The key to ancient Roman mortars hydraulicity: ceramic fragments or volcanic materials? A lesson from the Phlegrean archaeological area (southern Italy)

This research deals with mineralogical, petrographic, chemical, and microstructural characterization of raw materials from the Roman Dragonara cave (Phlegraean Fields, Campania region, Italy) through Polarized Light Microscopy (PLM), X-ray Powder Diffraction (XRPD), Field Emission Scanning Electron Microscopy coupled with an Energy Dispersive Spectrometer (FESEM/EDS), Simultaneous Thermal Analysis (STA) and Mercury Intrusion Porosimetry (MIP). The aim of this study was to asses mortars hydraulic properties along with provenance of raw materials and mortars recipe. A focus on the reactions between the surface of the ceramic aggregates and volcanic materials with binding matrix proved to be crucial to define their role in the mix design of mortars. Representative results showed that hydraulicity of the analyzed mortars is mainly due to volcanic materials rather than ceramic fragments. In fact, reaction active elements such as Reaction Rims (RR), Interfacial Transition Zones (ITZ) and particularly Ca-rich rims were usually found at the matrix-volcanic fragments interface. Moreover, porosity tests evidence that bedding mortars, which contain mainly volcanic material and only sporadically ceramic fragments have a higher closed porosity of binder matrix due to the good pozzolanic reactivity.