Constitutive response of calcium-silicate-hydrate layers under combined loading

Calcium-silicate-hydrate (C-S-H) is the main hydration product for ordinary Portland cement (OPC) materials that exhibits a layered structure containing interfaces that controls the system response to shear deformation at the nanometer scale. In this work, we used molecular statics simulations to study the mechanical behavior of an atomistic model of C-S-H under combined loading conditions that are typical of structural applications of these materials. Combined loading is implemented by first compressing or stretching the atomistic structure to impose an external hydrostatic pressure, and then loading the system through both heterogeneous and homogeneous shear deformation. By utilizing two different shear methodologies, we were able to isolate the interface behavior from the bulk response. Our results show several qualitative similarities with that of macroscale cementitious materials including pressure sensitivity of the maximum shear strength and strength asymmetry in compression and tension. This indicates that the well-known cohesive-frictional behavior of cementitious materials is fundamental to interfaces between C-S-H grains at the nanoscale. Comparing differences in our results with nanoindentation experiments motivate future investigations of the effect of C-S-H particle size and morphology on strength scaling properties at the mesoscale. These mesoscale model interactions should include the normal-stress or pressure dependency that we observe.