Nanoscale Insights into the Formation Reaction Mechanism of Si–Al Oligomers in Alkali-Activated Materials

Alkali-activated materials (AAM) have garnered significant attention as environmentally friendly alternatives to cement due to their potential to mitigate greenhouse gas emissions and facilitate the effective utilization of industrial waste streams. Silicon–aluminum (Si–Al) monomers serve as the cornerstone units within the nanocomposite structure of AAMs, playing a pivotal role in the polycondensation reactions (PR) that govern their formation. Despite extensive research on the PR processes within AAM, the nanoscale reaction mechanisms remain elusive. In this study, MD simulations based on the ReaxFF were employed to delve into the structural evolution and reaction mechanisms of PR processes at the nanoscale. Analyses employing radial distribution functions, bond lengths, and bond angles demonstrated the robustness of the models developed in this investigation. The simulations revealed that when three Si–Al nanomolecular monomers, namely [SiO₂(OH)₂]^2–, [SiO(OH)₃]⁻, and [Al(OH)₄]⁻, undergo pairwise PR, distinct reaction pathways emerge, leading to the formation of various Si–Al oligomers that collectively constitute the framework of the AAM gel. During the assembly of Si–Al oligomers, we aim to shed light on the crucial role played by Al monomers in driving the reaction forward and the subsequent polymerization of Si–Al oligomers. This disparity underscores Al’s pivotal function in the PR mechanism, illuminating its indispensable role in governing the molecular architecture and kinetics of these complexes.