The nanogranular behavior of C-S-H at elevated temperatures (up to 700 °C)

Cement-based materials are non-combustible, but the complex chemo-physical mechanisms that drive at elevated temperatures the thermal degradation of the mechanical properties (stiffness, strength) are still an enigma that have deceived many decoding attempts. This paper presents, for the first time, results from a new experimental technique that allows one to rationally assess the evolution of the nano-mechanical behavior of cement paste at elevated temperatures. Specifically, the thermal degradation of the two distinct calcium-silicate hydrate (C-S-H) phases, Low Density (LD) C-S-H and High Density (HD) C-S-H, is assessed based on a statistical analysis of massive nanoindentation tests. From a combination of nanoindentation, thermogravimetry and micromechanical modeling, we identify a new mechanism, the thermally induced change of the packing density of the two C-S-H phases, as the dominant mechanism that drives the thermal degradation of cementitious materials. We argue that this loosening of the packing density results from the shrinkage of C-S-H nanoparticles that occurs at high temperatures, most probably due to the loss of chemically bound water.