NaCl-Related Weathering of Stone: The Importance of Kinetics and Salt Mixtures in Environmental Risk Assessment

Abstract

Salt weathering is one of the most important causes of deterioration in the built environment. Two crucial aspects need further investigation to understand the processes and find suitable measures: the impact of different climatic environments and the properties of salt mixture crystallization. We demonstrate the importance of kinetics in quantifying crystallization and dissolution cycles by combining droplet and capillary laboratory experiments with climate data analysis. The results proved that dissolution times for pure NaCl were much slower than crystallization, while thermodynamic modelling showed a lower RHeq of NaCl (65.5%) in a salt mixture (commonly found in the built heritage) compared to its RHeq as a single salt (75.5%). Following the results, a minimum time of 0.5 hour is considered for dissolution and the two main RHeq thresholds could be applied to climate data analysis. The predicted number of dissolution/crystallization cycles was significantly dependent on the measurement frequency (or equivalent averaging period) of the climatic data. An analysis of corresponding rural and urban climate demonstrated the impact of spatial phenomena (such as the urban heat island) on the predicted frequency cycles. The findings are fundamental to improve appropriate timescale windows and to illustrate a methodology with specific points of interest to quantify salt crystallization cycles in realistic environments as a risk assessment procedure that can be applied to climate data. The results are the basis for future work to improve the accuracy of salt risk assessment by including the kinetics of salt mixtures. This will improve the understanding of past and future salt weathering mechanisms and enable scientifically informed conservation strategies.

Modeled versus Experimental Salt Mixture Behavior under Variable Humidity

“…Additionally, the dissolution of niter under drying conditions and its recrystallization as seen in mix T2 VI has also been observed in mix T2 V , as further detailed in. 33 Time-Lapse Micrographs under Rapid Changing RH. The time-lapse micrographs (Figure 5A,B) illustrate the dissolution and crystallization times of salt mixtures under rapidly changing relative humidity conditions (0.6% s −1 ).…”

Salts in the built environment

“…In response, an initial paper was published on NaCl-related weathering of stone, focusing on the importance of crystallization and dissolution kinetics of salt mixtures in environmental risk assessment (Godts et al, 2021). This paper includes results of an investigation into salt weathering assessment methodology, considering the temporal and kinetic behavior of salts present in porous media in both urban or rural environments.…”

“…However, some flexibility should be considered, specifically toward the lower RH range as the solid-state reactions might have limited effect on the substrates. Additionally, an RH increase over a limited time should be acceptable due to the kinetics considering dissolution/crystallization rates (Godts et al, 2021). However, more research is needed to further understand these processes for common mixture compositions, see Chapter 5.…”

“…Although a limited amount of literature is available (such as, Godts et al, 2012Godts et al, , 2014Godts et al, , 2017Godts et al, , 2021De Clercq et al, 2014;Heinrichs and Azzam, 2015;De Clercq and Godts, 2016;Laue et al, 2017;Menéndez, 2017Menéndez, , 2018Gibeaux et al, 2018;Gómez-Laserna et al, 2018;Godts and Hayen, 2019;Godts and Clercq, 2020;Scatigno et al, 2021;Chabas et al, 2022;Pintér, 2022), it is important to note that its use is mostly undocumented in peer-reviewed literature; for example, in Belgium the software has been used for over 300 heritage sites (Godts et al, 2022a). Like any model, ECOS and RUNSALT have limitations and pitfalls, with ineffective or non-existent transfer from research to practice, a challenge faced more widely in heritage science, as recently explored by Richards and Brimblecombe (2022).…”

“…Investigations on this subject have been carried out to understand the chemo-mechanical processes, as detailed by Flatt et al (2014). However, most studies focus on individual salts (Espinosa-Marzal and Scherer, 2010;Balboni et al, 2011;Flatt et al, 2017;Mancigotti and Hamilton, 2017;Caruso et al, 2018;Çelik and Sert, 2020;Meldrum and O'Shaughnessy, 2020;Oguchi and Yu, 2021), overlooking the reality that salts often exist in mixtures and behave differently when compared to single salts (Price and Brimblecombe, 1994;Lubelli et al, 2004;Linnow et al, 2007;Sawdy and Heritage, 2007;De Clercq et al, 2013;Steiger et al, 2014;Lindström et al, 2015;De Clercq and Godts, 2016;Godts et al, 2017Godts et al, , 2021Menéndez, 2018;Shen et al, 2020). Salt mixture behavior is the key driver of decay and is caused by crystallization and dissolution cycles triggered by environmental conditions.…”