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.
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 (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.