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Heat Curing and Delayed Ettringite Formation

Published online by Cambridge University Press:  21 February 2011

M.C. Lewis,
Karen L. Scrivener and
S. Kelham
M.C. Lewis
Affiliation:
Imperial College of Science, Technology and Medicine, London, U.K.
Karen L. Scrivener
Affiliation:
Imperial College of Science, Technology and Medicine, London, U.K.
S. Kelham
Affiliation:
Blue Circle Industries PLC, Kent, U.K.
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Abstract

This paper reports some preliminary results from a study of the effect of elevated temperature curing on mortars and the phenomenon of delayed ettringite formation (DEF). Mortars made from cements with sulphate levels of 3%, 4%, and 5% and with 5% sulphate and added alkali were cured at 20 and 90° C and subsequently stored in water. Expansion measurements showed a pessimum effect with increasing S03 content. Mortars which expanded showed a corresponding decrease in strength. X-ray diffraction (XRD) studies indicated that no ettringite is present after heat treatment but re-forms over time within the material. However, the ultimate levels of ettringite reached do not correspond to the magnitude of expansion observed. X-ray microanalysis shows that immediately after the heat treatment the aluminate species and most of the sulphate species are incorporated within the C-S-H gel. The concentrations of these species decrease during expansion, such that at the end of expansion the amounts remaining correspond to the presence of AFm phase mixed with C-S-H.

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