Water chemical potential: A key parameter to determine the thermodynamic stability of some hydrated cement phases in concrete?

The CaO–Al2O3–SO3–H2O system at 25 °C under 1 bar of pressure has been investigated with phase diagrams software (Zen + k) based on chemical potentials (or activities). The reported invariant points are similar to those obtained previously using equilibrium calculations. However Zen + k enables us to calculate systems at different relative humidities, and in conditions where water is not in excess, calcium monosulfoaluminate could be a stable phase and thus, as observed experimentally, remain for long times in an ordinary Portland cement paste.