###### Abstract
In addition, the usage of FA in concrete not only improves concrete performance but also provides an economical and effective way to consume FA [28]. The existing study found that the main chemical components of FA, as a pozzolanic material, are SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), CaO, Fe\({}_{2}\)O\({}_{3}\), and a small number of unburned carbon particles [29]. Its major phases are vitreous aluminum silica and a small amount of quartz, multite, and other minerals. A large number of vitreous substances are the main source of the gelling activity of FA and have potential hydration reactivity [30]. The activation methods of FA mainly include calcium treatment, single alkali excitation (time, sodium silicate), sulfate excitation (CaSO\({}_{4}\), Na\({}_{2}\)SO\({}_{4}\)), physical fine grinding or physical fine grinding combined with chemical activators, etc. [PERSON] and [PERSON] [31] obtained fine ash with a particle size of less than 45 \(\upmu\)m by a wind separation method, which can improve the strength and working performance of concrete. [PERSON], et al. [32] demonstrated that the slurry fluidity and backfill strength of the cement material mixed with FA can meet the mining requirements of filling cement. [PERSON] and [PERSON] [33] suggested that the strength of FA concrete may be obviously improved by activating the activity of FA with chemical admixtures. [PERSON], et al. [34] found that the method of the combination of composite activation, mechanical activation, and chemical activator could enhance the activity of FA more significantly. Concrete consumption creates a large carbon footprint in the environment. Fly ash (FA), rice husk ash (RHA), and silica fume (SF) can replace cement in proper proportions to reduce CO\({}_{2}\) emissions. FA affects the set-up time and sulfate resistance of concrete and can improve the workability of concrete mixtures. Conversely, the high surface area of SF particles reduces workability. A high LOI ratio indicates that RHA may evaporate when the concrete is exposed to high temperatures. FA and RHA, as substitutes for cement, have no adverse effect on the compressive strength of concrete, whereas the incorporation of SF into concrete can improve the overall compressive strength, tensile strength, and elastic modulus of concrete. SF not only improves the bond between the cement paste and the aggregate but also has a key advantage as an alternative to cement due to its ability to reduce the permeability of concrete [35]. It has been established that the mechanical strength of concrete will be affected when the replacement level of RHA and SF synergies is above 30%, and that concrete with a continued increase in RHA content will have lower mechanical strength than ordinary concrete. When the RHA content is increased to a certain extent, higher flexural strength can be obtained, and the durability of the concrete can be improved [36]. The combination of FA and nS is adopted to replace ordinary Portland cement (OPC), which significantly improves the mechanical properties of environmentally friendly concrete and the form of the cement matrix. At the same time, homogenization of the cement matrix promotes structural alignment of the composite materials mixed with FA and nS and reduces the carbon footprint of cement-based materials [37]. In addition, the research on the properties of FA concrete mainly focuses on the physical and chemical properties of the material, whereas the research and test data on the mechanical properties of concrete are relatively few. Therefore, the mechanical properties of FA concrete are investigated in depth in this study. This study aimed to explore the high safety performance of green and low-carbon concrete. The improvement effect of FA on the performance of the composite Portland-cement-FA system was investigated. Under the condition that the amount of binder and water, the ratio of water to the binder, and the total amount of material used in the specimen are equal, the concrete was prepared by adding the FA content instead of cement.