Mechanistic understanding of geopolymerization at the initial stage: Ab initio molecular dynamics simulations

Geopolymers with a zeolite-like structure are attractive inorganic binder materials for addressing the critical challenge of reducing the carbon footprint of the cement industry. Despite many efforts, the exact geopolymerization mechanism remains a topic of investigation. In the present work, we determined the dimerization processes between Si/Al-bearing monomers through metadynamics. The free energy profiles were obtained using ab initio constrained molecular dynamics simulations. The results reveal that metastable pentacoordinate states formed after exposure to the alkaline medium, followed by the release of the hydroxyl on the rotational axis. The presence of Al(OH)4− monomers would significantly reduce the reaction barriers of dimerization, implying that Al(OH)4− monomers promote the crosslinking degree of the geopolymer gel. Increasing the pH value of the alkali-activated solution would facilitate the reaction between Si-bearing monomers and Al(OH)4− monomers. However, the reaction between anionic Si-bearing monomers presents a much higher barrier because of intensive electrostatic repulsion. Therefore, increasing the pH value decreases the Si/Al ratio of the geopolymer gel at the initial stage, which is consistent with the experimental results. This study refines the understanding of the oligomerization process of aluminosilicates by ab initio molecular simulations.