The successful application of class F fly ash in oil and gas wells can not only solve the problem of waste product pollution, but also can modify the quality of well cementing under some critical downhole conditions. However, class F fly ash is not normally used for well cementing directly because of the short setting time. The aim of this paper is designed to reveal the influence of alkali activator on the setting time of class F fly ash and develop one new type of composite activator to extend the setting time to realize the application of class F fly ash in oil and gas well. Results show that the influence of alkali activator on setting time could be expressed by the effect of activator on the concentration of [AlO4] and [SiO4] tetrahedrons. High concentration of [AlO4] tetrahedron plays a key role in setting time and act as polymerization inhibitor. Due to the repulsion between [BO4] and [AlO4] tetrahedrons and [BO4] tetrahedrons themselves, the setting time of class F fly ash with composite activator of sodium hydration and anhydrous borax is long enough for well cementing at 60 and 75 degrees C. The effect of anhydrous borax on setting time is related to the concentration of alkali and temperature. Although anhydrous borax has negative effect on the hardening of the considered specimens, the compressive strength of class F fly ash with proper composite activator still meets the demand of cementing engineering. (C) 2017 Elsevier Ltd. All rights reserved.