Insight into the mechanism of sulfate and magnesium ions on chloride diffusion and phase assemblage in limestone calcined clay cement (LC3)

Limestone calcined clay cement (LC³), a low-carbon cementitious material, has demonstrated outstanding resistance to chloride ion penetration. However, the real environment is rich in many harmful ions beyond chloride ions, including sulfate and magnesium ions, making the interactions among these ions highly complex. This study systematically explores the impact of sulfate ions and the synergistic effects of sulfate and magnesium ions on the chloride ion transport properties and chloride ions binding capacity of LC³. The characteristics of phase assemblage, pore structure, and microstructure vary differently under different environments. Furthermore, a thermodynamic model was employed to determine the impact of ion concentration and type on the phase assemblage. The findings suggest that within LC³, sulfate ions significantly affect Friedel's salt stability, converting it into ettringite (Ett). In addition, sulfate ions can combine with monosulfate (Ms) or undergo ion exchange with hemicarboaluminate (Hc)/monocarboaluminate (Mc) to form Ett, refining the pore structure and reducing the diffusion coefficient of chloride ions. However, prolonged exposure leads to sample expansion, accelerating the diffusion rate of chloride ions. Furthermore, the duration of sulfate ion inhibition on chloride diffusion is associated with the content of calcium aluminate phases. The combined attack of sulfate and magnesium ions lead to C-(A)-S-H decalcification, generation of gypsum and brucite, and decomposition of Friedel's salt and Ett, which further degrades matrix and promotes chloride ion diffusion. However, reducing the clinker amount in LC³ effectively enhances chloride binding capacity and reduces gypsum and brucite formation.