| Summary: | This investigation studies the physical and chemical effect of salt weathering on biocalcarenites and biocalcrudites in the Basilica of Our Lady of Succour (Aspe, Spain). Weathering patterns are the result of salty rising capillary water and water lixiviated from pigeon droppings. Surface modifications and features induced by material loss are observable in the monument. Formation of gypsum, hexahydrite, halite, aphthitalite and arcanite is associated with rising capillary water, and niter, hydroxyapatite, brushite, struvite, weddellite, oxammite and halite with pigeon droppings. Humberstonite is related to the interaction of both types of waters. Analysis of crystal shapes reveals different saturation degree conditions. Single salts show non-equilibrium shapes, implying higher crystallisation pressures. Single salts have undergone dissolution and/or dehydration processes enhancing the deterioration process, particularly in the presence of magnesium sulphate. Double salts (humberstonite) have crystals corresponding to near-equilibrium form, implying lower crystallisation pressures. This geochemical study suggests salts precipitate via incongruent reactions rather than congruent precipitation, where hexahydrite is the precursor and limiting reactant of humberstonite. Chemical dissolution of limestone is driven mainly by the presence of acidic water lixiviated from pigeon droppings and is a critical weathering process affecting the most valuable architectural elements present in the façades.
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