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Calcareous fly ashes are high-potential reactive residues for blended cements, but their qualification and use in concrete are hindered by heterogeneity and variability. Current characterization often fails to identify the dominant, most reactive, amorphous fraction of the ashes. We developed an approach to characterize ashes using electron microscopy. EDS element composition of millions of points is plotted in a ternary frequency plot. A visual analysis reveals number and ranges of chemical composition of populations: silicate, calcium-silicate, aluminosilicate, and calcium-rich aluminosilicate. We quantified these populations in four ashes and followed their hydration in two Portland-ash systems. One ash reacted at a moderate rate: it was composed of 70 vol.% of aluminosilicates and calcium-silicates and reached 60% reaction at 90 days. The other reacted faster, reaching 60% at 28 days due to 55 vol.% of calcium-rich aluminosilicates, but further reaction was slower and 15 vol.% of phases, the silica-rich ones, did not react. (C) 2015 Elsevier Ltd. All rights reserved.

Model Ca-Mg-Na-aluminosilicate glasses based on compositions found in a calcareous fly ash were synthesised and their reactivity assessed in NaOH solution and in Portland cement paste. It was found that the reactivity followed the same trend in both systems and that the reaction of the glasses in pastes was very similar to that of the components of similar composition in the real fly ash. This finding indicates that the reactivity of glass in cement can be directly linked to the chemical composition of the glass. Further, when the reactivity of the glasses was normalized to their surface area, it was found that there exists a strong correlation with the NBOff, the ratio of non-bridging oxygens and tetrahedral ions in the glass. (C) 2015 Elsevier Ltd. All rights reserved.

In this paper, we show a new technique to characterize fly ashes using scanning electron microscopy (SEM). Chemical composition of millions of points is measured by SEM-energy dispersive X-ray spectroscopy and plotted in a ternary frequency plot. This new and straightforward way of displaying microscope data makes it easy to understand and assess the number, the approximate chemical composition, and the relative proportions of populations that constitute the fly ash. For quantitative characterization of the raw fly ash, these populations are grouped as predominantly silicate, calcium-silicate, aluminosilicate, and calcium-rich aluminosilicate. The same technique is then used to track these anhydrous fly ash fractions in blended cements during hydration and thus provide crucial information on the reactivity of different glasses in cementitious systems.

The phase assemblage of binary Portland cements with 45 wt% replacement by calcareous fly ash and slag has been investigated by experiment, mass balance and thermodynamic simulations. The input for these calculations was based on clinker reaction measured by XRD-Rietveld refinement, on SCM reaction measured by SEM-EDS full element mapping and on C-S-H composition by SEM-EDS point analyses. A sensitivity analysis estimates the uncertainty of determination of phase volume as at best +/- 2 cm(3)/100 cm(3), and thus of rather semi-quantitative character. The differences between experiment and calculations regarding AFt/AFm, C-S-H and portlandite were assessed. Gel/space ratios computed using the volumes of all hydrates showed a clear correlation to compressive strength, which was unaffected by the mix composition. This suggests that the type of hydrates formed has little or no influence on the compressive strength and that the key factor is the space filling.