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 (pyroclastics/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 favor an easy absorption of circulating solutions, thus avoiding the water-rock interaction and hydrolysis. In a few cases, a physical intracrystalline decohesion, that causes a rounding of the sharp edges of ashlars, can occur. The presence of oxidation patinas and the chloritization of micas are imputable to the action of chemical alteration phenomena. The pyroclastics/ignimbritites, widely used in medi eval architecture for the excellent workability, are affected by greater alteration due to the different mineral 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 low porosity (<10%) limestones show a typical a lteration which preferentially occurs on the outer surface of ashlars (with solubilization-reprecipitation processes), while sandstones and calcarenites (porosity >25%) generally show an advanced stages of decay, giving rise to physical macroscopic forms similar to those of the pyroclastic rocks. The chemical alteration preferentially acts on the carbonatic cement. The paper makes a contribution to the preservation of Sardinian monuments, suggesting a new approach to define the different alteration-types of rocks in relation with their local exposure to the weather alteration phenomena. In particular, data on i) the changes of physical properties on surface of stone (porosity, water absorption, micro-morphology), acquired by laboratory tests and photogrammetry observations, ii) the newly-formed phases observed at the ashlars' surface (e.g., secondary minerals, soluble salts), by mineral (XRD) and chemical (XRF) investigations, will be reported and discussed.