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.