The role of moisture in sandstone decay in the Monumental Cemetery of Bologna (Italy)

One of the main causes of materials’ decay in the “Certosa” Monumental Cemetery of Bologna (XIX Cent.), as well as in many other historic cemeteries, is capillary water rise from ground. The presence of moisture in porous materials is directly or indirectly responsible for all their damage processes (e.g. solubilisation, freeze-thaw cycles, crystallization of soluble salts, chemical and biological attack, clay swelling). Despite the acknowledged role of moisture in materials’ decay, restoration and conservation works often didn’t take into proper account the problem of structures’ dehumidification, due to its challenging nature, cost and often uncertain outcome. As a result, even well performed consolidation and protection treatments are unable to stop stone decay or, in worst cases, they enhance it. The present contribution deals with the effects of moisture on soft sandstones, which suffer from deep pulverization, crumbling and flaking, although some conservation and maintenance works were carried out some years ago. Samples were withdrawn from the base of a column in the hemicycle of the Fourth Cloister, the base of a pillar in the Third Cloister and a courtyard wall (Fig. 1). Samples’ nature and microstructure were investigated by means of mercury intrusion porosimetry (MIP), XRD, TGA/DSC, FT-IR, SEM-EDS and soluble salts determination (ion chromatography). The results show that soluble salts amount in the investigated samples is not particularly high, while the analysis of climatic data around the cemetery points out the occurrence of frequent freeze-thaw cycles, hence frost damage seems one of the most probable decay process. In samples exhibiting some hydrophobic behaviour, rising damp from ground keep the stone permanently wet, despite the quite high total porosity (20÷22%) and large pore mean radius (12÷16μm), thus leading to frost occurrence and material pulverization. In stones were a consolidated layer is present, such layer exhibits a significant reduction in porosity (~14%), hence some water might remain trapped behind it and cause, when freezing occurs, the detachment of the entire consolidated layer in large portions. When rising water is not removed, the risks connected to microstructural changes due to conservation treatments were thus highlighted.