Chloride binding capacity and phase modification of alumina compound blended cement paste under chloride attack

In order to prolong the service life of reinforced concrete structures either serving in chloride-rich environment or prepared with chloride contaminated ingredients, the chloride binding behaviors of cementitious materials with alumina compound addition are investigated in this work. The chloride binding capacity, phase conversion and solution chemistry of mixtures during chloride exposure were discussed. The applied alumina compounds are γ-Al2O3, γ-Al(OH)3 and colloidal nano-alumina, and metakaolin is used as a reference additive. The results show that alumina compounds exhibit obviously higher potential of forming AFm-type phases than metakaolin and therefore significantly improve the chloride binding capacity. During chloride exposure, the charge-balancing anions like hydroxyls, carbonates and sulfates can be replaced by chlorides to form Friedel'salt and increases the pH of exposure solution. The released sulfates reacting with SO4-AFm lead to delayed ettringite formation, binding large amount of mixing water and reducing calcium hydroxide content. Besides, the transformation of carboaluminate to Friedel'salt also accounts for partial reduction of calcium hydroxide in hydrated blended after chloride exposure, due to the released carbonates precipitating with calcium ions in the system.