Property-Guided Selection of Fly Ash Across Binder Chemistry Windows

Fly ash (FA) recycling into cementitious binders offers a promising route to simultaneously reduce the environmental burden of high-impact construction materials, divert industrial waste from disposal pathways, and generate engineering value from an existing aluminosilicate residue. FA has therefore been widely incorporated into ordinary Portland cement (OPC), calcium sulfoaluminate (CSA) cement, magnesium potassium phosphate cement (MKPC), and alkali-activated/geopolymer systems, where it can improve workability, mechanical strength, durability and promote hydration reactions, depending on the binder chemistry. However, these benefits are not always successful: FA addition may also reduce early-age strength, delay setting, limit reactivity, impair fluidity, or produce under-reacted matrices when ash properties are mismatched with the chemistry window of the target binder. Rather than revisiting these systems as isolated application categories, this review develops a property-guided framework for interpreting and selecting FA across major cementitious routes. It first highlights why FA should not be treated as a single material. Key descriptors include glass content, fineness, calcium level, carbon residue, mineralogy, and beneficiation state. The review then compares four representative binder environments to clarify the role of FA shifts from pozzolanic contributor to filler-dominated hydration modifier to functional regulator, and finally to reactive precursor, including the calcium hydroxide (CH)-rich Portland systems, CH-poor CSA systems, phosphate-bonded MKPC systems, and alkali-activated/geopolymer systems. The central conclusion is that the key question is not simply where FA can be used, but which FA is best matched with which binder chemistry and for what performance objective.