Intrinsic chloride ion transport behavior in porous space of pure C-S-H

Knowing the chloride ingress rate is crucial in predicting the service life of reinforced concrete in chlorine environment. While empirical diffusion model is available, fundamental knowledge of Cl− diffusion in calcium silicate hydrate (C-S-H) is largely lacking as it is experimentally challenging to prepare a bulk pure C-S-H. In this study, synthesized C-S-H powder was compacted to form pellets of controlled pore structure and Ca/Si. Effective diffusion coefficients of Cl− in these C-S-H matrixes were measured using steady-state natural diffusion and electric-migration tests, as a function of Ca/Si and porosity. Increased pore tortuosity significantly reduced diffusivity even when porosity remains nearly unchanged. The influence of pore-solution pH to Cl− diffusion is distinct between natural-diffusion and electric-migration. Our data provides the first direct measure of Cl− diffusion coefficient in pure C-S-H matrix, and new insights into the surface effect for diffusion in extremely small pores.