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 22 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.
Alteration of stone materials on Sardinian medieval monuments: physical, chemical and petrographic analysis for their restoration and preservation
M Palomba;
2014
Abstract
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 22 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
