Preparation of coal gangue-based multicomponent solid waste geopolymer mortar and strength formation mechanism

Coal gangue is commonly activated through physical or chemical methods to enhance its reactivity for geopolymer production. However, these methods are energy-intensive and costly, limiting their environmental viability. In contrast, raw coal gangue is abundant and holds potential for direct use, provided its reactivity can be effectively stimulated. This study developed a multicomponent geopolymer using untreated coal gangue as the primary aluminosilicate precursor, along with slag, fly ash, and desulfurized gypsum. An orthogonal experimental design was employed to examine how variations in coal gangue content, sodium silicate modulus, alkali activator dosage, and gypsum affected compressive strength. The optimal mix (CG1) achieved compressive strengths of 10.8, 17.6, and 23.2 MPa at 3, 7, and 28 days, respectively, surpassing other formulations. Aiming to elucidate the strength development mechanism, CG1 was analyzed using XRD, FTIR, TG–DTG, SEM–EDS, and 27Al/29Si SS NMR. Results revealed that the formation of an amorphous (C,N)-A-S-H gel network was the main factor driving strength gain, while early ettringite (AFt) formation contributed to initial strength. NMR analysis indicated that aluminum primarily existed in tetrahedral coordination, and silicon transitioned from Q1/Q2 to Q3/Q4 species. Between 7 and 28 days, an increase in the Si/Al ratio and mean chain length (MCL) reflected enhanced polymerization. A seven-stage geopolymerization mechanism—activation, dissolution, bond breaking, polycondensation, gelation, cross-linking, and hardening—was proposed to describe the transformation of raw coal gangue under alkaline conditions. These findings offer valuable insights into promoting the low-carbon utilization of coal gangue, particularly in cold or low-temperature environments.