Implementation of a non-destructive method to assess weathering deterioration of sandstones in cultural heritage

The problem of deterioration of building stones is particularly important in Cultural Heritage rich countries; Italy is one of these, with the highest number of UNESCO World Heritage Sites. To reach sustainable monument preservation, proper countermeasures are required to protect stone monuments from weathering or to provide more efficient restoration actions on damaged buildings from catastrophic events (i.e., earthquakes). In this framework, together with granite and limestones, sandstones are the most used construction materials in Central Italy and all over the world. Due to their high compositional heterogeneity, the characterisation of weathered sandstone is difficult. For this reason, the development of an innovative non-invasive methodology is essential to better understand the behaviour of weathering on protected monuments. The sandstone columns of Palazzo Ducale, one of the main historical buildings in Camerino town (Central Italy), are protected by the Superintendent for cultural heritage of Marche Region (peripheral body of the current MiBAC Ministero per i Beni e le Attività Culturali) and display a different range of weathering in terms of discolouration, significant scaling and loss of large volumes of stone. In this study, an innovative methodology which couples non-destructive rebound measurements with Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscopy (SEM), X-Ray diffraction and grain size analyses, has been proposed and tested on some columns of the palace. The methodology has been further validated on a few rock cores from the original extraction site. It was found that weathering action on the columns’ surface has caused a 25% strength reduction with respect to the core samples. Based on the physicochemical analyses, it was identified that freeze-thaw cycles have caused a reduction in volume of micropores in favour of an increase in macropores, weakening the rock surface. In ochre zones pyrite oxidation has occurred, releasing iron and sulphur, promoting dissolution of calcium carbonate and its reprecipitation and reorganization in the outer and superficial macropores. This results in a significant decohesion of the material in crusts and scales as well as an important loss of material, which leads to a strong heterogeneity in the alteration of the artefacts. In fact, the lithic material's surface alteration does not occur in a predictable and uniform manner; instead, it can have different speeds in adjacent points depending on the mineral-petrographic properties. Eventually, our study aims to provide a valuable contribution to future restoration work in light of the damages suffered by the building after the 2016–2017 seismic sequence of Central Italy and it might be a representative example for understanding sandstone decay in historical artworks.