The use of fly ash, a type of industrial by-product, as a binder material in concrete mixing has been widely adopted in civil engineering. This is beneficial to developing environmentally-friendly constructions due to the recycling of fly ash waste and reduction of cement consumption. In this paper, the effects of high-volume fly ash on early-age characteristics and hardening properties of concrete are investigated. A series of test specimens with different proportions of fly ash (0%−60%) as a replacement of cement were fabricated. In the first stage, the early-age characteristics, including flowability, initial and final setting, and hydration reaction, were studied with the particular emphasis of the effects of the replacement level of fly ash as cement. Subsequently, the hardening properties, including mechanical properties and microstructures, were investigated for each concrete mixture by mechanical tests, scanning electron microscope (SEM) test and energy dispersive X-ray spectrometry (EDS) analysis. It can be concluded that the hydration reaction is delayed and the mechanical properties of concrete at early age are declined by increasing the proportion of fly ash. However, the excellent mechanical properties at a later stage are discovered when fly ash with a proper proportion is incorporated into concrete. This means that it is possible to achieve environmentally-friendly construction by mixing concrete incorporating fly ash of a suitable proportion. Furthermore, based on the consideration of environmental benefits, economic benefits and good performance of the concrete, an optimum replacement level of cement by fly ash is recommended to be 40%.