Alteration of stone materials on Sardinian medieval monuments: physical, chemical and petrographic analysis for their restoration and preservation

Sardinian medieval monuments are mainly made up by volcanic rocks (pyroclastites/ignimbrites), minor granitoids and sedimentary rocks, that show a more or less significant chemical-physical alteration. Mineral-petrographic features, physical properties related to petrogenetic processes, as well as manufacturing, strongly influence type and intensity of stone-decay. The granitoids show an alteration degree less than the other rock-types, due to low porosity (<10%) that does not favour an easy absorption of circulating solutions, thus avoiding the water-rock interaction and hydrolysis. In a few cases, a physical intra-crystalline decohesion, that involves a rounding of the sharp edges of ashlars, can occur, while, in chemical/mineralogical terms, oxidation patinas and the chloritization of mica minerals can be observed. The pyroclastites/ignimbritites, widely used in medieval architecture for the excellent workability, are affected by greater alteration due to the different mineral/chemical composition, mainly including glass (>70%), and physical features (higher porosity, 20-45%) due to a variable welding degree. Chemical alteration is not always evident since is more slow than the physical degradation (with macroscopic forms of pitting, exfoliation, alveolation). The limestones with low porosity (<10%) show an alteration that generally occurs on the outer surface of ashlars (with solubilization-reprecipitation processes), while sandstones and calcarenites (porosity >25%) generally show an advanced stages of decay, with physical macroscopic forms similar to those of the pyroclastites. The chemical alteration is also present, in particular on carbonate cement. To make a contribution to the preservation of sardinian monuments, this paper suggests a new approach to define the different alteration-modes of rocks in function of their local exposure to the weather, studying: 1) the changes of physical properties on surface of stone (porosity, water absorption, micro-morphology) determined through laboratory tests and photogrammetry observations, 2) the alteration phases present on surface (e.g., secondary minerals, soluble salts) determined by mineralogical (XRD) and chemical (XRF) investigations.