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.

The application of hydrophobic treatments as a means of protecting vulnerable stone heritage has been a topic of research for decades. The findings of previous research have shown that there are a number of factors that influence the efficiency of a treatment and that sometimes, if used incorrectly, such treatments can even accelerate stone weathering and decay. In this study, we revisit a hydrophobic treatment test area at Arbroath Abbey where the product was applied over 40 years ago, thus providing a rare opportunity to investigate the long-term efficiency of hydrophobic treatments. As well as assessing the condition of the treated area in situ by means of moisture analyses, lab-based accelerated salt weathering experiments are conducted to better understand the impact of silane-based treatments on sandstone durability. Moreover, the petrography and petrophysical properties of weathered sandstone (open porosity, capillary absorption, and vapour diffusion) before and after treatment are also characterised to provide a better understanding of how stone properties may influence the compatibility of the treatment. The field-based results show that the treated area has maintained a degree of hydrophobicity since its application over 40 years ago. Both field-based and lab-based analyses suggest that silane-based treatments can be used successfully in protecting sandstone when applied correctly, both in reducing the rate of decay and functioning over long periods of time. However, sandstone heterogeneity may mean that some individual stones are less compatible with the hydrophobic treatment tested than others. Further field-based analyses (including methods such as XRF and in situ vp) of the treated area is required in order to determine the state of conservation more accurately. These results highlight the complexity in selecting a suitable hydrophobic treatment, especially at built sites where the mineralogy and petrophysical properties of the stone may vary between blocks. However, such treatments may still be important to consider as many climates, including Scotland’s, are becoming progressively wetter, increasing the vulnerability of stone heritage to moisture ingress, accelerated decay, and eventual ruin.

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