Critical evaluation of strength prediction methods for alkali-activated fly ash

Abstract

The literature contains many proposed methods for proportioning alkali-activated binders for maximum compressive strength. Many of these methods have been developed using metakaolin, which is a relatively pure aluminosilicate powder. In recent years, fly ash has become a more common aluminosilicate source for alkali activation. However, fly ash is a more complex material than metakaolin, and activated fly ash may not follow the same trends as activated metakaolin. In this study, literature-recommended strength prediction methods for alkali-activated binders, using metakaolin and fly ash, are reviewed and compared with the compressive strengths measured for eight Class F fly ash-based binders made by activation with sodium hydroxide solution. Of the eight fly ash binders in the study, six had correct performance predictions considering SiO 2 / Al 2 O 3 and Na 2 O/Al 2 O 3 optimal ratios developed for metakaolin. A published empirical equation developed to predict alkali-activated fly ash concrete strength correctly predicted relative strengths for six of the eight fly ash binders. Modifier element content is another possible indicator of reactivity, and the fly ashes in this study generally showed that fly ashes with high contents of Ca 2? , Mg 2? , Na ? , and K ? were likely to produce strong binders, although the correlation shown here was not as strong as that shown in prior studies. This work demonstrates that, while the proposed prediction methods are generally adequate, they do not cover all fly ashes and more work is needed improve prediction methods and account for the behavior of outliers.

Fly ash as an assemblage of model Ca–Mg–Na-aluminosilicate glasses

“…However, Al can also act as a network modifier, with no need for charge compensation, even if cations are still present. The actual proportion between network forming and modifying Al could potentially be estimated by Raman spectroscopy, 27 Al NMR or 17 O NMR. But, it may be difficult to extract individual glasses from fly ash to be able to test them.…”

“…A study by Bumrongjaroen [26] has shown that the reactivity estimated from depth profiles of fly ash-type glasses leached in pH 12 LiOH largely corresponded to the NBO/T. Recently published results [27] on pastes prepared by alkali activation of siliceous fly ashes show a trend to higher compressive strength with increasing NBO/T of fly ash.…”

Quantitative assessment of parameters that affect strength development in alkali activated fly ash binders

“…There have been recent attempts [14,15] to evaluate the effect of fly ash composition and properties (e.g., bulk composition, mineralogy, and particle size) on compressive strength of AAFA. However, there has not been a systematic effort to explicitly assess the effect of items (2)-(4) on the strength development.…”

Identifying Glass Compositions in Fly Ash

“…MSIA does not distinguish between glassy and crystalline phases, so the crystalline phases were included in the analysis. From the X-ray diffraction data, which were presented for each fly ash in Aughenbaugh et al (2014), crystalline phases present in these ashes include quartz, lime, and iron-bearing phases, such as maghemite, magnetite, or hematite. The compositions of the crystalline phases are not listed in the tables, as they correspond to those for crystalline quartz (SiO2), lime (CaO), or iron oxides, such as hematite, magnetite, or maghemite (Fe2O3 or Fe3O4).…”