Effect of trace elements (Fe3 +, Mg2+, S6+) on the hydration mechanisms of alkali-activated coal gangue

To better understand the influence of trace elements in coal gangue (CG) on the hardening mechanism of alkali-activated coal gangue cementitious materials (AACG), this study selected Fe, Mg, and S as the primary trace elements based on variations in the chemical composition of CG from different origins. AACG was prepared by adding Fe 2 O 3 , MgO, and SO 3 into coal gangue at proportions of 0 %, 0.5 %, 1.0 %, 1.5 %, and 2.0 %. The compressive strength test, nuclear magnetic resonance (NMR) porosity analysis, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TG-DTG), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) were employed to systematically investigate the compressive strength, hydration products, and microscopic morphology of AACG cementitious materials. The results indicated that Fe₂O₃ partially facilitated the formation of zeolite-like aluminosilicates in AACG cementitious materials and significantly improved the pore structure, reducing porosity by 38.2 % compared to the control group at a 2 % dosage. Increasing the MgO concentration enhanced the activity of CG, raising the pyrolysis temperature of N-A-S-H and aluminosilicate gels from 110.19 °C to 117.86 °C in TG-DTG analysis at 2 % MgO doping. Additionally, SEM images revealed a better polymerization state, and the 28-day compressive strength reached 93 MPa, an increase of 30.9 % compared to the undoped group. The formation of zeolite-like aluminosilicates in AACG cementitious materials peaked at 1 % SO₃ doping. Additionally, at 2 % doping, the proportion of capillary and macropores decreased from 6 % to 3 % compared to the undoped group. This study offers new insights into the role of Fe 2 O 3 , MgO, and SO 3 in AACG and establishes a scientific foundation for the efficient utilization of coal gangue from various sources.