Speaker
Description
In technical applications, the hydration kinetics of calcium aluminate cement (CAC) are commonly modified by chemical admixtures. In this phenomenological study, we investigate the influence of five strong acids - HCl, HNO₃, HClO₄, H₂SO₄, and H₃PO₄ - on CAC hydration under two conditions: dilute suspensions (w/s = 100) and cement pastes (w/s = 0.3). In dilute systems, in-situ pH and conductivity measurements show that all acids inhibit the initial dissolution of monocalcium aluminate (CA). The inhibition time increases exponentially with proton concentration. Thermodynamic modeling suggests that early precipitation of Al(OH)₃ plays a key role. Notably, the acids differ in their inhibiting efficiency at equivalent proton dosages. Phosphoric acid stands out, which is attributed to the simultaneous formation of hydroxylapatite and Al(OH)₃. For the other acids, the inhibition follows the order SO₄²⁻ > Cl⁻ > NO₃⁻ > ClO₄⁻. This sequence is in line with the Hofmeister series, indicating that anion-specific effects influence calcium solvation. In cement pastes, isothermal calorimetry reveals that all acids delay the CAC main reaction, though the retardation order differs significantly from that in dilute suspensions. The different hydration kinetics with H₃PO₄ suggests a unique retardation mechanism, likely due to early formation of hydroxylapatite and Al(OH)₃. For the other acids, thermodynamic calculations suggest that Al(OH)₃ and anion-specific AFm phase formation during the dormant period may explain the observed differences in retardation behavior. In summary, this work provides new insights into acid-specific mechanisms during initial CAC dissolution and subsequent phase formation by combining experimental data with thermodynamic modeling.
Affiliations
Chair for the Chemistry of Construction Materials, TUM School of Natural Sciences, Technical University of Munich, Lichtenbergstraße 4, Garching, Germany, 85748, Contact: torben.gaedt@tum.de
| Keywords | Calcium Aluminate Cement, Hydration, Mineral Acids, Thermodynamic Modeling |
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