Geopolymer is considered to be an eco-friendly binder and replacement for ordinary Portland cement as it leads to a reduction in CO2 emissions and energy saving. A strong alkaline solution is required to activate the silicon and aluminum present in the fly ash and setting additives. The main objectives of this work are to determine the effects of the alkaline solution type on the fresh and mechanical properties of alkali-activated fly ash (class F) based geopolymer mortar. Four different alkaline solutions were prepared. The first and the second solutions were made by mixing sodium silicate liquid with sodium hydroxide and with potassium hydroxide, respectively. For the other two solutions, potassium silicate liquid was used instead of sodium silicate liquid. The investigated parameters included molarity of NaOH and KOH (10M, 12M, 14M and 16M) as well as different ratios of alkaline solutions to the binder (0.40, 0.45 and 0.50), silicate to hydroxide solutions ratio (2, 2.5 and 3) and sand to fly ash ratio (1.5, 2.0 and 2.5). Test results revealed that the workability significantly increased by using potassium silicate with respect to sodium silicate. In addition to increasing the silicate to hydroxide ratio has a negative impact on the workability of potassium silicate-based geopolymer mortar prepared by NH or KH. In case of sodium silicate based geopolymer mortars, the concentration of NH or KH of 16M achieves the maximum compressive strength but the concentration of 14M was remarked in case of potassium silicate-based geopolymer.