Unraveling the reaction mechanism of alkali-activated materials through multi-scale modeling

As an environmentally friendly alternative to cement-based materials, alkali-activated materials (AAM) have attracted much attention due to their significant ability to reduce greenhouse gas emissions and effectively utilize industrial waste. Three Si-Al monomers ([SiO₂(OH)₂]^2-, [SiO(OH)₃]^-, and [Al(OH)₄]^-) serve as the fundamental building blocks for the formation of AAM gels. Simulating their polycondensation reactions (PR) among these monomers is of paramount importance but faces a delicate balance between precision and efficiency. To unravel the PR mechanisms, we have integrated reactive force field molecular dynamics (MD) simulations with first-principles calculations based on density functional theory. Our 100ps MD simulations reveal that Al monomers exhibit faster reaction rates and lead to the formation of Si-Al oligomers with diverse structures. Subsequent first-principles calculations on transition state models derived from MD results uncover that PR pathways involving Al monomers possess lower energy barriers, which are correlated with changes in chemical bond strength arising from electronic orbital transitions. Our study, for the first time, establishes an intrinsic link among five hierarchical levels: electronic orbital structure, chemical bond strength, reaction energy barrier, reaction rate, and reaction scale. This provides invaluable theoretical guidance for designing energy-efficient and low-barrier reaction pathways for AAM gel formation.