New insights into the mechanism governing the elasticity of calcium silicate hydrate gels exposed to high temperature: A molecular dynamics study

When exposed to fire, the integrity of cement-based materials is governed by thermally-induced changes in the mechanical properties of their binding phase, i.e., the calcium-silicate-hydrate (C-S-H) gel. However, the effect of temperature on the structure, density, and mechanical properties of C-S-H remains only partially known. Here, based on reactive molecular dynamics simulations, we reveal the nature of thermally-induced damage in C-S-H gels. In general, we show that, at the atomic scale, exposure to high temperature results in partial dehydration, volumetric shrinkage, disordering, and stiffening in the C-S-H grains. However, we show that the thermal response of C-S-H strongly depends on its chemical composition, wherein C-S-H systems associated with lower Ca/Si molar ratios are able to undergo higher temperatures before amorphization. Based on these results, we demonstrate that the stiffness of C-S-H gels (i.e., including porosity-as probed by nanoindentation) is governed by a competition between the stiffening of the grains and the decrease in packing density-wherein the latter eventually become predominant.

Automated summary

This study using reactive molecular dynamics simulations revealed the nature of thermally-induced damage in C-S-H gels, showing that exposure to high temperature leads to partial dehydration, volumetric shrinkage, disordering, and stiffening in the C-S-H grains. The thermal response of C-S-H strongly depends on its chemical composition, where systems with lower Ca/Si molar ratios can withstand higher temperatures before amorphization occurs.