Unveiling the secrets of Roman craftsmanship: mortars from Piscina Mirabilis (Campi Flegrei, Italy)

Aim of this study is the characterisation of ancient Roman mortars collected in Piscina Mirabilis, located in the important geological, archaeological and historical area of the Campania Region (southern Italy): the Campi Flegrei. Goals of this research were (a) improving knowledge of Roman construction techniques by means of detailed microstructural and compositional examination of cementitious binding matrix and aggregates, to point out both mortar mix-design and provenance of raw materials, (b) the study of secondary minerogenetic processes and (c) comparison with modern mortars. Thanks to the permission by the former Soprintendenza Archeologia della Campania (authority of the archaeological heritage) current “Parco Archeologico dei Campi Flegrei”, it was possible to collect small, non-invasive, but representative samples of mortars. Samples were studied by combined methodologies such as optical microscopy (OM) on thin sections, X-ray powder diffraction (XRPD), scanning electron microscopy analysis (SEM), energy-dispersion X-ray spectroscopy (EDS), simultaneous thermal analyses (STA) and mercury intrusion porosimetry (MIP). Results showed that local geomaterials were used in this archaeological site, as they are well consistent with the surrounding geological setting. A relevant characteristic is the hydraulicity of these mortars shown by the reaction rims of pozzolanic materials. Composition of the cementitious binding matrix is characterized by various products of reaction, including amorphous C-A-S-H gel, calcite and Al-tobermorite. Results also highlighted that porosity represents the main difference between ancient Roman mortars and modern hydraulic ones.