Comparative Study on the Performance and Hydration Mechanism of Coal Gangue Cementitious Materials with Different Alkali Activators

Coal gangue (CG) is one of ranks among China's most significant industrial by-products. In response to China's carbon neutrality commitments and the growing emphasis on resource recycling, finding effective ways to valorize CG has emerged as a pressing concern. Based on the mineral composition and chemical composition characteristics of CG, this study systematically investigated the enhancement effects of three alkali activators (sodium silicate, NaOH, and Ca(OH)2) on the cementitious properties of CG. Through different dosage and compressive strength tests, the efficiency ranking of the three activators was determined as: sodium silicate > Ca(OH)2 > NaOH. A 10% sodium silicate dosage combined with 28-day curing was identified as the optimal condition for achieving sufficient reaction and structural densification. Under these conditions, the compressive strength of CG cementitious material reached 6.4 MPa, representing an increase of 190.9% compared to the blank group (2.2 MPa), significantly superior to Ca(OH)2 (69.55%) and NaOH (62.27%). X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analyses revealed thatalkali activators function primarily by disrupting the crystalline framework of CG, promote the cross-linking polymerization of silicon-aluminum monomers to generate dense cementitious products, thereby improving material performance. The sodium silicate is attributed to its “dual activation effect”, providing OH− to create an alkaline environment while supplying reactive silicate ions (SiO32−) to accelerate N-A-S-H gel and C-A-S-H gel formation. These findings offer guidance for optimizing CG-based cementitious formulations for the formula optimization and large-scale utilization of CG cementitious materials.