On the retardation mechanisms of citric acid in ettringite-based binders

This study aims to obtain insights into the retardation mechanism of citric acid in an ettringite-based binder from ladle slag and gypsum. The hydration kinetics and phase assemblage of the binder were experimentally investigated and thermodynamically modelled. Additionally, the effects of citric acid on synthetic ettringite were studied to obtain further understanding of the interaction between this organic ligand and the crystal. Experimental results reveal that citric acid works as an inhibitor of ettringite's formation leading to the precipitation of monosulfate and gypsum; the ettringite surface blockage by citrate ligand effectively prevents precipitation of this crystal. This leads to an overestimation in the precipitation of ettringite in the thermodynamic model due to this kinetic barrier imposed by the ligand. Thermodynamic modelling suggests ettringite, monosulfate, aluminum hydroxide, and strätlingite as main hydrates in this binder, whereas an intermixed C–(A–)S–H gel was observed experimentally instead of strätlingite.