Nanoindentation Study of Calcium-Silicate-Hydrate Gel via Molecular Dynamics Simulations

Abstract

The mechanical properties of calcium-silicate-hydrate (C-S-H) gels in cementitious materials are mainly realized by nanoindentation experiments. There is limited research on the dynamic response of the molecular structure of C-S-H under nanoindentation conditions. This study simulated the nanoindentation on the C-S-H gel samples by the molecular dynamics method considering the essential factors of modeling and loading process. The results demonstrate that the averaged elastic moduli we obtained had slight differences from those by experiments. In contrast to the experimental results, the gels showed bi-modulus and transverse isotropic with the material principal direction perpendicular to the C-S-H layers. The modulus in a direction increased with the loading speed, which indicates that C-S-H behaves viscous due to the water motion in the sample and the propagation of stress wave. The saturation of water influenced the moduli differently because more water in C-S-H will reduce the polymerization of silicon chains and then weaken the local stiffness. The conclusions provide a deeper understanding of the mechanism on the unique mechanical response of C-S-H gels.

Nanoscale Insights into the Mechanical Behavior of Interfacial Composite Structures between Calcium Silicate Hydrate/Calcium Hydroxide and Silica

“…Moreover, the interior of the CSH bulk displays a distinct layer-like structure with periodic density fluctuations. This layered structural characteristic of the CSH molecular model has also been observed in numerous previous studies, corroborating our experimental findings [31,33]. Notably, these studies have identified the regions where water molecules tend to aggregate during the stretching of CSH alone and its involvement in composite structures as potential fracture sites [18,38].…”

“…The Q n distributions of the silicate chains in the CSH molecule used in this paper were Q 0 = 9.4%, Q 1 = 68.2% and Q 2 = 22.4%, which are close to the distributions of the experimental samples (Q 0 = 10%, Q 1 = 67%, Q 2 = 23%) [32]. Furthermore, the influence of the oxhydryl (-OH) group on the mechanical properties of CSH was not taken into account here due to its limited effect [33]. The saturated C-S-H structure exhibits the chemical formula of (CaO) 1.69 (SiO 2 )•1.81H 2 O, which closely resembles the composition (CaO) 1.7 (SiO 2 )•1.8H 2 O obtained through the small-angle neutron scattering (SANS) test [34].…”