible (arrowed). Aluminate and ferrite phases 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 principal minerals:
The balance is made up of alkali sulfates and minor impurities. The typical mineral contents shown are subject to wide variation.
In the polished section image above, brown crystals are alite, blue crystals are belite, bright interstitial material is mainly ferrite, with small dark inclusions of aluminate. Gray regions are 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 with hydrofluoric acid to show the crystals more clearly.