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 are typically 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 30 min 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 that can be applied to climate data and to illustrate a methodology to quantify salt crystallization cycles in realistic environments as a risk assessment procedure. The results are the basis for future work to improve the accuracy of salt risk assessment by including the kinetics of salt mixtures.

Modeling Salt Behavior with ECOS/RUNSALT: Terminology, Methodology, Limitations, and Solutions

“…Since its development, the model has been used extensively to aid management decisions for the preservation of built heritage worldwide. Although a limited amount of literature is available [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50], 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 [51]. 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 [52].…”

The transfer of heritage modelling from research to practice

“…A range of model types were present, including computational fluid dynamic [4,[21][22][23][24], thermodynamic [25,26], biological [27] and fuzzy logic [28][29][30] models. The models described had been used to capture processes such as moisture regimes [31][32][33], pollutant deposition [4,34], wind-driven sediment abrasion [5,21,23] and salt weathering [35,36]. Our selection does not capture all process-based models that relate to heritage, but our dataset encompasses a range of model types, applications and approaches that give a reasonable understanding of the current state of modelling within heritage science.…”

The Forerunners on Heritage Stones Investigation: Historical Synthesis and Evolution

“…In this time, there is a growing interest in the investigation of deterioration and conservation of building stones [227][228][229][230][231][232][233][234]. The development of analytical techniques allows scientists to quickly obtain increasingly reliable results and non-destructive and portable diagnostic techniques have improved considerably and play an important role in the conservation of built heritage [235][236][237].…”