Multi-scale quantitative investigation of the early structural transformation mechanism in alkali-activated materials: insights from molecular dynamics and first-principles calculations

Abstract

At the heart of the AAM gel structure lie Si–Al monomers, whose polycondensation reactions (PRs) play a pivotal role in gel formation. Despite extensive research on AAM's PR processes, nuances of its nanoscale structural evolution remain elusive. This study delves into the PR processes and their structural evolution mechanisms through a combined approach of molecular dynamics simulations and first-principles calculations. The simulation findings elucidate that upon engaging in paired polymerization reactions (PRs), the trio of Si–Al monomers, comprising [SiO₂(OH)₂]²⁻, [SiO(OH)₃]⁻, and [Al(OH)₄]⁻, yield a total of nine unique Si–Al oligomers, showcasing a diverse array of structural outcomes. Notably, Al monomers exhibit significantly faster reaction rates compared to Si monomers. Moreover, the bond angles and bond length distributions within these Si–Al oligomers differ markedly from those of the individual Si–Al monomers. The diffusion rate of Na ions is substantially higher than that of other atoms, suggesting its primary role in charge balancing within the AAM gel. Furthermore, first-principles calculations elucidate that during the alkali activation process, the initial relative positions of Si–Al monomers directly influence the PR pathways, resulting in variations in the molecular composition and structure of the Si–Al oligomers formed.