Abstract
The disposal of fly ash by incorporation in concrete and other cementitious materials is not only environmentally desirable and highly energy-efficient, but leads to improvements in the properties of the material, in particular durability. The partial replacement of cement by fly ash is recognised as effective in giving increased resistance to attack by sulphate in groundwater and soils. BRE Digest 363 allows fly ash/cement for almost all groundwater and soil conditions for which sulphate-resisting cement is allowed, and draft ENV 197-X also allows fly ash/cement within defined composition limits. In order to give confidence in the use of fly ash/cement concrete in aggressive conditions, the mechanisms by which resistance is conferred must be well understood. Evidence is presented of a considerably decreased rate of diffusion where fly ash is present, and also of stabilisation of the calcium aluminate hydrates, resulting in increased resistance to sulphate attack. The results are consistent for cements with C 3A contents of 9–12%, but further work is required with lower C 3A cements. The test method used, developed from the work of Mehta, is considered to offer the most likely route to an acceptable accelerated test which is consistent with long-term data.
Published Version
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