  Cement is the most widely used manufacturing material in the world and
improving its toughness would allow for the design of slender infrastructure,
requiring less material. To this end, we investigate by means of molecular
dynamics simulations the fracture of calcium-silicate-hydrate (C-S-H), the
binding phase of cement, responsible for its mechanical properties. For the
first time, we report values of the fracture toughness, critical energy release
rate, and surface energy of C-S-H grains. This allows us to discuss the
brittleness of the material at the atomic scale. We show that, at this scale,
C-S-H breaks in a ductile way, which prevents from using methods based on
linear elastic fracture mechanics. Knowledge of the fracture properties of
C-S-H at the nanoscale opens the way for an upscaling approach to the design of
tougher cement.
