Effects of unconventional fly ashes on cementitious paste properties

The availability of fly ash for concrete has decreased due to the shutdowns of coal-fired power plants. Alternatives to conventional fly ash are needed to ensure sustainable and durable concrete. In this study, the impacts of numerous unconventional fly ashes - reclaimed, beneficiated off-spec, and marginal fly ashes on early and later-age properties of cementitious paste were assessed. Heat release characteristics, bulk resistivity, compressive strength, calcium hydroxide content and bound water content were studied. Unconventional fly ash performance was compared to conventional fly ashes, inert materials and a cement control. Substantial differences between Class C and Class F fly ashes were observed, even for unconventional fly ashes. The fly ash CaO content is positively correlated with several early-age paste properties, however, certain other properties did not show significant early-age differences based on fly ash type. Class F fly ashes showed significantly higher bulk resistivity at 91-days, while Class C fly ashes showed higher bound water contents. All tested unconventional fly ashes were reactive, contributed to strength development, and did not negatively affect cement hydration. Some fly ashes resulted in properties that varied significantly from those expected for conventional fly ashes, but there did not appear to be a simple correlation between a fly ash being either off-spec, marginal, reclaimed, or beneficiated and its performance being different from what is expected from similar class fly ashes. The results suggest that fly ash specifications need to be broadened to include unconventional fly ashes.

Automated summary

This study evaluated the impacts of various unconventional fly ashes on the properties of cementitious paste, comparing them to traditional fly ashes and inert materials. Differences between Class C and Class F fly ashes were observed, with some properties positively correlated with CaO content. Unconventional fly ashes were found to be reactive and contribute to strength development without negatively affecting cement hydration. The results suggest a need to broaden fly ash specifications to include unconventional types.