Toward understanding the activation and hydration mechanisms of composite activated coal gangue geopolymer

•Coal gangue can be employed as primary cementitious material after being modified by the proposed activation method;•The activated coal gangue geopolymer displayed high early and uniaxial compressive strength;•An optimum calcination strategy was proposed for the formation of high reactivity amorphous SiO2 and Al2O3;•Microstructure evolutions and hydration productions at different stages were experimentally verified. The feasibility of coal gangue as primary cementitious material was experimentally investigated in this paper. An Orthogonal method was employed to investigate the optimum activation strategy for coal gangue-based geopolymer. Experiment parameters included the modulus of sodium silicate (M), alkali content (A), calcination temperature (T), holding time (H), and water-to-binder ratio (W/B). Meanwhile, the phase transitions, chemical compositions, and microstructure evolution were investigated by supplementary techniques, such as ESEM, XRD, and FTIR. Experimental results indicated that the hydration reactivity of coal gangue was markedly enhanced. And the formations of high contents of amorphous SiO2 and Al2O3 phases are the critical factors for the high reactivity, which highly depends on the proper calcination temperature and holding time. The hydration productions of alkali-activated coal gangue, mainly N-A-S-H gels, can rapidly precipitate around the unreacted particles and fill the existing cavities. Meanwhile, the average surface area, diameter, and porosity considerably declined with the hydration degree of geopolymer. In summary, coal gangue can be viewed as a new eco-friendly cementitious material via the proposed composite activation strategy.