Effects of seawater on the formation and mechanical properties of Friedel's salt associated with tricalcium aluminate

This work examines the micromechanical properties of AFm phases formed in seawater and deionized water. As the interlayer SO₄ ²⁻ in the AFm phase is gradually replaced by Cl⁻ in seawater, the basal spacing of the AFm crystal narrows, which promotes the packing density of nanocrystals and their indentation modulus and hardness, i.e., Cl-AFm (Friedel's salt) > SO₄-Cl-AFm (Kuzel's salt) > SO₄-AFm (monosulphate). It is found that Friedel's salt formed in seawater feature multiple structural and compositional defects. First, the anions (Cl⁻, OH⁻, SO₄ ²⁻) are bound in the interlayer of Friedel's salt, and the bound OH⁻ contents have negative relations with the Cl⁻ concentration in the seawater. Second, the incorporation of Mg ions in the Friedel's salt barely changes its micromechanical properties based on the experimental data and molecular dynamics simulations. Third, when portlandite is present in the seawater, some Ca vacancies would be formed in the Friedel's salt, which would decrease its Young's modulus significantly. This accounts for the decrease in the indentation modulus of Friedel's salt as observed in the experiments. These findings enable us to better understand and control the micromechanical properties of Cl-containing AFm phase formed in seawater-mixed cementitious materials.