Modification of thermal-activated recycled cement with calcined coal gangue

Unmodified thermal-activated recycled cement (RC) has short setting time, low fluidity and high water demand. Substitution of 30 wt.% of RC with coal gangue (CG) thermally treated under 600, 700, 800, and 900 °C was explored, aiming to explore the modification of RC with CG through fluidity, setting time, compressive strength, X-ray diffraction (XRD), thermogravimetry (TGA), micron indentation (MI), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) tests. The findings show that amorphous Al2O3 and SiO2 in calcined CG quickly reacted with CaO in RC and formed C(–A)–S–H. The inactive ingredients in CG could fill into the loose C–S–H gel, which was rapidly formed at the initial stage. RC paste modified with CG could reach a maximum 28-day compressive strength of 28.6 MPa anda 27% higher fluidity and 70% longer setting time than unmodified RC paste. In addition, more than 400% higher strength development from 3 d to 28 d was observed in CG modified RC compared to the unmodified RC. The study highlights a dual mechanism of chemical reactions and physical pore-filling, which significantly improved the performance of RC.