Modelling of diffusion behavior of ions in low-density and high-density calcium silicate hydrate

To investigate the relationship between morphology and specific ion transport properties in low-density (LD) and high-density (HD) calcium-silicate-hydrate (C-S-H), a numerical method of directly achieving diffusivities is developed. Firstly, a multiscale structural model is proposed to reconstruct C-S-Hs in which self-similarity and quantitative physical properties are compared to experimental results. Subsequently, electrical double layer (EDL) theory is introduced into analysis of ion transport in pore solution. The quantitative analysis of various influence factors of EDL on ion diffusivities in pore solution is carried out and average diffusivity in pore solution is also obtained. Moreover, a numerical method for solving Fick's first law equation is performed on the simulated C-S-H structures to obtain numerous diffusivities of common ions in C-S-H. Finally, the significant parameter for durability in reinforcement concrete, chloride diffusivity in hardened cement paste, is predicted using simulated diffusivities in C-S-Hs, based on a multiscale method. It is found that compared to the atomic force microscope image and surface area of C-S-H, this self-similar model can well reconstruct real C-S-H structures. In addition, the influence factors of EDL including overlapping diffuse layer, relative potential, cation/anion, valence, zeta potential and bulk solution have great effects on ion diffusivities, which demonstrates EDL effects cannot be ignored when investigating ion transport behavior in C-S-H. Furthermore, using an appropriate case of pore solution with diffusivities of 8.2 × 10−12 m2/s in LD C-S-H and 2.0 × 10−13 m2/s in HD C-S-H, estimated chloride diffusivities in cement pastes agree well with measured values, which can indirectly indicate this modelling work is reasonable.