Weathering detection of granite from three asynchronous historical quarries of Sabrosa municipally (North Portugal)

Abstract

Vale das Gatas granite is the main traditional building granite of Sabrosa municipality (North of Portugal). It is a two-mica granite porphyroid of coarse-medium crystal-size, characterised by the presence of pseudo-oriented phenocrystals of feldspars.

Quarries deepen as extraction techniques evolve over time. They were shallow in the Iron Age, and the granite extracted from them is more weathered and has more developed exfoliation microcracks than the one extracted from medieval quarries, when new techniques allowed the extraction of deeper and fresher granites. Current quarries can go tens of meters deep and produce fresh granite, which is less weathered and has exfoliation microcracks that are less developed.

Prehistoric, medieval and current quarries of the Vale das Gatas granite exist in Sabrosa municipality. Therefore, a great variety of monuments have been built with the same granite but with different weathering degrees. Replacement granite must have the same weathering degree as the original granite present in the monument to avoid different responses to the same decay agent.

Different weathering degrees of Vale das Gatas granite were assessed by non-destructive techniques to correlate their physical properties with the weathering degree. Thus, in addition to a microscopic study, density, porosity, capillary water uptake, colour, ultrasonic pulse velocity and thermal properties were determined. These properties allowed determining the weathering degree of Vale das Gatas granite and correlating it with the original historical quarries. In this way, replacement granites can be obtained for future restorations.