Formation of Ettringite from Waste Concrete and Its Application to Water Treatment

A huge amount of waste concrete has been generated from demolished concrete buildings and structures. Effective processes for recycling and reuse of the waste concrete should be developed. In this study, we focused on the calcium contents in waste concrete, and we examined the potential use of waste concrete as a raw material for the synthesis of ettringite, a calcium aluminum sulfate, of which the chemical formula is given by Ca₆Al₂(SO₄)₃(OH) ₁₂· 26H₂O. Sulfate ions SO₄^- in the crystalline structure of ettringite can be exchangeable with anions in the aqueous phase, and ettringite is useful for removal of harmful anions in waste water such as boron, fluorine, and arsenic. In this study, we developed and tested two types of ettringite formation methods for removal of boron from water; (1) addition of synthesized ettringite into water containing boron, and (2) in-situ formation of ettringite and boron removal from model waste water by adding raw materials, namely, waste concrete, aluminum and sulfate sources. Aluminum dross was tested as an aluminum source for ettringite formation. Ettringite was successfully synthesized from waste concrete as a calcium source and aluminum dross as an aluminum source. Boron concentration was significantly reduced by adding the synthesized ettringite into boron containing model waste water. Higher boron removal performances were observed by the in-situ formation process of ettringite in the aqueous phase, where boric ions can access more easily to the ettringite structure.