Enhanced Sulfate Resistance: The Importance of Iron in Aluminate Hydrates

Sulfate erosion is one of the main durability issue of ordinary Portland cement (OPC) while used in a sulfate-rich environment. Two aluminate phases in OPC, tricalcium aluminate (C ₃ A) and tetracalcium aluminoferrite (C ₄ AF), are primarily responsible for sulfate attack but with different sulfate resistant performance. This paper therefore focuses on the internal sulfate invasion to hydration products of these two aluminate minerals and attempts to explain the role of irons in hydrates. Chemical shrinkage coupled with X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectra (EDS) are employed to determine the overall development of internal sulfate erosion of these hydrates. The results suggested that the doped iron in hydrogarnet (C ₃ (A,F)H ₆ ) dramatically changes crystal growth and leads to large size of polyhedral particle to tiny cubic grain of C ₃ AH ₆ , which contributes superior performance to sulfate erosion. As to another hydrate monosulfate, the introduction of Fe in C ₄ (A,F)SH ₁₂ causes a slightly worse sulfate resistance to C ₄ ASH ₁₂ . The substitution of iron to aluminum in hydrogarnet is the radical reason for better sulfate resistance of C ₄ AF to C ₃ A. This study greatly enhances the understanding of sulfate resistance to OPC.