The carbonate building stone of the Ortygia Island (Syracuse, Italy): characterization and decay processes

During the XVIII century, the historic centre of Syracuse (Ortygia) was interested by intense building activity, which took place after the severe earthquake of 1693. Ortygia and the other cities of the Hyblean area (south-eastern Sicily) were characterized by a close relationship between building stone and geological context, since the wide use of local calcarenites characterized and defined their urban planning and architecture. This contribution, dealing with the study of limestones used in the historic buildings of Ortygia, is part of a wider research project aimed at the characterization of different types of carbonate rocks, employed in the Baroque architecture of the Hyblean area, and their decay processes. Different shades of colour and textural features characterize the Ortygia façades, due to the use over time of several types of limestone taken out from diverse extraction sites or different layers of quarrying. Specifically, six local calcarenites, belonging to four geological formations (Monti Climiti Formation: Pietra di Siracusa and Pietra Bianca di Melilli; Carrubba Formation: Calcare a lumachelle and Calcare oolitico; Palazzolo Formation: Pietra di Noto; Panchina pleistocenica: Pietra giuggiulena), were used both as structural component and decorative element in the historic buildings. These carbonate rocks have different chemical, physical and mechanical characteristics which lead, over time, at different and more or less intense alteration and degradation processes of the façades. Here, we focus on the study of Pietra di Siracusa, Pietra Bianca di Melilli, Calcare oolitico and Pietra giuggiulena, through the analysis of rock samples collected from historical quarries. These four lithotypes underwent mineralogicalpetrographic and physical-mechanical investigations with two aims: 1) to frame the different types of limestone used in the historic centre of Syracuse; b) to individuate the causes responsible for long or short term durability of each limestone when placed in the same outdoor environment at identical climatic conditions. The obtained results were also compared with literature data concerning the other two limestones (Noto stone and Calcare a lumachelle) used in the façades of historic buildings in Ortygia, whose behaviour with respect to weathering is markedly different. Results obtained are consistent with deterioration types observed in the monuments, and highlight the close relationship between textural characteristics of the stone and its damage. In addition, such information are extremely useful for both the safeguard of monuments and their restoration.