Unveiling hydration pathways and kinetics of red mud-derived cementitious materials via tailored alkali activation systems.

Red mud (RM) is a high-alkalinity bulk solid waste with complex composition, posing environmental pressures yet holding resource potential for alkali-activated cements. Bayer RM was used as the primary precursor and blended with ground granulated blast furnace slag (GGBS), clinker (PC) and gypsum to prepare alkali-activated cementitious materials. The Box-Behnken response surface methodology was applied to optimize the proportions of RM, GGBS and PC, with 7d and 28d uniaxial compressive strengths (UCS) as evaluation indicators. The regulation effects of three alkali activators (NaOH, Ca(OH)2, and water glass) on mechanical performance and hydration heat evolution were systematically characterized. The optimized mixture (65% RM, 30% GGBS, 10% PC and 8% gypsum) achieved a 28d UCS of 6.815 MPa. Ca(OH)2 delivered optimal activation; its 7% optimal dosage achieved a 28d UCS of 18.816 MPa. In contrast, NaOH induced an intense and sustained early-stage hydration reaction, whereas water glass resulted in an extremely shortened nucleation and crystal growth stage, featuring a burst-like initial reaction rate followed by rapid decay due to surface passivation. Hydration kinetics analysis based on the Krstulovic-Dabic model reveals that all systems follow a three-stage mechanism involving nucleation and crystal growth, phase boundary reaction, and diffusion control. Ca (OH)2 fosters a balanced hydration process by facilitating nucleation while moderating reaction intensity, which underlies its superior strength development. This work provides insights into mix design optimization, activator selection, and hydration mechanisms for sustainable utilization of RM-based alkali-activated cementitious materials.