Hydration, Microstructure, and Properties of Fly Ash–Based Geopolymer: A Review

Geopolymers have gained attention as a potential eco-friendly alternative to Portland cement, primarily due to their reduced carbon dioxide emissions and the opportunity to repurpose industrial waste materials. Fly ash (FA), a byproduct of coal combustion, has been favored as a raw material for geopolymer concrete owing to its widespread availability and high concentrations of alumina and silica. The development and application of fly ash–based geopolymer concrete can contribute significantly to production of sustainable construction materials. An in-depth analysis of fly ash–based geopolymer concrete has been conducted to explore its potential as a substitute for traditional concrete. This review encompasses the underlying reaction mechanism, strength, long-term durability, and microstructural characteristics of geopolymer concrete. The present review paper shows that adding the optimal quantity of fly ash improves the performance of fly ash–based geopolymer when exposed to extreme durability conditions, as well as improving strength properties. The microstructural analysis shows that when fly ash is added, the microstructure of the concrete matrix would be dense and packed. However, challenges remain in adopting fly ash–based geopolymer concrete for large-scale construction projects, as the existing literature presents inconsistencies in the reported strength, durability, and test results. Further research is necessary to consolidate knowledge on the behavior and mechanism of fly ash–based geopolymer concrete and to ultimately provide comprehensive data to support its widespread implementation in the construction industry.