Dissolution and early hydration of tetracalcium aluminoferrite (C4AF) in water and in aqueous sulfate solutions

Fe-rich cementitious phases, such as tetracalcium aluminate (C₄AF), are likely the least characterized and understood components of ordinary cements, in terms of their reactivity and early-age reactions. In this study, we provided the first in-situ measurements of C₄AF's absolute dissolution rates in water and aqueous sulfate solutions (Na₂SO₄, MgSO₄ and CaSO₄) along with its early-age hydration characterization. Upon contact with water, C₄AF dissolved at a rate of 1.74 ± 0.23 μmol·m⁻²·s⁻¹, and sulfates were found to inhibit its reaction. Among the three types of sulfates tested, CaSO₄ exhibited the strongest inhibition effect, while Na₂SO₄ showed the weakest inhibition. Soon after the initial dissolution, the precipitation of hydrates occurred, and the presence of sulfates affected the overall hydration process of C₄AF. These results enhance our understanding of C₄AF's dissolution and early-age hydration behaviours, thereby advancing our knowledge of its hydration mechanisms.