Reactivity of coal fly ash used in cementitious binder systems: A state-of-the-art overview

Abstract Coal fly ash is a type of combustion residue from electric power plants that burn coal. Conventionally acting as supplementary cementitious material to produce economically and environmentally sustainable concrete, coal fly ash has been increasingly demonstrated as an alternative binder (in place of Portland cement) in cementitious composites. Reactivity of fly ash is a crucial factor for its appropriate use in the concrete industry. An in-depth understanding of the reactivity of fly ashes would not only facilitate their value-added utilization but also benefit their transformation from waste to commodity. This is particularly the case for the marginal fly ashes (e.g., ponded and landfilled fly ashes) that fail to meet the requirement of AASHTO M295 (or ASTM C618) and have been considered unsuitable for direct use in concrete. To unlock the potential and enable beneficial uses of coal fly ashes, this work presents a comprehensive review of recent literature related to the reactivity of coal fly ash in cementitious binder systems, with the focus on reactivity indices, reactivity test methods, treatments employed for improving reactivity, and strength prediction of fly ash-based binder systems. The urgent research needs are also identified to support the enhanced utilization of fly ashes for sustainable construction.