Insight into the sulfate resistance mechanism of Portland cement with different Fe/Al ratios of ferrite phases

This study investigates the mechanisms governing sulfate resistance in Portland cements with varying Fe/Al ferrite ratios, with particular emphasis on elucidating the distinct corrosion mechanisms. The results demonstrate a descending hierarchy of sulfate resistance: C 2 FPC > C 6 AF 2 PC > C 4 AFPC. In comparison with the pore structure of hardened cement pastes, the differing components of hydration products in C 6 AF 2 PC and C 4 AFPC samples play a crucial role in determining their sulfate resistance. Notably, the C 6 AF 2 PC sample exhibited two critical superiority factors compared to C 4 AFPC: (1) The solid solubility of Fe 3 + in ettringite (AFt) phase is as high as 39 %, significantly higher than the 21 % in the C 4 AFPC specimen. The increased Fe 3+ incorporation into AFt enhances its crystallographic stability and inhibits transformation to AFm under sulfate exposure; (2) More Fe(OH) 3 gel was generated in the C 6 AF 2 PC specimen during hydration, acting as a diffusion barrier that retards ingress of sulfate ions and limits volumetric instability. These are the main mechanisms for the outstanding sulfate resistance of the C 6 AF 2 PC specimen. These findings provide theoretical support and technical pathways for the development of new Portland cement with high sulfate resistance. • Systematic study on sulfate resistance of Portland cement with varied ferrite phase Fe/Al ratios. • C 2 FPC and C 6 AF 2 PC enhanced sulfate resistance compared to C 4 AFPC. • Composition and stability of AFt in C 6 AF 2 PC dominate the sulfate resistance compared with the pore structure.