Alteration processes of geomaterials used on the pentagonal tower of Serravalle Castle (central-west Sardinia, Italy)

The pentagonal tower belong to medieval Serravalle castle area (81 m. above sea level), near the mouth of the Temo River. The castle is important medieval fortifications of Sardinia and was built in various stages. The oldest part was constructed by Marchesi Malaspina of Villafranca which arrived in Sardinia at the 12th century (1112-1121). The earliest fortification was the four corner towers about 10 m tall, linked by a thick wall. The pentagonal tower, located in the western corner of the boundary dates about 1330, probably was consisted of two storeys with wooden beams, now missing, and one stone one with a longitudinal arch. In the structure was used local volcanic rocks (i.e., pyroclastites) belonging to the Oligo-Miocenic volcanic cycle of Sardinia (32-11 Ma). According to De La Roche classification (1980), the pyroclastic rocks have a composition varying from dacite to rhyolite and show a porphyritic structure (I.P. from 10 to 20%) for phenocrystals of opaque (ilmenite, magnetite and/or titanomagnetite), plagioclase, ± biotite, and rare hornblend and quartz. Two main type of volcanics are present with different physical properties (porosity, density, etc.) and petro-volcanological characteristics (e.g., welding degree): cineritic pyroclastites, little welded, with average values of open porosity and bulk density of 36.3±2.6% and 1.50±0.07 g/cm3, respectively; lava-like ignimbrites, from medium to high welded, and average values of open porosity and bulk density of 22.5±5.9% and 1.99±0.15 g/cm3, respectively. As function of these different features and extremely heterogeneous, due to variable incidence of pumice, lithic- and crystal-clasts, the alteration is present mainly on volcanics with low welding and exposed to the sea-winds. Due to marine aerosol, salt efflorescences are present. These latter, together thermal and hydric cyclic dilatation, lead to various macroscopic physical alterations (decohesion, chromatic alteration, pitting, exfoliation, flaking, alveolation, differential degradation between the lithic-clasts and the vitreous matrix).