Effect of activated coal gangue on the hydration and hardening of Portland cement

Herein, the effect of ball milling on the crystal structure of coal gangue (CG) was quantitatively analysed based on the Scheele and Hinckley formulas，and the trend of the crystals from 700–800 ℃ was analysed. The results indicate that ball milling decreases the crystallinity and increases the lattice distortion rate of kaolinite in CG. As the calcination temperature increases, the [SiO₄] and [ AlO₆] groups increase and so does the crystallinity. The conductivity change of the CG/ portlandite (CH) system was tested, and the volcanic ash activity of CG with different activation modes was quantified using the diffusion kinetic model. The results indicated that the volcanic ash activity of CG was higher as the calcination temperature decreased and as the ball milling time increased. CG blended cement (CGC) was analysed for compressive strength, heat of hydration, derivative thermogravimetry and X-ray diffraction tests, nuclear magnetic resonance tests as well as scanning electronic microscope tests. The results indicated that CGC exhibited higher strength than the pure cement test group at an early stage, and that their activity index (H₂₈) was greater than 1 (standard requirement is greater than 0.6) at 28 days at 30% dosing for each test group. In addition, the greater the lattice distortion rate of CG after ball milling, the more apparent the promotion effect on cement hydration. The higher the temperature after calcination, the more [SiO₄] and [AlO₆] groups occur as a result of recrystallisation in CG, leading to a weaker promotion of cement hydration. Response surface methodology was utilized to predict the optimum parameters of CG based on strength, The model has an R² of 0.975 and shows good predictive performance.