Performance Evaluation and Microstructure Characterization of Steel Fiber–Reinforced Alkali-Activated Slag Concrete Incorporating Fly Ash

This paper investigates the performance of steel fiber–reinforced alkali-activated slag concrete incorporating different fly ash replacement percentages. Three different molarities of sodium hydroxide (SH) were combined with sodium silicate to activate the binding phase. Double hooked-end steel fibers were incorporated into the alkali-activated mix in varying volumetric proportions up to 3% to enhance its ductility. Blended binder, alkali-activator solution, dune sand, and coarse aggregate contents were proportioned and samples were cured at ambient conditions. Results showed that higher slag content, molarity of SH, and fiber addition led to less-workable concretes but with improved mechanical properties, especially at early ages. Fly ash replacement of 25% could enhance mechanical performance after 28 days. Analytical models correlating mechanical properties were developed for alkali-activated slag concretes with fly ash. Scanning electron microscopy, differential scanning calorimetry, and Fourier transform infrared spectroscopy highlighted the coexistence of calcium aluminosilicate hydrate and sodium aluminosilicate hydrate gels.