Valorization of coal gangue as a sustainable supplementary cementitious material for immersed tube tunnel construction: equivalent strength performance in LC3 systems

The cement industry is a major source of global CO₂ emissions, and limestone calcined clay cement (LC³) is a promising low-carbon alternative, but its large-scale application is limited by the dependence on high-cost commercial calcined clays. Coal gangue (CG) is a massive industrial solid waste with potential pozzolanic activity after thermal activation, yet its reaction behavior, synergistic mechanisms, and performance equivalence to commercial calcined clays in LC³ systems remain unclear, with a lack of quantitative assessment of its carbon emission reduction potential. To address these gaps, this study conducts a multi-scale investigation on thermally activated coal gangue (CCG)-based LC³ cement, combining experimental characterization including X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, mercury intrusion porosimetry, isothermal calorimetry and life cycle assessment. The results show that the optimal mass ratio of CCG to limestone is 2:1, and the LC³ formulation with 20% CCG replacement achieves a 28-day compressive strength comparable to that of commercial calcined clay-based LC³ and ordinary Portland cement. A synergistic reaction mechanism involving sulfate, carbonate, and aluminosilicate is clarified: gypsum regulates early aluminate hydration, while limestone reacts with reactive aluminates derived from CCG to form stable carboaluminate phases, which refine the pore structure and densifying the microstructure. LCA results indicate that the optimal CCG-based LC³ formulation reduces the global warming potential by 24.4% compared to ordinary Portland cement. This study realizes the high-value utilization of CG and provides a low-cost, sustainable alternative to commercial calcined clays for LC³ technology, offering technical support for the decarbonization of the cement industry and the disposal of industrial solid waste.