Nanoengineering of the mechanical properties of crystalline calcium-silicate-hydrate phases via molecular dynamics simulations

Abstract

The nanoengineering of the mechanical properties of crystalline calcium-silicate-hydrates (i.e., tobermorite 9 Å (T9) and 14 Å (T14)) using single-layer graphene sheets (GS) as nanoreinforcement was studied. The effects of geometrical assembly of GS in tobermorite, surface structure of tobermorite interfacing with the GS (i.e., water, calcium, and silicate surfaces), and –OH functionalization of GS (percent surface coverage and arrangement of –OH functionalization in random versus clustered line patterns) on the strength, stiffness, strain energy density, directional elasticity, and compressibility of tobermorite/graphene nanocomposites were investigated. The normal and shear traction-separation relations for tobermorite/graphene interfaces were determined. The anisotropic structure-topology-property relations of –OH functionalized graphene and the normal and shear traction-separation relations for graphene bilayers with and without nanoconfined water monolayer were also studied. Results showed that the GS functionalized with –OH in clustered line pattern arrangements acted similar to a nanoscale mechanical spring (increased compressibility perpendicular and increased stiffness parallel to the line patterns). The stacked geometrical assembly of GSs in the T9 matrix increased the in-plane tensile and shear strengths, strain energy density, and stiffness more than the hierarchical assembly due to the bundle effect of the GSs and fewer T9/GS interfaces. The GS enhanced the in-plane tensile and shear strengths, stiffness, and strain energy density while degrading the out-of-plane properties of the T14 structures, with more in-plane strengthening when graphene interfaced with water (molecular friction) than with the calcium or silicate surfaces of T14. Functionalization of the GS with –OH groups in a random arrangement increased the out-of-plane tensile strength (Z-direction) of the T14 matrix by transitioning the fracture mechanisms from interface-dominated (without functionalization) to interlayer-dominated while clustered line pattern arrangements modulated the directional elasticity of the nanocomposites. The presence of confined water monolayer in graphene bilayer systems was shown to reduce the interfacial shear adhesion, thereby increasing the lubrication ability during shear traction. Functionalization with random arrangement of the –OH groups (10% surface coverage and higher) increased the shear traction strength of the T14/GS interface compared to without functionalization.