Elucidating the behaviours and mechanisms of enforced carbonation in ferrite

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.