Fly ash is a by-product of coal-fired electric generating plants and has been widely used as mineral admixtures in normal and high strength concrete.The addition of fly ash mainly represents dilution effect and chemical effect on cement hydration.The dilution effect is a consequence of the replacement of cement by fly ash and results in an increase in the water/cement ratio.The chemical effect is pozzolanic reaction between mineral admixture and calcium hydroxide.This paper presents an analytical model to evaluate the properties evolution of cement-fly ash blends considering both dilution effect and chemical effect.It is found that the stoichiometric ratio of fly ash to calcium hydroxide relates with fly ash replacement ratio.For cement-fly ash blends with a lower to binder ratio and high volume fly ash, the dilution effect is much more significant, the hydration degree of cement is significantly improved, and the compressive strength of cement-fly ash blends is comparable to that of control specimens.