Salt and Heritage: prediction and remediation of salt- induced damage to cultural heritage
Overview
The presence of salt and moisture is a major factor in the physical weathering of stone, causing structural damage to buildings, stone artefacts, archaeological sites and wall paintings. The presence of salts alone does not necessarily pose a threat, but rather the repetitive cycles of crystallization and dissolution when the salts are in the presence of moisture.
When salts dissolve in water, they turn into ions that can travel through porous stone materials. Changes in temperature or humidity can then lead to crystallization, where the salt ions form crystals in the pores of the stone. Once the crystals form, they can exert pressure on the pore walls. For instance, when sodium sulphate transforms from anhydrite or thenardite (without moisture) to decahydrate or mirabilite (with moisture), it expands more than 300% and thus the crystallization pressure exerted on the pores of the stone increases dramatically. This can then cause cracking and weakening of the stone, which in turn will compromise the integrity of the structure and lead to loss of both the material and the cultural heritage it represents. It is therefore very important to research the behaviour of salts and the effect of climatic changes on the conservation and restoration of stone.
Examples of decay due to salt crystallisation
The ISAAC mobile lab using remote SWIR and Raman to map and identify salt deposits at Fort Brockhurst, an English Heritage property in Hampshire
This study aims to develop a model that would serve as a benchmark for large scale monitoring of heritage materials alteration due to salt action, and also to establish an experimental set up that would express realistic conditions at a multi scale level, allowing the forecasting of real salt-damage scenarios on heritage sites. The project will be divided into two parts:
1. Studying salt behaviour to understand the relationship between physical parameters of porous materials, such as stone, and the kinetics of salts under environmental changes. This will allow a better understanding of salt efflorescence patterns- the powdery deposits we often see on monuments and buildings- which are formed by sodium sulphate in the presence of moisture. Raman spectroscopy data will be used to identify the phase transitions of sodium sulphate, such as those which occur during changes in humidity.
2. Designing a framework to predict and investigate salt transport dynamics. This will allow better understanding of the reversibility of reactions (salt precipitation- dissolution cycles) throughout porous materials such as stone, taking into account salt subflorescence- when salt-containing moisture migrates to the interior of the stone material and crystallizes within its pores.
Addressing the challenge
Both efflorescence and sub-florescence can occur due to a variety of factors, including the presence of soluble salts in the material itself, the use of salt- contaminated water during construction, and the presence of salt-contaminated soil or groundwater in the surrounding environment.
Prevention of crystallization and phase transitions of salts in stone materials due to climatic variations is therefore an important part of their conservation. To detect sites and areas that are at risk or to choose the most appropriate treatment, it is necessary to conduct preliminary research that includes identifying salts and determining moisture and salt contents, by conducting salt accelerated weathering tests, and employing various analytical techniques such as gravimetric analysis, ion chromatography, X-ray diffraction or Raman spectroscopy. ECOS/RUNSALT software will be used to predict the crystallization behaviour of a salt mixture at different temperatures and relative humidities.
The information collected experimentally will be transported to a pore network model (PNM), which is a simplified representation of the complex pore-space geometry of porous materials, and the model will be used to extend the experimental results to test different exposure conditions, such as different concentrations of salt, and a range of relative humidities.
making a difference
Many cultural heritage sites suffer from damage induced by salt crystallization, making the investigation of its impact on natural building stones essential. This project will revolve around the study of the key parameters that lead to such damage, causing the volume expansion of salts in porous building materials, and a real time analysis of the behaviour of salts on historical sites, undertaken via a field analysis campaign. A particular novelty of the project will be to develop a numerical approach to dynamically simulate in-situ salt behaviour. Such a model is intended to serve as a benchmark for structural health monitoring of cultural buildings and as a predictive measure for remediation of salt crystallization in cultural heritage sites.
people
Supervisory team:
Lucas Goehring (Nottingham Trent University)
Haida Liang (Nottingham Trent University)
Ran Holtzman (External Supervisor, Coventry University)
David Thickett (External Advisor, English Heritage)
PhD student:
Fatima- Zohra Sahraoui (Nottingham Trent University)