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Concrete Examination by SEM

At a basic level, SEM can been used as an adjunct to optical microscopy, for example to obtain high-magnification images of concrete fracture surfaces or to analyse material not identifiable by optical microscopy. SEM does this very well, but the full potential of SEM goes much further, especially in non-routine examinations.

SEM is especially useful in studies of deleterious processes such as sulfate attack, including the particular form of sulfate attack known as delayed ettringite formation (DEF), alkali-silica reaction (ASR), alkali carbonate reaction and any other situation where the microstructural or microcompositional characteristics of the concrete need to be examined.

As with the examination of cement clinker, SEM with quantitative X-ray microanalysis lifts concrete petrography to a higher plane, especially where the examination of the cement paste is needed. Typically, a large number of analyses is carried out of the paste in a polished section of concrete or mortar. The data is represented graphically in the form of atomic ratios of selected elements to show the compositions of the phases present; these plots can show subtle changes in composition and can be used in many ways.

Examples include testing the condition of concrete in highway structures to show how the concrete paste composition may have changed since construction and how it may evolve in the future; showing how the use of cement replacement materials such as fly ash, slag, microsilica or metakaolin affect paste composition; testing the effect of admixtures on paste composition and determining the composition and microstructure of autoclaved concrete.

Concrete troubleshooting: we have countless examples of where we have applied SEM to concrete or mortar microstructure, mainly in the context of bridges, tunnels, floors, masonry or concrete blocks, but also to birdbaths and faience.

SEM can be used to distinguish between different types of cement in older concrete such as 'ordinary' Portland cement and sulfate-resisting cement (ASTM Types I/II and V).

SEM in conjunction with digital image analysis can determine the aggregate and cement content of concrete. Using polished concrete sections, the areas occupied by aggregate, paste and air are measured, the water-cement ratio is estimated and the mix proportions calculated. This technique can be particularly useful where conventional chemical methods cannot be used, perhaps because of the presence of limestone and soluble silica in the aggregate.

The scope for using SEM in examining concrete, mortar and other cementitious systems is almost infinite. If you want to investigate the microstructure or composition of a cementitious system, we can probably help. Contact us and we can discuss how best to move your investigation forwards.

Alkali-reactive silica aggregate, with alkali-silica gel exuding into cracks in the concrete. Ettringite is also present.

Delayed ettringite formation: ettringite (e) surrounds the coarse limetone aggregate (L). Fine aggregate is silica sand (s).

Concrete: concrete petrography / concrete by SEM / cement / chromium (VI)

 

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