Modeled versus Experimental Salt Mixture Behavior under Variable Humidity.

This study investigates the kinetics of salt mixture crystallization under relative humidity (RH) conditions, varying between 15 and 95% (at 20 °C), to inform applications in built heritage preservation, geology, and environmental sciences. We focused on commonly found, sulfate-rich and calcium-rich salt mixtures containing five to six ions, Cl-, NO3-, Na+, and K+, including or excluding less common Mg2+, and including either an excess of SO42- or Ca2+, with respect to gypsum. Using time-lapse micrographs and dynamic vapor sorption, we explore how crystallization and dissolution behavior depend on RH and mixture composition under constant temperature. A range of RH change rates were studied to simulate realistic weather events. Microstructural analyses through environmental scanning electron microscopy (ESEM) confirmed the crystal habit corresponding with RH transitions. Phases predicted from thermodynamic modeling (ECOS/RUNSALT) were confirmed using micro-Raman spectroscopy, X-ray diffraction (XRD), and elemental mapping via energy-dispersive X-ray spectroscopy (EDX). We identify a strong correlation between phase transition kinetics and RH change rates, with crystallization deviating by -15% and dissolution by +7% from modeled values under rapid (several seconds) and slow (several days) RH changes. These insights are important for preservation strategies in built heritage, salt deposition, and dissolution mechanisms in diverse geological and realistic environmental contexts, laboratory experiments, future modeling efforts, and the understanding of stone decay in general.

Salt mixtures in stone weathering.

Salt related weathering of stones has been attributed to pressures exerted by repeated cycles of crystallization within pores. Relative Humidity (RH) is a key driver for dissolution and crystallization processes. Despite the prevalence of salt mixtures in natural environments, most experimental work has focused on single salts. Thus, the identification of salt mixture composition and their behavior is necessary to understand weathering. Thermodynamic calculations are used to analyze several thousand realistic salt mixtures found in weathered stone. We identify two common mixture types and their behavior. From at least 85 salt species theoretically present, 14 common salts are identified that occur most frequently and their critical RH points are discussed. These findings have wide-reaching implications for understanding salt weathering processes and informing the design of experimental stone weathering research.

Charge balance calculations for mixed salt systems applied to a large dataset from the built environment.

Understanding salt mixtures in the built environment is crucial to evaluate damage phenomena. This contribution presents charge balance calculations applied to a dataset of 11412 samples taken from 338 sites, building materials showing signs of salt deterioration. Each sample includes ion concentrations of Na+, K+, Mg2+, Ca2+, Cl-, NO3-, and SO42- adjusted to reach charge balance for data evaluation. The calculation procedure follows two distinct pathways: i) an equal adjustment of all ions, ii) adjustments to the cations in sequence related to the solubility of the theoretical solids. The procedure applied to the dataset illustrates the quantification of salt mixture compositions and highlights the extent of adjustments applied in relation to the sample mass to aid interpretation. The data analysis allows the identification of theoretical carbonates that could influence the mixture behavior. Applying the charge balance calculations to the dataset validated common ions found in the built environment and the identification of three typical mixture compositions. Additionally, the data can be used as direct input for thermodynamic modeling.