Cement microscopy

Details of the history of a clinker can be seen - raw material fineness and homogeneity, clinker composition, temperature profile in the kiln, for example. From this information, the likely performance of the cement can be predicted or the cause of production problems identified such as poor combinability, or low grindability.

Some cement manufacturers use microscopy as a technique for kiln control, with clinker samples being examined continuously. Other manufacturers use it occasionally on an 'as required' basis while some manufacturers never use it at all. (As cement microscopists ourselves, if we may let a little bias creep in here, we would say these cement producers who don't use microscopy are really missing out on making better cement at lower cost.)

A petrographic microscope is used to examine cement clinker and related materials. Specimens are usually prepared as polished sections or as powder mounts, or sometimes as thin-sections. Important characteristics the microscopist examines are:

Overall nodule microstructure - dense, porous, dense micronodules interconnected by tenuous 'bridges'. This gives a broad relative indication of burning conditions.

Alite crystal size - coarse alite may indicate a slow heating rate, excessive burning or coarse silica in the raw feed; silicate reactivity may be lower than it could be with improved burning conditions.

Belite crystal size - larger belite crystals suggest longer time in the burning zone.

Aluminate and ferrite crystal size - coarse flux phases suggest slow cooling; finer, intergrown, flux phases indicate faster cooling. Belite colour also indicates the cooling rate; fast-cooled crystals are clear while slower cooling allows impurities to crystallize out along lattice planes imparting a yellow colour.

Large clusters of belite or free lime - these may indicate coarse particles in the raw feed.

Overall distribution of silicates - ideally, belite crystals will be dispersed evenly throughout each clinker nodule, either as individual crystals or in small clusters. If large clusters of belite are present, burnability is likely to be less good and the clinker will be harder to grind.

Other clinker mineral characteristics can indicate very slow cooling, reducing conditions, an excess of alkali over sulfate in the clinker and other adverse conditions.

Coming soon! We are preparing a book, an introduction to scanning electron microscopy of cement and concrete. Click here for more details.

Our own dedicated cement microscopy website has more information on both optical and scanning electron microscopy.

The International Cement Microscopy Association (ICMA) holds an annual meeting, usually in the USA. The proceedings of the meetings are a valuable source of reference.