Calcium aluminate cement hydration under the influence of mineral acids

In technical mortars and refractory concrete, the reactivity of calcium aluminate cement (CAC) is typically controlled by retarders and accelerators. This study examines the retarding impact of five strong acids (HCl, HNO3, HClO4, H2SO4, and H3PO4) on CAC hydration (1) in dilute suspensions at a water-to-solid (w/s) ratio of 100, and (2) in pastes at a w/s of 0.3. Doing so, we identified several factors that affect the dissolution and hydration of CAC. In dilute suspensions, in-situ monitoring of the pH and conductivity reveals that the acids delay the initial dissolution of monocalcium aluminate (CA). The inhibition time depends exponentially on the proton concentration, and linearly on the available CA surface area, which is partially explained by the preferential formation of Al(OH)3 suggested by thermodynamic modeling. Notably, the acids’ efficiency varies at equimolar proton dosages. For H3PO4, we explain the outstanding inhibition efficiency by the joint formation of hydroxylapatite and Al(OH)3. For the other acids, the dissolution inhibition power follows an anion-specific sequence sulfate > chloride > nitrate > perchlorate, which aligns with the Hofmeister series and suggests a stabilization of calcium ion solvation by the anion. Isothermal calorimetry experiments performed on pastes show that the retardation order of these acids (H3PO4 > HClO4 > HNO3 > H2SO4 > HCl) differs from that observed at w/s of 100. Phosphoric acid still displays exceptional retardation properties suggesting a unique mechanism of action most likely related to early hydroxylapatite and Al(OH)3 formation. For the other acids, Al(OH)3 and anion-containing AFm phases formation may explain the differences in retardation maximum.