The application of Portland cement as a cementitious binder has been used widely in the construction industry. However, the application of geopolymer has equal popularity nowadays, especially for concrete repair works and rehabilitation. Research and development of high-performance geopolymer binders or known as alkali-activated materials is increasing worldwide. There are various precursors that have been investigated to optimize the performance of the geopolymer binders such as kaolin, slag, and many others. The application of fly ash is popularly being tested and proved to be the most efficient precursor owing to its excellent properties and performance. The performance of this precursor is being reviewed and presented in this paper and the high calcium fly ash is notably the most suitable precursor for fast setting and high early strength geopolymer. This paper presents an overview of fly ash, in terms of its properties, reaction, and mechanism. Several significant properties affecting the performances of fly ash-based geopolymer were discussed. It is being highlighted in this paper that fly ash has various microstructures which produced different geopolymerization reaction mechanisms.
