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30 Development of sustainable construction materials has been the focus of research efforts 31 worldwide in recent years. Concrete is a major construction material; hence, finding alternatives 32 to ordinary Portland cement is of extreme importance due to high levels of carbon dioxide 33 emissions associated with its manufacturing process. This study investigates the effects of 34 activating solution type, curing procedure, and source of fly ash in relation to the resulting 35 compressive strength of fly ash-based geopolymer concrete. The fly ash-based geopolymer paste 36 microstructure was observed and density, absorption and voids were measured. Two activating 37 solutions were used: a) a mixture of sodium hydroxide, silica fume, and water; and b) a mixture 38 of sodium hydroxide solution, sodium silicate, and water. Test results indicate that the resulting 39 concrete has the potential for high compressive strength and the compressive strength is directly 40 affected by the source of fly ash. Results further indicate that compressive strength is not 41 significantly affected by the curing condition when silica fume is used in the activating solution 42 in comparison to the use of sodium silicate. 43 Keywords: alkali-activated fly ash concrete, geopolymer concrete, early compressive strength, 44 silica fume activating solution, sodium silicate activating solution 45 1 2 3 4 46 47 1 FGC = Fly ash-based Geopolymer concrete 2 FGC-silica fume = FGC with silica fume in the activating solution 3 FGC-sodium silicate = FGC with sodium silicate in the activating solution 4 SEM = Scanning Electron Microscopy Density, absorption and voids for FGC (silica fume and sodium silicate based) were measured 130 according to ASTM (C 642-06) at 7 days after casting. 131 132

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