Chemical model for cement-based materials: Temperature dependence of thermodynamic functions for nanocrystalline and crystalline C-S-H phases

In the context of waste confinement and, more specifically, waste from the nuclear industry, concrete is used both as a confinement and as a building material Its exposure to high temperatures makes its geochemical behavior difficult to predict over large periods of time The present work aims to elucidate the solubility constants, as a function of temperature, for the phases of the system CaO-SiO2-H2O For the nanocrystalline phases, the present work investigates the existence of compounds of fixed composition, possibly in a metastable state The question of whether the nanocrystalline C-S-H phases correspond to a series of phases of discrete composition or a solid solution is discussed and compared to the possible influence of impurities in the solid phases. For the crystalline phases, having established that the currently available values of thermodynamic properties were not consistent, we show that their refinement leads to a better agreement with the literature data From the refined thermodynamic properties of crystalline C-S-H, a polyhedral decomposition model is developed It enables to estimate the enthalpy of formation and the heat capacity of nanocrystalline C-S-H phases. Finally, verification shows that such phases remain unstable compared to the crystalline phases, at room or higher temperatures A comparison, based on reaction enthalpies derived from experimental data indicates that predicted values for nanocrystalline C-S-H are in close agreement with experimental data By estimating the properties of okenite and truscottite with the model developed in this study, we have been able to complete the CaO-SiO2-H2O phase diagram with a reasonable agreement with the literature The case of jaffeite remains open to discussion Finally, for the hydrate C2SH,alpha, the model predicts a transition with hillebrandite at 159 degrees C. in contradiction with the hypothesis of C2SH,alpha metastability (C) 2009 Elsevier Ltd All rights reserved