The influence of admixture on chloride time-varying diffusivity and microstructure of concrete by low-field NMR

The permeability of chloride can be used as an important indicator of durability. In this article, based on the ingress test in an artificial simulating dry-wet cycling chloride environment, the influence of four minerals, namely basalt fiber (BF), fly ash (FA), silica fume (SF), and combined FA and SF mixture on the penetration of chloride ions and microstructure in concrete were examined. In addition, the chloride diffusion coefficients of concrete were fitted with Fick's diffusion law, and pore characteristic parameters and pore-size distribution with exposure time were measured by a nuclear magnetic resonance (NMR) technology. The results show that the addition of these mineral admixtures can increase anti-permeability and decrease properties of concrete at the same exposure time. Moreover, the addition of SF has the most obvious decreasing effect on porosity, and the decreased effect of FA on chloride diffusion coefficient of concrete is the most significant, especially at the later stage. Minerals can refine the pore-size distribution in concrete, in which the content of aperture radius 100 ~1000 nm pores decreases after the same exposure time. At last, we can also see that the influence of contributive porosity of aperture radius 100-1000 nm in concrete on chloride diffusion coefficient is more than that of total porosity of concrete, there is a better positive correlation between contributive porosities of radius 100~ 1000 nm in concrete and chloride diffusion coefficients.