Steel slag used as binding materials often shows volumetric instability because of the existence of f-CaO (free calcium oxide). According to the composition analysis of steel slag, the tri-component f-CaO (CaO center dot aFeO(1.5)center dot bMnO(2)) was synthesized. The single phase CaO and some solid solution phases were detected in the tri-component f-CaO by XRD. The single phase CaO was found to be completely hydrated at room temperature after 18 h. The solid solution phase could not be hydrated at room temperature, while it could be hydrated partly via a boiling treatment. The hydration activity of CaO in the solid solution phase was lower than that of the single phase CaO. The f-CaO was mixed into Portland cement to study the expansibility by autoclave test. The single phase CaO had no contribution to the expansion when the content was not more than 1%. The solid solution phase in f-CaO was the main cause of the expansion. The relationship between the autoclave expansion rate and the f-CaO content conformed to the exponential equation. For the tri-component f-CaO, the smaller the molar ratio of Ca to Mn and Fe was, the more solid solution phase was present and the greater was the expansibility.
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