It is widely known that the production of Portland cement consumes considerable energy and at the same time contributes a large volume of CO2 to the atmosphere.It is estimated as being responsible for 5-7% of current anthropogenic CO2 emissions world-wide.The environmental impact from the production of cement has prompted research into the development of concretes using 100% replacement materials activated by alkali solutions, termed geopolymers.It has been estimated that reduction of CO2 emission due to the replacement of Portland cement with geopolymer is between 26-45%.While the use of geopolymers is now becoming widespread there are significant variations in the chemical and physical properties of the fly ashes available.This study investigated the microstructure and durability characteristics of geopolymer concrete synthesized from a range of class F fly ash obtained in different power plants in Australia.The optimum mix designs for each fly ash based geopolymer concrete were determined by.activating the fly ash with sodium silicate and sodium hydroxide using different ratios.A number of key durability parameters were then analysed for the optimum concrete mix for each fly ash.Properties investigated included compressive strength, workability,ultrasonic pulse velocity, rebound hammer and resistivity.Scanning electron microscopy and energy dispersive X-ray spectroscopy were employed to examine the geopolymerization, composition and microstructure of the end products.A variation in 28-day compressive strength for the five fly ash based geopolymers was observed between 24.1MPa and 48.7MPa.All geopolymers contained unreacted/partially reacted phases as inactive fillers within the geopolymer binder, resulting in variability in their durability characteristics.The difference in strength, durability and microstructure among the five geopolymer concretes are attributed to the different reactivity of the source materials and the amount of nonreactive fillers.