Geochemical and mineralogical alteration on the stone and mortar surfaces of medieval churches (Sardinia, Italy): weathering processes, ancient conservation treatments and their microstratigraphy

The geomaterials (e.g., stones and mortars) used on the monuments when exposed to the atmosphere subjected frequently weathering and bio-deterioration processes. Thus, these latter produce various chemical and mineralogical transformations of substrate at the interface with the agents as function of intrinsic compositional and physical characteristics of the material. Moreover, if there also are ancient treatments on the surface of geomaterials, the investigation field is further complicated. This research aims to analyse the surfaces of different lithology (basalts, pyroclastic rocks, limestone, sandstone, marbles) and mortars used in the medieval churches of Sardinia (XI-XIV Cent.) having high historical-cultural relevance. By mineralogical and petrographic analysis and other different analytical methods (SEM-EDS, LIBS, XRD) the investigations addressed to define: i) chemical and mineralogical analysis of coatings/crusts (with different composition from the unaltered stone substrate) and the salt fano- and cryptoefflorescence; ii) physical-mechanical decay and Ca-binder dissolution of bedding mortars used for the masonry stone ashlars; iii) compositional and microstratigraphic characterization of ancient treatments (i.e., Ca-oxalate films) on the surface of facades; iv) chromatic modifications of surface with respect to the substrate due to the alteration processes. The results show the incidence of several factors in the alteration process, as function of the occurred time, petrophysical features and composition of geomaterials, their position in the monument structure, exposition to the weathering processes, microclimatic characteristics, environmental conditions. The presence of organic and inorganic substances frequently is observes as depositions on the surface (e.g., atmospheric particles) or as ancient treatments (i.e., weddellite and whevellite). These secondary crystalline phases has been find in several monuments (e.g., St. Antioco di Bisarcio, San Mamiliano di Simassi, Santa Maria di Tergu, San Pantaleo di Dolianova, etc.), independently from the kind of substrate (volcanic rocks, sandstone, marble). In fact, the presence of the Ca-oxalate on the facades of ancient monuments, as noted, is a testimony of application of organic substances on the surfaces themselves, with aesthetic reason, to standardize or improve the tone colour of the stone, or conservative reason, to limit the negative effects of weathering.