3D imaging and analysis of the inner structure of the "Tufo Giallo Napoletano": a contribution to the study of the architectural decay of Castel Nuovo

Façades of Castel Nuovo, a medieval castle located in front of the city hall in central Naples, Italy, are made with bricks of "Tufo Giallo Napoletano" (TGN), a widespread yellow tuff of that area. Castel Nuovo is one of the main architectural landmarks of the city, and shows, among other types of stone decay, a certain extent of black crusts. Such a kind of degradation, independently from its origin is affected by the typical porous structure of that rock material.Multiscale scanning of samples of TGN from the south-western façade of the castle has been carried out by means of the X-ray microCT technique in order to reconstruct images of its natural and degraded inner structure. The 3D image analysis algorithm of "successive opening" has then been applied to determine the size distribution and the connectivity of the outer connected pores of the TGN. The crust thickness distribution and its development inside the pores under the surface of the rock material has also been evaluated.In the not degraded material the open macroporosity was 27.6% with size ranging between 20 and 660 microns and a percolation threshold of 96 microns. Closed porosity was about 10% of the open one and porosity inside the black crusts reduced to 8.2% with an increase in the layer just below the crusts to 36.1%. Black crusts exhibited an average thickness of 39.3 microns with a propagation below the surface of the stones of about 1.2 millimetres. Analyses in thin sections of the crusted layes by transmited light microscopi allowed to identify as gypsum as main composition of the black crusts.Overall results allowed to recognize the X-ray microCT as a useful technique of investigation of stone decay in the framework of a multidisciplinary approach aiming at addressing the restoration treatments of porous construction materials.