Hydration and physical characteristics of ultrahigh-volume fly ash-cement systems with low water/binder ratio

Replacing Portland cement by fly ash in concrete has attracted extensive attention, as this approach is effective in controlling heat release rate, reducing material cost and enhancing greenness. However, only limited studies have been reported on the hydration and physical characteristics of ultrahigh-volume fly ash (UHVFA, fly ash/binder > 60 wt%) concrete. This study aims to explore these characteristics of fly ash–cement systems with low water/binder ratios and a wide range of fly ash replacement levels (from 20% to almost 100%). Even if 80% of the cement was replaced by fly ash, the 28-day compressive strength of the mortar reached over 65 MPa under normal curing conditions, and the total hydration heat was 70% less than that of conventional cement mortar. Moreover, the morphological and micro-aggregate effects of the fly ash were found to play important roles in maintaining adequate strength of the systems, especially for those cases with UHVFA. The findings of this study can support the future designs and applications of sustainable UHVFA concrete.