The influence of aluminum from metakaolin on chemical binding of chloride ions in hydrated cement pastes

In this study, chloride ions binding capacity of Portland cement blended with 10-30% of metakaolin was investigated. Chemical binding of chloride ions was found depending more on phase assemblage of pastes rather than merely on available aluminum content in the blends. More monosulfoaluminate formed at low replacement levels, while at high replacement levels of metakaolin, strätlingite formation in the blends was favored. When exposed to 0.53 M NaCl solution at ambient temperature, monosulfoaluminate converted to Friedel's salt completely, and played a principal role in chemical binding of chloride ions while ettringite and strätlingite did not participate in the chemical binding of chloride ions. Considering the exposure condition, the contribution of monocarboaluminate in chemical binding was less significant than that of monosulfoaluminate. Results demonstrated that well formulated Portland cement-metakaolin blends with appreciate metakaolin replacement level and sulfate supply are expected to have good chloride binding capacity.