Preparation and characterization of coal gangue geopolymers

•Coal gangue is silicon-aluminium materials, which can be used to prepare geopolymer after high-temperature calcination.•Coal gangue geopolymer has a high early strength, higher than that of OPC.45 cement paste specimens.•The generation of the zeolite facies crystallization plays an obvious role on the compressive strength of coal geopolymer.•This paper provides experimental and theoretical basis for the development and wide application of coal gangue cementitious materials. This paper explores the impacts of sodium hydroxide modulus, alkali lye amount and liquid–solid ratio on the strength and microstructure of coal gangue geopolymer materials, involving a total of five sodium hydroxide moduli, four NH/NS mass ratios and seven liquid–solid ratios. To characterize the morphology and structure of coal gangue geopolymer materials, we carried out XRD, TG-DTG, FT-IR and SEM analysis on the specimens respectively. The results show that NH concentration has no impact on paste fluidity and has significant impact on compressive strength of geopolymers. Both paste fluidity and compressive strength of geopolymers increase with the increase in liquid–solid ratio. However, it should be noted that the compressive strength decreases with the increase in liquid–solid ratio when the liquid–solid ratio is greater than 0.32. The optimal NH/NS mass ratio is 1:1.5 ∼ 1:2. In comparison with P.O42.5 cement specimens, coal gangue geopolymers have higher initial strength and lower 28 d compressive strength. The analytical results of microstructure are consistent with those of compressive strength, which demonstrates that the polymerization products of coal gangue geopolymers are N-A-S-H gels and some other aluminosilicate zeolite crystals.