Complementary and Potentially Synergistic Interaction of Fly Ash, Ultrafine Fly Ash, and Fiber in Cementitious Composites: A Multi-Scale Mechanistic Review

Ordinary Portland cement (OPC) is associated with considerable energy demand for its production and accounts for approximately 8% of the global CO2 emissions. The extensive extraction of natural aggregates further undermines the sustainability of concrete. The use of supplementary cementitious materials (SCMs) as a partial replacement for clinker, along with improved material optimization, is considered a fundamental measure. Fly ash (FA), owing to its pozzolanic behavior and its positive influence on improving workability and long-term durability, is extensively utilized. Ultrafine fly ash (UFFA) contributes to improved packing density and microstructural refinement due to its finer particle size and enhanced reactivity, whereas fiber inclusion bridges micro- and macrocracks by promoting structural ductility. This study reviews and integrates the recent research on HVFA systems, UFFA-based cementitious binders, and fiber-reinforced cement-based composites. Based on the governing mechanisms of each component, this review develops a rational synthesis of their potential interactions and dosage-related limitations. Although the FA-UFFA, FA–fiber, and UFFA–fiber binary systems have been individually explored, their simultaneous integration (FA-UFFA–fiber) remains unexplored. Based on the synthesized evidence, a conceptual multi-scale interaction framework and a preliminary evidence-informed design window are proposed to illustrate the complementary interactions and potentially synergistic interactions among FA, UFFA, and fibers, thereby providing a mechanistic basis for future experimental investigations of FA-UFFA–fiber cementitious systems.