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A MULTISDICIPLINARY APPROACH TO CHARACTERIZE GRANITE BUILT HERITAGE IN NORTHWESTERN PORTUGAL

Angela Almeida, Fabiana Dias

Abstract

The evaluation of the behaviour of natural stone applied in the built heritage of the historical centre of Porto city, Northwestern Portugal, classified World Heritage by UNESCO in 1996, has been carried out using a multidisciplinary approach in order to characterize both the rock that constitutes the main body of the building and the deterioration processes that have developed on the stone as a consequence of environmental and anthropogenic factors. Among an extensive and monumental historical heritage, built with the local granite, the Rectory of the Porto University has been selected as it constitutes a paradigmatic building that hosted the roots of Porto Academy in the 18th century and has been a teaching establishment of areas that contribute to the cultural and scientific development of the country. The dimension rock is a Variscan peraluminous two-mica granite exploited from quarries within the city and whose emplacement was controlled by the third Variscan tectonic phase responsible for a NW-SE deformation trend and the occurrence of shear zones. The intrinsic properties of the granite have been studied based on petrographic, geochemical and petrophysics procedures. U-Pb and Rb-Sr geochronological dating suggest an age in the range 312 and 318 Ma. The specific petrogenetic properties are in the origin of the susceptibility of the granite to alteration processes that are enhanced by posterior aggravation when the building is exposed to climate, environment and anthropogenic activity. The deterioration products, particularly resulting from granular disintegration, black crusts and thin black films, were submitted to scan electron microscopy (SEM) coupled with an energy dispersive X-ray system (EDS) and to Fourier transformed infrared spectroscopy (FTIR). SEM-EDS analysis allowed the identification and the composition of soluble salts namely halite, gypsum, calcite and nitratite, as well as of porous carbon-rich and smooth aluminium-rich fly ashes. FTIR analysis revealed the presence of nahcolite, trona, thenardite and ammonium salts, and confirmed the presence of halite, gypsum and calcite as the main minerals in the stone deterioration types.

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DOI resolver: doi.org/10.5593/sgem2017/11/s01.001

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