Role of particle fineness and reactive silicon-aluminum ratio in mechanical properties and microstructure of geopolymers

•Reactive silicon and aluminum components increased by alkali thermal activation.•High compressive strength geopolymers were prepared from three solid wastes.•The silicon-aluminum ratios of raw materials are well linearly related to the reactive components.•The looser structure of geopolymers produced by finer particles of fly ash is related to agglomeration. The main two factors of the mechanical properties and structure of geopolymers including reactive silicon-aluminum ratios and particle fineness were investigated. Vanadium-bearing shale residue (VSR), fly ash (FA), and silica fume (SF) were subjected to alkali thermal activation (ATA) to increase the content of reactive silicon and aluminum, then a leaching experiment was carried out to quantify the content of active silica and aluminum in the raw materials, which indicated that the reactive silicon-aluminum ratios are well linearly related with those in the raw materials. High compressive strength (82 MPa at 28 days) geopolymers were prepared at ambient temperature curing of the three solid wastes after ATA, and the effect of FA fineness to geopolymers was also investigated during the preparation. As the FA content in the raw materials increases within a certain range, the compressive strength of the geopolymer is significantly enhanced, along with the main band is relevant to asymmetric stretching of Si-O-Si/Al has shifted towards a lower frequency, which is results from structural alteration of the Al-Si network. The appropriate reactive silicon-aluminum ratios accelerate the geopolymerization reaction process, which is conducive to the formation of the silicon-aluminum gel microstructure. The fineness of FA has a notable effect on the mechanical properties of geopolymers. It is interesting to found that the geopolymers with finer FA fineness have lower compressive strength, which can be explained by the looser structure and more pores due to the agglomeration of fine particles, resulting in difficult geopolymerization reactions.