Technological transfers in the Mediterranean on the verge of Romanization: Insights from the waterproofing renders of Nora (Sardinia, Italy)

The paper reports the results of a multi-analytical characterization study on the renders constituting the waterproofing coatings of 11 cisterns located in the Punic-Roman archaeological site of Nora (Southern Sardinia, Italy). The multi-layer layout of the coatings is related both to the practice of putting several strata of render in a synchronic plastering phase, and to frequent restoration activities related to the prolonged use of the cisterns. The aims of this study are (a) to determine the compositional and textural properties of the binding materials, (b) to define the coating systems employed in diachronic plastering phases, and (c) to identify the role of diverse cultural influences in the modification of the render recipes along time. The combined interpretation of the results obtained through petrographic, mineralogical, microstructural and microchemical analyses allowed discriminating different render types employing various pozzolanic additives, whose utilization is related both to Punic and Roman technological practices. Furthermore, the cementation processes of the renders were studied through an in-detail mineralogical and spectroscopic characterization of their binder fractions. The results demonstrated that hydraulic reactions were often enhanced through specific treatments of the binding mixtures, such as the combined use of different pozzolanic additives, the adoption of comminution processes to increase their reactive specific surface and the use of saltwater to enhance silica and alumina activity in solution through pH increase. Such technological practices promoted in some cases not only the precipitation of Ca-based pozzolanic reaction products (C-S-H and AFm phases), but also the formation of nanostructured anthropogenic magnesium-silicate-hydrate (M-S-H) phyllosilicate gels, leading to a further increase of the cohesive properties of the binding mixtures. A combined interpretation of the results with the cistern construction chronologies confirmed that cistern coating techniques in Nora were still largely influenced by Punic traditions even two centuries after the Roman conquest of Sardinia.