The Transmission Properties Of Graphene Oxide Modified Concrete Cementitious Materials: A Molecular Dynamics Study

Calcium silicate hydrate is the main product of cement hydration.The transport of water and ions in the gel pores of calcium silicate hydrate(C-S-H)determines the durability of cement materials.Molecular dynamics simulation is an effective mean to study the microcosm.It is used to fit the Lucas-Washburn equation and to calculate the permeability curve of water and ions in the C-S-H nanopore.The transport and adsorption of water and ions in graphene and graphene oxide(GO)sheets were further investigated by fitting the Lucas-Washburn equation.The adsorption and dynamic properties of graphene oxide interfacial ions were studied by density distribution,orientation distribution,coordination number environment,diffusion coefficient and time correlation function.Finally,Study on the ion resistance of nanocomposites with calcium silicate silicate(C-S-H)matrix embedded with different single-layer graphene sheets and graphene oxide sheets.The transport rate of water and ion,the intrusion depth curve and the diffusion coefficient were calculated.The impermeability of graphene makes it a potential film against the intrusion of harmful ions.The main conclusions of the research are followed:(1)Molecular dynamics(MD)simulation was used to study the transport of chloride salt solutions in C-S-H nanochannels.We find that the simulation results do not conform to the classical Lucas-Washburn equation,but the ion permeability curve in solution can be well described by combining the effects of slip length,dynamic contact angle and effective viscosity into the modified theoretical equation in the L-W equation.The validity of the theoretical equations was confirmed by other models with different ion concentrations.In addition,the local structure of the ions is further analyzed to clarify the effect of ion concentration on the capillary adsorption process.(2)The effects of graphene and graphene oxide(GO)interface on the adsorption and dynamic properties of calcium sulfate solution were studied by MD simulation.Four typical surfaces were chosen to study the effects of GO surface structure: deprotonated carboxyl groups(-COO-),carboxyl groups(-COOH),hydroxyl groups(-OH),and unfunctionalized graphene(GNS).We found that the presence of oxygen-containing functional groups restrained the layered packing of water molecules,reduced the water density and changed the dipolar angle distribution in the vicinity of graphene sheet.Due to the hydrophilicity of the surface,the OH bonds of the water molecules are preferably oriented towards the graphene oxide sheets.As the polarity of the functional groups increases,more surface water molecules provide their hydrogen bonds to the-COO-,-COOH and-OH oxygen sites.In addition,to a certian extent,the adsorption properties of calcium ions and sulfate ions depend on the polarity of the GO surface.The adsorption rate of ions is arranged in the following order: GO-COO->GO-COOH > G-OH > G.The GO sheet fixes calcium ions by forming a Ca-O ion bond with a functional oxygen atom.As the polarity of the functional group increases,the intensity of the Ca-O interaction also increases,and its behavior can be showed by a time correlation function.The adsorbed surface calcium ions can further fix the sulfate ions by forming a Ca-SO42-ion pair.Comparing to the water and ions in the vicinity of the graphene sheets,the diffusivity of the solution on the surface of the GO-COO-was reduced by 73%.The decreasing in fluidity is attributed to the longer residence time of ions on the surface of the GO and the longer hydration time of the ions.(3)Molecular dynamics(MD)was used to study that the water and ion migration in the nanochannels of a calcium silicate hydrate(C-S-H)substrate embedded with a single layer of graphene sheets.On the one hand,the rate of transport and diffusivity of the fluid is highly dependent on the type of functional groups in the coated sheet.The van der Waals interaction between the graphene sheet and the C-S-H gel is significantly attenuated by the intrusion of ions and water molecules,resulting in dissociation of the graphene sheets from the C-S-H surface.The separated graphene sheets hardly contribute to repelling water and ions.On the other hand,the hydroxyl and carboxyl groups in the GO sheet provide sufficient oxygen sites to accept the H bond and bind to adjacent sodium ions,which immobilize water molecules and ions on the GO surface.The GO-COOH plate,deeply rooted in C-S-H,further blocks the connectivity of the transport channels and restrains the water and ions in the inlet region of the gel pores.Hopefully,this paper will provide valuable insights on the design of graphene oxide films into improve the durability of sustainable cementitious composites.