pH-dependent carbonation behavior of tricalcium aluminate

Abstract

This study systematically investigates the enforced carbonation behavior of tricalcium aluminate (C₃A) across a precisely controlled pH range of 5.8–12.5. The results indicate that C₃A carbonation is thermodynamically spontaneous; its overall rate, reaction pathway, and phase assemblage are significantly influenced by solution pH. The accumulation rate of calcium carbonate (Cc) increases sharply below pH 11.0 and peaks at pH 9.5–10.0, where only 4.1 wt% of the initial C₃A remains after 10 min of carbonation. Phase analysis reveals a distinct pH-dependent transition: CO₃²⁻-AFm dominates when pH > 11.0, whereas Cc is the primary product when pH < 11.0. Mechanistically, pH governs C₃A carbonation via three coupled effects: (i) by modulating Al dissolution, it alters the aqueous Ca/Al ratio, thereby adjusting the relative supersaturation of Cc and CO₃²⁻--AFm; (ii) it determines the precipitation threshold of Al(OH)₃, enabling dissolved Al(OH)₄− to react with nascent Cc and form CO₃²⁻--AFm; and (iii) at pH < 6, an Al-rich amorphous film rapidly forms on the surface, effectively halting further carbonation. These findings enhance our understanding of aluminate carbonation mechanisms in cementitious systems and provide insights into tailoring pH to optimize CO₂ uptake in cement.