Characterization of carbonate building stones of monuments in the Mediterranean coats

The carbonatic stone degradation in marine environment is one of the most important problem on the conservation of the monuments of the Mediterranean Basin. The behaviour of the building carbonate stones close to the sea is influenced by an environment which is particularly aggressive and often causes a high degree of degradation of the shallower parts of the masonry structures. In this context a non-destructive evaluation to identify damages and degradation of the materials is of great importance both in assessing their status and planning their restoration. In this study the control of the carbonate building stones of two monumental structures close to the sea was based on non-destructive ultrasonic investigation integrated with mineralogical and petrographical analyses. The ultrasonic techniques using different data acquisition procedures (step by step and refraction) were effective in detecting the shallow altered areas of the investigated masonries and in determining the thickness of the alteration. Considering the nature of the investigated materials and the constructive types of the masonry, which is characterised by shallow and deep dishomogeneity, the ultrasonic investigation was carried out in the low frequency ultrasonic range (24-54 Khz) with the aim to detect damages and degradation zones through the study of the propagation of the longitudinal ultrasonic pulses. In fact alterations in the materials normally cause a decrease in the longitudinal pulse velocity values. Therefore the longitudinal velocity values can be considered representative of the elasto-mechanical behaviour of the materials. The ultrasonic measurements were also integrated by a mineralogical-petrographical study of the investigated carbonate stones to correlate their petrophysical features with the elastic ones. The studied materials were extensively used as building materials of the monumental structures in the town of Cagliari (Italy). They can be distinguished in three different types of rocks dated to Tortonian-Messinian periods. These rocks are named from top to bottom Pietra Forte, Tramezzario and Pietra Cantone. Pietra Forte is a bioclastic white yellowish speckled calcareous rock of littoral and infralittoral environments (Fig.1). Tramezzario is a bioclastic limestones white coloured. Pietra Cantone is a avana yellow coloured fine-grained calcareous rock, with calcareous fossil remains of bivalves and gasteropodae. From this study results that the modifications in the elasto-mechanical properties and in the mineralogical-petrographical composition of the investigated carbonate materials are the main cause which reduce their quality as building materials. The integration of the ultrasonic, mineralogical and petrographical data provided a good knowledge of the investigated materials and gave an efficaceous contribution both in the diagnostics on the preservation state of the carbonate building materials of the investigated monuments and in checking the effectiveness of the restoration actions. Anyway the conclusion of this study cannot be easily extrapolated to other monuments of the Mediterranean area, because any changes in the local climate factors, in the environmental conditions of the monuments, in the characteristics of the carbonate stones can lead to a different diagnosis.