History, recent progress, and future challenges of alkali-activated binders – An overview

Alkali-activated binders (AAB) are low-carbon alternatives to cement used in traditional concrete manufacture. This paper presents a state-of-the-art survey of the historical background, synthesis development, reaction mechanism, and recent advances of the AAB. The parameters influencing the engineering performance of the AABs, including the physicochemical characteristics of the precursor materials, formulation and dosage of alkaline activators, and type and period of curing, were critically reviewed. In this regard, three different precursor material sources are identified: industrial by-products, natural aluminosilicates, and agricultural waste materials, which have various availability, reactivity, cost, and value worldwide. In addition, recent advances in the life cycle assessment of AAB mixtures are compiled. The challenges that AABs face towards commercial applications are also discussed. The selection of appropriate precursors, the production of low-cost alkaline activators, mix design parameters, fundamental mechanisms, engineering properties, construction processing and application, performance standards, and sustainability aspects constitute the challenges. Alkali-activated concrete can be a potential low-carbon substitute for conventional concrete if it ensures true sustainability and not just greenwashing.