Reactive molecular dynamics and experimental study of graphene-cement composites: Structure, dynamics and reinforcement mechanisms

The remarkable properties of graphene and graphene oxide (GO) make it as an ideal candidate for high performance cement-based composites. This paper firstly investigated the effects of graphene and GO on the hydration, microstructures and mechanical properties of cement paste. The incorporation of 0.16 wt % GO into the cement matrix can enhance the flexural strength of the material by 11.62% due to the higher hydration degree, nano-filler effect and cracking-bridging effect. On the other hand, graphene reduces the hydration development and mechanical behavior of cement paste due to its poor dispersibility in alkaline environments. Furthermore, the different interaction mechanisms between graphene/GO and cement hydrates have been deeply studied by reactive force field molecular dynamics (MD), revealing that the functional hydroxyl groups in GO provide non-bridging oxygen (NBO) sites that accept hydrogen-bonds of interlayer water molecules in the calcium silicate hydrate (CSH). In the presence of interface counter-ions, protons transfer from the OH in GO to the NBO sites in CSH, which further contributes to the polarity of the GO surface and enhances the bonding with neighboring species. Besides the H-bond connections, the Ca2+ and Al3+ ions near the surface of CSH play a mediating role in bridging oxygen atoms in silicate chains and hydroxyl groups in GO, which increases the silicate chain length and heals the defective GO structure. Dynamically, the aluminate-silicate chains, calcium ions and functional hydroxyl groups establish the “cages”, and strongly prevent the freely diffusion of the interface water molecules, stabilizing the connections between CSH and GO structures. Finally, the uniaxial tensile simulation indicated that while the high cohesive force and enhance plasticity in the GO modified cement composite is mainly contributed by the strong structural H-bonds and calcium aluminate skeleton, the weakest mechanical behavior of the graphene/CSH composite is attributed to poor bonding and the instability of atoms in the interface region. Reactive force field couples the chemical and mechanical responses that the water dissociation, protons exchange between CSH and GO structures and silicate-aluminate-carbon network de-polymerization occurred to resist the tensile loading.