Rh. Yang et Nr. Buenfeld, Microstructural identification of thaumasite in concrete by backscattered electron imaging at low vacuum, CEM CONCR R, 30(5), 2000, pp. 775-779
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 d
etailed 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 larg
e quantities in a concrete sample. However, the usefulness of optical micro
scopy is limited by its resolution. When ettringite is abundant, the identi
fication of thaumasite by XRD analysis is very difficult as the d-spacings
between thaumasite and ettringite are so similar that the XRD peaks of thau
masite in small quantity may be overwhelmed by the adjacent strong peaks of
ettringite. In this study, concrete specimens containing bands of both ett
ringite and thaumasite were examined by backscattered electron imaging, fir
st under low vacuum (9 Pa) and then under high vacuum. Prior to examination
, the specimens were dried at 50 degrees C, resin impregnated, then polishe
d. Under high vacuum, thaumasite and ettringite bands appeared heavily crac
ked and of similar gray scale; they could not be differentiated visually, o
nly by X-ray microanalysis, which is laborious and sometimes imprecise. Und
er low vacuum, the appearance of ettringite was similar to that in high vac
uum, but thaumasite was different, with very few cracks and darker gray sca
le. 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. (C) 2000
Elsevier Science Ltd. All rights reserved.