Abstract
Natural stones have represented one of the main building materials since ancient times. In recent decades, a worsening in degradation phenomena related mostly to environmental pollution was observed, threatening their conservation. The present work is focused on the minero-petrographic and geochemical characterization of black crust (BC) samples taken from the historical center of Naples, after selecting two pilot monumental areas. The latter were chosen based on their historical importance, type of material, state of preservation and position in the urban context (i.e., high vehicular traffic area, limited traffic area, industrial area, etc.). The building materials used and their interaction with environmental pollutions were studied comparing the results obtained by means of different analytical techniques such as polarized light Optical Microscopy (OM), scanning electron microscopy with energy dispersion system (SEM-EDS), X-ray powder diffraction (XRPD) and laser ablation coupled with inductive plasma mass spectrometry (LA-ICP-MS).
Highlights
Air pollution strongly affects the integrity of stone materials, since it promotes their degradation over time, especially in an urban context [1,2,3,4,5,6,7,8,9]
Characterization was performed by optical microscopy (OM) of the samples collected at the complex of San Domenico Maggiore (SD series) and the cloister of San Marcellino e Festo (SM-P, SM-A, SM-S series)
Embedded iron oxides and hydroxides were observed, together with black combustion particles of spherical, sub-spherical and prismatic shape; b) a layer of scialbo which shows a good adherence to the substrate, with relatively homogeneous morphology, and a thickness ranging from 3 mm to 800 μm, dark brown in color, a cryptocrystalline aspect, a very fine granulometry and a secondary porosity of about 10% related to the dissolution of some portions
Summary
Air pollution strongly affects the integrity of stone materials, since it promotes their degradation over time, especially in an urban context [1,2,3,4,5,6,7,8,9]. The formation of black crusts, which occurs mainly on carbonate rocks, represents one of the most dangerous degradation forms caused by air pollution [10,11,12,13,14,15,16,17,18] Their formation is due to calcium carbonate (CaCO3 ) sulphation, as a consequence of pH value decrease caused by SO2 in the polluted atmosphere [19]. This dissolution allows gypsum (CaSO4 ·2H2 O) to precipitate, which because of its low water solubility of 2.4 g/L at 25 ◦ C [20] remains as a crust that becomes black (due to soot particles) on surfaces protected from intense wash-out [21,22,23,24].
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