Experimental Study on the Mechanical Properties and Microstructural Mechanisms of Coal Gangue-Based Cementitious Materials Synergistically Activated by Desulfurization Gypsum and Lime

Coal gangue and flue gas desulfurization gypsum (DG) are industrial by-products whose utilization is critical for sustainable development. This study explores the synergistic activation of coal gangue using DG and lime to develop eco-friendly cementitious materials. Three systems-CF (coal gangue-DG), CL (coal gangue-lime), and CFL (coal gangue-DG-lime)-were designed to investigate the effects of DG and lime on coal gangue reactivity. Unconfined compressive strength tests identified the optimal dosages of DG and lime, while XRD, SEM, TGA-DTG, and NMR analyses were employed to characterize the hydration products and microstructure. The results demonstrated that calcium ions from both lime and DG facilitated the formation of C-(A)-S-H gel, enhancing strength development. However, excessive DG led to undesirable volume expansion due to ettringite overproduction, compromising material stability. Excess lime forms weak Ca(OH)₂ binding phases and causes expansion and cracking from excessive hydration heat, reducing strength and durability. Thus, the optimal DG dosage in the CF system is 70%, and the optimal lime dosage in the CL system is 6%. In the CFL system, 70% DG and 4% lime achieve the best performance. The combined use of DG and lime not only improved coal gangue reactivity but also achieved an optimal balance between strength enhancement and long-term stability, offering a promising approach for sustainable construction materials.