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.