This research was conducted to compare class C and class F fly ash as cementitious materials. The fly ash was supplied by local power plants in Malaysia. The chemical composition analysis showed that class F fly ash has more Si and Al content compared to class C fly ash which affects the amount of the produced gel during the pozzolanic reactivity or/and geopolymerization. The chemical bonding analysis showed that class F fly ash has more bond structure (C-S-H, C-A-H, -Si-O-Al-, and -Si-O-Si-) than class C fly ash. Finally, the microstructure images indicated the similarity of the fly ash particles shape on both classes. Furthermore, the microstructure images showed a correlation between the chemical composition and the bond structure in terms of the high element content (Si and Al) forming the gel and the bond structure in the cementitious process during pozzolanic reaction or/and geopolymerization. © 2016 The Authors. Published by Elsevier Ltd.