Mineralogical evolution of salt over nine years, after removal of efflorescence and saline crusts from Elche’s Old Bridge (Spain)

Abstract

The Old Bridge Elche’s showed abundant efflorescence and salt crusts that were eliminated in an intervention in 2005. We have followed the development of efflorescence after the intervention. And nine years later the efflorescence mineralogy was quite similar to the mineralogy of the pre-restoration efflorescence. The efflorescence on Elche’s Old Bridge (Puente Viejo) shows a wide mineralogical variation in the vertical profile, which is the result of a balance between two diffusive processes: the evaporation–condensation of atmospheric water vapor on/inside the wall, and the capillary flow of saline groundwater. Diurnal variations in atmospheric relative humidity (RH) and the low deliquescence relative humidity (DRH) values of some saline minerals (nitrate & chloride) play an important role in the vertical distribution of minerals. The mineral composition of efflorescence and saline crusts prior to cleaning the monument was similar to that identified nine years later. The chemical composition of groundwater, with a high sodium chloride component and much lower concentrations of sulfate and bicarbonate, explained the presence of halite, while the presence of demolition waste of a gypsiferous nature near the bridge explained the presence of sulfate salts, thenardite, konyaite, aphthitalite, and arcanite. The dissolution of airborne particles in condensation water probably contributed to the genesis of nitrate-rich minerals such as nitratine, darapskite, and especially humberstonite.