Climate change impact of salt weathering on vernacular and archaeological cultural heritage building materials in Europe and Latin America

Salts are widely present in all porous building materials. Their chemical composition, degree of hydration and crystalline phase as well as their abundance and location in the building are highly variable. In addition, these parameters depend on the type of material, the location and environmental factors such as climate, air pollution or groundwater composition.  The crystallization of salts inside porous materials depends on the nature of the solutions present in the pores and the conditions under which it occurs. Environmental, climatic and pollution conditions have changed in the past and continue to change today.

In this work we will estimate the changes of the potential salt weathering in vernacular cultural heritage in Europe and archaeological sites in Latin America, in particular in the selected regions of the European project SCORE (Sustainable COnservation and REstoration of built cultural heritage 2021-2024). In order to determine how future climatic conditions may affect salt weathering in these sites, climatic conditions in different models, based on scientific literature, will be used. Salt weathering will be estimated for different salts: Na₂SO₄, NaCl, and mixture of salt.

For temperatures between 0 and 30 ° C, the solubility of sodium chloride is almost constant (around 26% by mass) while that of sodium sulfate is very variable (between 5% and 20% by mass). Changes in temperature will induce precipitation/dissolution more easily for sodium sulphate than for sodium chloride. Similar conclusions can be drawn for changes in relative humidity in the case of crystals. In the case of sodium sulphate, the crystalline phase changes between the anhydrous salt (thenardite) and the decahydrated salt (mirabilite) and the deliquescence will depend on both temperature and relative humidity. In the case of sodium chloride with a single degree of hydration at temperatures above 0°C, the deliquescence is almost independent of temperature, with a relative humidity of almost constant equilibrium around 75%. In general, salts that have only one state of hydration have a lower capacity of degradation than salts with several phases of hydration. Phase diagrams will be employed to quantify the weathering induced by thee salts.

In nature as well as in buildings, it is common to find associations of salts more than pure salts, which complicates the study of the dependence of salt crystallization on environmental conditions. The variety of salts that can be formed by crystallization of solutions containing several different anions and cations is extremely important. The behavior of mixed solutions is much more complicated than that of solutions containing a single species of cation and anion. For complex solutions, the crystallization pathways as a function of composition and environmental conditions cannot be directly deduced from that of the salts taken separately. Thermodynamic models can be very useful for modeling the sequence and conditions of salt crystallization in a solution. We used the ECOS-RUNSALT model to calculate the evolution of salt volume as a function of temperature and relative humidity conditions to estimate the weathering produced by complex solutions.