Binding of external chlorides by cement pastes.

摘要

Chloride-induced reinforcement corrosion is the dominant cause of premature deterioration of reinforced concrete structures worldwide. The need to quantify the durability of new and existing structures led to the development of service life prediction models. This requires a clear understanding of the mechanisms of chloride penetration into concrete cover. This research looks into chloride binding when chlorides are introduced after the hydration of cement.; Highlights of this research include the X-ray diffraction (XRD) results which provide insight into some mechanisms of chloride binding. They show that several calcium aluminate hydrates (C-A-H), including monosulphate, convert to Friedel's salt. The monosulphate converts Kuzel's salt at low concentrations before transforming into Friedel's salt at higher concentrations. Evidence suggests that ettringite starts converting to Friedel's salt at high chloride concentration. Friedel's salt peaks kept increasing at chloride exposures above 1.0 M, indicating that chemical binding is not exhausted at low chloride concentrations.; The results indicate the chloride binding capacity of cement paste results from the contribution of hydrates of different cement phases. The C₃ A has a strong effect on chloride binding, especially at high chloride concentrations (1.0– 3.0 M). Evidence from synthetic compound pastes and cement pastes indicate that C₄AF, C₃S, and C ₂S bind chlorides and significantly contribute to the binding capacity. The binding isotherms of cement pastes can be predicted from the chemical composition of cements.; The results show that chloride binding isotherms are non-linear, and the Freundlich isotherm is the best fit in the chloride concentration range 0.1 M–3.0 M. In general, the partial replacement of cement with fly ash, ground granulated blast furnace slag (GGBFS), or metakaolin, increased chloride binding. Partial replacement with silica fume reduced chloride binding. An increase in the pH (13–14) of the storage solution reduced chloride binding. The presence of sulphate ions in the storage solution at 0.1 M concentration reduced chloride binding. Pre-carbonation of cement pastes greatly reduced chloride binding. An increase in temperature between 7°C and 38°C reduced chloride binding at 0.1 M chloride exposure, but increased it at 3 M exposure.