Effects of different composite activator systems on mechanical properties and hydration behavior of large amounts of Bayer red mud based low carbon cementitious materials

Bayer red mud (BRM) exhibits inherently low pozzolanic activity and requires activation by chemical activators. However, the excitation effects and underlying mechanisms of different composite activator systems on large amounts of BRM based cementitious materials remain insufficiently understood. In this study, the mechanical properties and hydration behavior of BRM based cementitious materials activated by various composite activator systems were systematically investigated. The hydration products and microstructures were characterized using XRD, FT-IR, and SEM to elucidate the activation mechanisms. The results show that at a BRM content of 50 wt%, the optimal mixing ratios under each composite activator system yield the maximum compressive strengths at 28 d of 44.3 MPa (cement-gypsum), 46.1 MPa (cement-CaCl₂), 46.6 MPa (NaOH-Na₂SiO₃), and 39.3 MPa (NaOH-Na₂CO₃), indicating that all systems except NaOH-Na₂CO₃ can meet the requirement strength of C40 concrete. The peak heat flow of BRM based cementitious materials is 56–89 % lower than that of cement, and the cumulative heat release within 72 h is reduced by more than 40 %. The primary hydration products include the zeolite and C-(A)-S-H gel, and BRM contributes to enhancing the formation of zeolite. The hydration process of BRM based system follows a sequence of ionization, dissociation, and polymerization, highlighting the distinct reaction pathways induced by various composite activators.