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Clinker mineral proportions

The key contribution of SEM is that it provides quantitative compositional data of individual clinker minerals. One application of this information is the calculation of the clinker mineral proportions in a modified version of the Bogue calculation. The Bogue calculation is a standard calculation used in the cement industry to estimate the quantity of alite, belite, aluminate and ferrite in the clinker.

Pure mineral compositions are assumed (eg: alite is assumed to be tricalcium silicate) and the bulk clinker composition is required. The calculation is the solution of four simultaneous equations to give the proportions of each of the four minerals. This is a widely-used and useful calculation, but it gives only very approximate results as the assumed clinker mineral compositions may differ appreciably from the true compositions. The modified calculation uses the compositions determined by X-ray microanalysis to give a more accurate result. The technique can be extended to calculate the proportions of clinker sulfate minerals such as arcanite (mainly potassium sulfate) and calcium langbeinite (mainly potassium calcium sulfate). These proportions are used to assess the balance of alkali and sulfate in the kiln and to predict the likely water-soluble soluble alkali content of the cement. The types of clinker sulfate present and the water-soluble alkali both influence the setting properties and strength development of the cement.

Aluminate and belite compositions

An excess of clinker alkali over sulfate causes increases in aluminate and belite alkali contents. Increasing alkali in aluminate adversely affects aluminate reactivity, with possible consequences on setting properties and behaviour with some admixtures. Increasing alkali in belite can result in alkali-stabilised belite, causing difficulty in achieving good combination. In extreme cases, this imbalance will be apparent from the clinker bulk analysis, but SEM with X-ray microanalysis can provide much more detailed information on mineral compositions, helping to clarify target clinker bulk compositions. This may be particularly useful where the raw materials are low in sulfate.

Mineralised clinker

Mineralised clinker can be produced at lower temperatures than normal clinker, resulting in fuel savings or increased output. X-ray microanalysis can monitor the composition of the clinker minerals to determine the extent of mineralisation; alite Al and F content are of particular interest.

Clinker: introduction / optical microscopy / scanning electron microscopy / examining your clinker

Clinker examination by SEM

SEM applied to cement clinkers opens up a completely new dimension of information unavailable by optical microscopy because X-ray microanalysis tells us the composition of the individual clinker minerals.

Small changes in mineral composition can have a direct effect on clinkering and on hydration characteristics, so the clinker mineral compositions form a major part of a clinker assessment by SEM.

Important physical parameters, such as crystal sizes, are also measured, just as they are using optical microscopy. Some information such as mineral colour or birefringence cannot be assessed by SEM; the loss of colour information is the price paid for the ability to quantify mineral compositions. Where the balance of advantage lies depends who you ask and what you are trying to achieve.

Our perspective is that we would not wish to be without the information given by quantitative X-ray microanalysis and, invaluable though optical microscopy will continue to be, we have found that once clients have used SEM with their clinkers, it often becomes their technique of choice for off-works trouble-shooting investigations and research.Examples of the application of SEM and X-ray microanalysis follow.

 

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