Diverging mechanisms of calcium aluminate in carbonated matrix: Passivation as micro-aggregate reinforcement vs monocarboaluminate-driven densification

The development of carbonatable clinkers represents a promising pathway for reducing the carbon footprint of cementitious materials. However, achieving a homogeneous microstructure through direct carbonation remains challenging. Incorporating hydraulic phases like calcium aluminates is a potential strategy to mitigate this issue, but their behavior and role in carbonatable systems are not well-understood. This study aims to elucidate the dual mechanisms of calcium aluminates in a γ-C₂S-dominated carbonatable clinker, specifically investigating their passivation during carbonation and their reactivation during subsequent rehydration. Results reveal that calcium aluminates (C₁₂A₇ and CA) undergo rapid passivation during carbonation, primarily due to physical encapsulation by carbonation products. These passivated phases function as reinforcing micro-aggregates, enhancing strength via their high modulus and strong interfacial bonding. Sequential carbonation-rehydration curing reactivates the aluminates through carboaluminate reactions at pH ≥ 11, forming monocarboaluminate (Mc) and amorphous Al(OH)₃. This reaction sequence refines the pore structure and significantly improves matrix homogeneity. However, the carboaluminate products possess a lower elastic modulus and hardness than the carbonate matrix, which slightly compromises the overall mechanical properties. The carbonation-rehydration approach yields matrices with competitive compressive strength (110 MPa at 28 days) and superior homogeneity compared to fully carbonated systems, while minimizing the demand for prolonged carbonation. This work clarifies the dual role of calcium aluminates and provides a novel strategy for designing efficient and sustainable carbonatable clinkers.