Synergistic action and effect mechanism of coal gangue powder and red mud on the properties of concretes

Red mud and coal gangue powder were employed as substitutes for cement to prepare red mud concrete, coal gangue concrete, and red mud-coal gangue concrete. The effects of red mud and coal gangue powder on the physical properties, workability, and mechanical properties of concretes were evaluated, and the hydration reaction mechanisms were elucidated through microanalysis. Furthermore, the reaction process was investigated through thermogravimetric analysis. The results indicated that the individual addition of red mud and coal gangue powder had negative effect on the strength, workability, and porosity of concretes. However, the combination of red mud and coal gangue powder significantly improved the strength and porosity of the concrete compared to red mud concrete and coal gangue concrete. The red mud-coal gangue concrete with 15 % red mud and 15 % coal gangue powder had lower porosity, and comparable compressive strength with the cement concrete. The microscopic results and thermogravimetric analyses indicated that the synergistic effect of red mud and coal gangue led to the improvement of the strength of red mud-coal gangue concrete. The red mud exhibited the promotion effect on the hydration of coal gangue powder to generate new C-S-H, filling the pores. The denser structure in the red mud-coal gangue concrete was formed, thereby improving the pore space and mechanical properties of the concretes.