Microstructural identification of thaumasite in concrete by backscattered electron imaging at low vacuum

Abstract

A few cases of sulfate attack, involving the formation of thaumasite, have been discovered in site concrete. A technique able to detect small amounts of thaumasite in the presence of ettringite is necessary for developing a detailed understanding of the mechanism of thaumasite sulfate attack and for assessing concrete structures exposed to sulfates. Optical microscopy and X-ray diffraction (XRD) analysis may be used to identify thaumasite in large quantities in a concrete sample. However, the usefulness of optical microscopy is limited by its resolution. When ettringite is abundant, the identification of thaumasite by XRD analysis is very difficult as the d-spacings between thaumasite and ettringite are so similar that the XRD peaks of thaumasite in small quantity may be overwhelmed by the adjacent strong peaks of ettringite. In this study, concrete specimens containing bands of both ettringite and thaumasite were examined by backscattered electron imaging, first under low vacuum (9 Pa) and then under high vacuum. Prior to examination, the specimens were dried at 50°C, resin impregnated, then polished. Under high vacuum, thaumasite and ettringite bands appeared heavily cracked and of similar gray scale; they could not be differentiated visually, only by X-ray microanalysis, which is laborious and sometimes imprecise. Under low vacuum, the appearance of ettringite was similar to that in high vacuum, but thaumasite was different, with very few cracks and darker gray scale. This suggests that thaumasite requires a higher vacuum than ettringite to lose its structural water. Thus, low vacuum SEM can be used to identify thaumasite and to differentiate between thaumasite and ettringite.