Characterization of Natural Stone from the Archaeological Site of Pella, Macedonia, Northern Greece

The goal of the study was to characterize the limestone that was used extensively in the ancient city of Pella (Macedonia, Greece), the birthplace of Alexander the Great. An on-site examination of the building material was carried out to record the types of damage and to select sampling areas. A variation in the nature of the stone and the degree of deterioration, even between the stones that comprise a specific monument structure, was observed, with water absorption and biological colonization being the main factors resulting in the deterioration of the stone. A comprehensive microanalysis and testing scheme was conducted to fully characterize the mineralogical, chemical, mechanical and thermal properties of the stones collected from various areas of the archaeological site. Optical microscopy, XRD and SEM-EDX were used to investigate the chemical composition and the structure of the stone samples. Finally, other properties, such as porosity, specific gravity and water absorption, were measured. Surface alterations, material degradation and biological deterioration were observed in most samples. The results obtained using XRD showed that the dominant mineral phase of the limestone is calcite, with quartz and clay minerals also detected in traces. The microscopic examination of the samples showed that the main natural stone at the archaeological site is a marly limestone. Thermographical measurements showed that the decay of the stones due to ambient temperature variation and corresponding contraction/expansion phenomena may be relatively limited, as the stone exhibited a low thermal diffusivity. Moreover, high porosity values (12.06-21.09%) and low compressive strength (11.3-27.7 MPa) were recorded, indicating the vulnerability of the stone and the need to take conservation measures.