Elucidating the behaviours and mechanisms of enforced carbonation in ferrite

Abstract

In this study, the carbonation behaviour and mechanisms of the ferrite (C4AF) phase in cement were investigated for the first time through enforced wet carbonation. The results confirm that ferrite carbonation is a spontaneous chemical reaction, the kinetics of which are predominantly controlled by monocarbonate precipitation. The carbonation process involved the initial precipitation of hemi/monocarbonate, followed by further carbonation to form Cc, amorphous Al(OH)3, and Fe(OH)3 as the final carbonation products. A unique products structure was identified, consisting of a Cc inner core enveloped by an iron-containing amorphous Al(OH)3 outer layer. The efficiency of carbonation was significantly enhanced by high initial pH, resulting in a >3.3-fold increase in the amount of calcium carbonate (Cc) compared to that observed in a neutral environment. Elevated initial pH levels enhance the dissolution of ferrite and CO₂, thereby increasing the concentration of Al(OH)₄− and prolonging the period of monocarbonate oversaturation, which substantially enhances the final carbonation degree. Additionally, higher initial pH suppresses the consumption of Al(OH)₄− by HCO₃−, favouring the precipitation of amorphous Al(OH)₃.