Utilization of off-specification fly ash in preparing ultra-high-performance concrete (UHPC): Mixture design, characterization, and life-cycle assessment

This paper presents feasibility and benefits of utilizing off-specification fly ash (OSFA), which would have otherwise been landfilled, in preparing ultra-high-performance concrete (UHPC). Effects of mixture design variables, including OSFA content, water-to-binder ratio, and slag content, on compressive and flexural properties of UHPC were tested. Experimental results showed that UHPC with proper combination of OSFA and slag achieved desired compressive and flexural strengths, as well as low autogenous shrinkage and leachability of heavy metals. The underlying mechanisms of property development were investigated through isothermal calorimetry, thermogravimetric analysis, and X-ray diffraction. Results indicated that use of OSFA retarded hydration reactions, but incorporation of slag effectively suppressed adverse effects of OSFA. Economic and environmental analysis showed that use of OSFA greatly reduced the life-cycle cost, carbon footprint, and embodied energy consumption of UHPC. This study develops a new avenue for valorization of OSFA and development of cost-effective and eco-friendly UHPC.