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 SiO2/Al2O3 and Na2O/Al2O3 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 Ca2+, Mg2+, 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.
Highlights
Alkali-activated binders, which include the subset of materials known as geopolymer cements, are cementing materials that require the addition of alkalis in order to react in the presence of water and can be used in place of ordinary Portland cement in concrete construction [40]
The study presented in this paper compared the measured compressive strengths of geopolymers mixed with 8 M NaOH solution and cured at 23 °C with the predicted properties of a geopolymer using several methods reported in the literature
The use of network modifier content demonstrated a general trend with regards to geopolymer compressive strength but should not be used alone to form precise predictions of strength since it does not account for total vitreous content, the distribution of network modifiers among the glassy phases present, the contribution of calcium toward calcium silicate hydrate (C-S-H) or calcium aluminosilicate hydrate (C-A-S-H) binding phases, or particle size
Summary
Alkali-activated binders, which include the subset of materials known as geopolymer cements, are cementing materials that require the addition of alkalis in order to react in the presence of water and can be used in place of ordinary Portland cement in concrete construction [40]. The non-crystalline portion is called the bulk glassy phase in the material and is generally assumed to be reactive for the purposes of molar oxide ratio proportioning [31] It does not does not fully describe the availability of the elements contained in the glassy material to dissolve in solution (nor the kinetics of dissolution), the minor elements contained in the material, or the presence of unburnt carbon or sulfate; use of the ratio proportioning method for alkaliactivated fly ash design must be used with these caveats
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