Computer modelling of tritiated water diffusion test for cement based materials

In durability-based design, diffusivity of ions through cement based materials is one of the most important factors that determine the service life of concrete structures. This paper gives a method and numerical tool to estimate and predict in a short time the effective diffusion coefficient of cement paste, mortar or concrete starting from the microstructure scale and taking into account important factors like calcium silicate hydrate (C-S-H) and interfacial transition zone (ITZ) phases. The numerical model works in two main steps. First, the CEMHYD3D model is used to generate a realistic 3D Representative Volume Element of the cement based material at different scales. Second, the finite element method is used to simulate diffusion of tritiated water (HTO) through a saturated material. The method is then applied to simulate the well-known HTO diffusion test and to derive the effective diffusion coefficient of the heterogeneous material based on Fick's law. In order to validate the proposed tool, the effective diffusion coefficient of two cement pastes and two mortars with water-to-cement ratio (w/c) of 0.25 and 0.45 are computed and successfully compared to experimental data. The model results provide an approximation for diffusivity through the C-S-H and ITZ phases. Finally, the model is applied to investigate the effect of changes in the microstructure due to the leaching of portlandite, hydrated aluminates and sulfoaluminate phases on the increase of the effective diffusion coefficient.