FLY ASH - CEMENT MICROSTRUCTURE OBSERVED BY SEM IMAGES

At a basic level, scanning electron microscope SEM can be used as an adjunct to optical microscopy, for example to obtain high-magnification images of concrete fracture surfaces or to analyze 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 micro compositional characteristics of the concrete need to be examined. SEM can be used to distinguish between different types of cement in older concrete such as 'ordinary' Portland cement and sulfate-resisting cement. 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.