The ancient mortars of Serravalle fortification (Bosa, Italy): a case study

The case study is the pentagonal tower from Serravalle Castle fortification, located near to the river Temo (west-cost of Sardinia), above the medieval Bosa village (XII-XIV cent.). The castle belongs to the medieval Giudicato di Torres (X cent.) and then to Malaspina from Lunigiana (XII cent.), Turrritani, Arborea, Aragonese, until to the Catalans (from Alghero) that led it to decline (XVI cent.). The aim of this proposal is to analyze from mineralogical-petrographic point of view the mortars used in the construction of the tower. Samples were taken between the volcanic ashlars at different heights with respect to the ground level, to assess any compositional variations during the construction of the building. Through microscopic analysis, the compositional aspects concerning the nature and petro-mineralogical features of the raw materials used for the aggregate were defined. By image analysis (on meso-microscopic photographs) the vol.% of binder/aggregate ratios and their grain-size characteristics were determined and compared with the wt% data obtained from disaggregation method of mortars and acid attack of carbonate binder, in order to understand if the mixing ratios used by constructors have respected the production standards of those historic period. By X-ray diffraction (XRD) and termo-gravimetric/differential analysis (TGA-DSC) of enriched-binder samples, the presence of C-S-H phases and the hydraulic degree of mortars have been studied, to understand the possible use of pozzolanic material. Finally, to define the physical-mechanical properties of mortars were also determined: porosity, real and bulk density, PLT punching-index, theoretical values of compression and traction strengths.