The synthesis of geopolymer mortars based on high calcium fly ash (HCFA) has been studied in order to assess the effects of the alkaline solution on the geopolymerization products and microstructure by means of SEM/EDS and compressive strength tests. The results showed that HCFA geopolymers can develop high strength binders with a dense homogeneous microstructure. The higher sodium silicate to sodium hydroxide (NS/NH) ratio in the range of 1�2.5 has increased the density and homogeneity of the microstructure. Similar behavior was observed for the effect of NH concentration with an optimum concentration of 10M. The microstructure of HCFA geopolymers can be described as porous with nanoporosity size and considerable amounts of unreacted fly ash particles depending on the study parameters. The formation of C�S�H crystals was not detectable. The Si/Al ratios of the produced gels were in the range 1.12-5.55 which was different than the initial mixing ratios. © 2016 Taylor & Francis Group, London.