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The use of fly ash as cement replacement material can overcome the environmental issues, especially the global warming problem caused by the greenhouse effect. This attributes to the CO₂ gas produced during the cement manufacturing process, which 1 ton of cement is equivalent to 1 ton CO₂. However, the major problem of fly ash is the need of activators to activate the polymer reactions. The most commonly activator used in non-cement material is the combination of sodium hydroxide (NaOH) and sodium silicate. This study aims to identify the effect of NaOH molarity as activator on strength development of non-cement class C fly ash geopolymer mortar. The molarity variations of NaOH were 6 Molar (M), 8M, 10M, 12M, 14M and 15M. The compressive strength test was performed at the age of 7, 14 and 28 days in accordance with ASTM standard, and the mortar specimens were cured at room temperature. The results showed that the highest compressive strength was achieved by geopolymer mortar with molarity of 12M. It exhibited a comparable strength to that normal mortar at 28 days. However, the use of NaOH molarity more than 12M tend to decrease the strength of non-cement geopolymer mortar specimens.