Molecular Dynamics Insights into the Synergistic Polymerization of C–(A)–S–H Networks Mediated by Al–O Tetrahedra and Graphene Oxide

This study investigates the polymerization mechanisms of calcium–(aluminum)–silicate–hydrate (C–(A)–S–H) gel using molecular dynamics (MD) simulations, with a focus on the roles of Al–O tetrahedra and graphene oxide (GO) nanosheets. Four representative gel models─C–S–H, C–A–S–H, GO/C–S–H, and GO/C–A–S–H─were constructed and simulated using the ReaxFF reactive force field. To elucidate atomic-level interactions, additional simulations of isolated Qn structural units were conducted to examine the evolution of atomic stress during polymerization. Results show that both Al atoms and GO nanosheets significantly increase polymerization, though via distinct mechanisms. Al incorporation enhances the formation of Si–O–Al bonds and improves cross-linking, increasing the proportion of high-order Q3 and Q4 units by 177.4% compared to pure C–S–H. GO accelerates early stage polymerization and stabilizes high-Qn clusters by forming interfacial GO–Ca2+–Si/Al layers, which also contribute to improved Ca2+ distribution and mobility. When both Al and GO are present, a synergistic effect emerges, yielding a more ordered and interconnected gel network. Atomic stress analysis reveals that Al primarily influences stress within Si atoms in both low- and high-Qn units, while GO significantly stabilizes stress distributions in high-Qn units for both Si and Al. These combined effects enhance the polymerization efficiency and structural regularity of the gel, offering insights into the design of advanced cementitious materials with improved mechanical and durability performance.