Synthesis of Ettringite in Suspension from Various Forms of Gypsum as Starting Material

Abstract

This study aimed to synthesize ettringite in suspension from various forms of used gypsum and by-product gypsum. We focus on the solubility of the raw material for ettringite. Heat treatment at various temperatures (400–1,000°C) was applied to gypsum and amorphous Al(OH)₃, and the effect of the solubility of Ca²⁺ and Al³⁺ as constituents of ettringite in NaOH solution at pH 12 on the ettringite formation in suspension was investigated using various heated raw materials. The removal of toxic anionic species (B, As(III), Cr(VI), Se(IV)) in dilute solution was tested for the ettringite synthesized under various conditions. By using soluble NaAlO₂ as an Al³⁺ source, ettringite was synthesized successfully after a shorter agitation time of 1 h from CaSO₄·2H₂O and its calcination product at 400°C. For calcination at 600°C and above, ettringite was formed by extending agitation time to 24 h. When amorphous Al(OH)₃ and its calcination products were used as an Al³⁺ source, longer agitation time is required for ettringite formation in suspension. From the viewpoint of ettringite formation, it implies that the solubility of Al³⁺ and the Al³⁺ sources is predominant compared with that of Ca²⁺ from gypsum. It was found that the ettringite as an anion removal agent was synthesized effectively by controlling the reaction conditions to improve the reactivity of Ca²⁺ in gypsum.