Most of fly ashes produced today are mainly used as the supplemental cementitutious material to partially replace cement used in many building and construction applications.How to increase the cement replacement is the most important task for fly ash utilization.Alkaline activation of fly ash has been used to completely replace cement in many cases.The effects of three key fundamental properties of fly ash, particle size and its distribution, chemical compositions, and mineral compositions, related to alkaline activation have been not well established.This study is to investigate the effects of alkaline and fly ash particle size on the strength development using a fly ash/cement blend at 70/30 by weight.For these high fly ash cement blends, water glass as the alkaline activator can significantly increase compressive and flexural strengths at all aging times.The use of finer fly ash under this alkaline activation can further enhance the strengths except at the early stage, such as 3 days.As expected, strength improvement by water glass is higher than that by fly ash fineness.However, the results show that alkaline activation is still not sufficient to achieve 3-and 28-day strengths of these high fly ash cement blends equivalent to the pure cement,except 28-day compressive strength.In addition, these blends, regardless of the use of water glass or water,seem to have two strength growth stages, while the pure cement has only one growth stage due to cement hydration.