Systematic Review of the Use of Fly Ash in Geopolymer Concrete: Impact on Its Physical, Mechanical, and Microstructural Properties

Abstract

The environmental impact of Portland cement has driven the adoption of alternative binders such as geopolymer concrete (GC) based on fly ash (FA). This review analyzes 69 studies published between 2021 and 2025 that examine how FA influences the physical, mechanical, and microstructural performance of GC. Workability is strongly affected by the spherical morphology of FA and by the concentration of NaOH, with reported slump values ranging from 62 to 220 mm. Alkaline solutions between 12 M and 18 M accelerate the reaction, reducing initial setting times to 30–150 minutes. Mechanically, mixtures incorporating 50–70% FA and supplementary materials such as slag or microsilica can surpass 60 MPa in compressive strength while also improving tensile and flexural behavior. Microstructural analyses using XRD, SEM, and FTIR confirm the formation of N–A–S–H and C–A–S–H gels that refine and densify the matrix. Overall, FA emerges as a sustainable, high-performance alternative for producing GC, with potential for future construction applications