The role of marine aerosol in the formation of (double) sulfate/nitrate salts in plasters

• Plasters inside a building close to the sea can be deteriorated due to the migration of the salts carried in marine aerosol.
• XRD in combination with Raman, ATR-FTIR and SEM-EDS allow avoiding inappropriate spectral assignations.
• Double sulphate/nitrates such as humberstonite and sulphates such as glauberite, syngenite and polyhalite were identified.
• Experimental results and thermodynamic simulations allow establishing the decay pathway useful to explain the reactivity.

During decades the use of gypsum in different buildings has been very common, especially in the Atlantic lands of Europe. Decay compounds like salt crystallizations are ones of the principal deterioration factors of such historical buildings. In this study, gypsum-based plasters from different inner rooms of the Igueldo Lighthouse (San Sebastian, Spain), a building dated back to 1860 that has been subjected to several repairs within these years, were investigated in order to ascertain the main mineral phases produced during the weathering process. A combination of Raman spectroscopy, X-ray diffraction, infrared spectroscopy in ATR mode, SEM-EDS imaging and Raman imaging was successfully applied to study the distribution of different decay compounds in the gypsum-based matrix and to establish the decay reaction pathway which leads to the formation of the identified decay compounds. Additionally thermodynamic chemical modeling was also performed to explain the formation of specific decaying compounds. According to the location of Igueldo Lighthouse (just above a cliff, close to the sea), this building experiments a wide influence of the marine aerosol (Na+, K+, Mg2 +, Cl− and NO3− input) and the influence of a high damp environment, giving rise to common efflorescence salts as well as to different mixed-calcium sulfates and mixed sulfate–nitrate salts, such as glauberite, syngenite, polyhalite and humberstonite. Dehydration process of gypsum from the plaster (leading to the presence of anhydrite and bassanite) was also identified.