Direct Measurements of the Deliquescence Relative Humidity in Salt Mixtures including the Contribution from Metastable Phases

Accelerated salt-induced deterioration occurs by frequent changes across the equilibrium relative humidity (RHeq). Therefore, knowledge of the actual RH_eq of a salt mixture has a major impact on preventive conservation to ensure that the relative humidity (RH) does not cause a salt-phase transition. In addition, knowledge of the RH_eq is essential in relation to in situ desalination as the dissolution of salt is an essential criterion to enable transport of salt (ions) in materials. For decades, it has been possible to determine the RH_eq in salt mixtures with thermodynamic-based ECOS-Runsalt software. However, the ECOS-Runsalt model is challenged by the influence of kinetics along with some limitations in regard to possible ion types and combinations. A dynamic vapor sorption (DVS) instrument is used for the direct measurement of RH_eq and to deduce knowledge on the physicochemical nonequilibrium process related to the phase changes in salt mixtures. The experimentally measured RH_eq values in this study of NaCl-Na₂SO₄-NaNO₃, NaNO₃-Na₂SO₄, NaCl-NaNO₃, NaCl-Na₂SO₄, and (NH₄)₂SO₄-Na₂SO₄ are in agreement with values from the literature. A comparison with thermodynamically calculated results makes it probable that the phase transition for some salts is significantly influenced by nonequilibrium conditions. The present work bridges some of the existing gaps in regard to improving the accuracy of ECOS-Runsalt, including the effects of kinetics and the possible ions and combinations that may be found in situ. The proposed method makes it possible to determine a more representative RH_eq in relation to real conditions for the improved treatment of salt-infected constructs.