LEARNING FROM THE PAST – RAW MATERIALS AND TECHNOLOGY OF ANCIENT ROMAN MORTARS USED IN THE ARCHAEOLOGICAL SITES OF CAMPANIA REGION (ITALY)

Ancient pozzolanic mortars testify the high technological quality achieved by Roman construction workers, due to their ‘excellent state’ of preservation in every environment. These workers well knew that thanks to the combination of lime with specific volcanic materials (pozzolana ss), mortar and concrete become hydraulic, allowing underwater hardening, and increasing mechanical strength.

This research focuses on the detailed characterization of mortars to improve the knowledge of Roman construction techniques by means of microstructural and compositional examination of 1) cementitious binding matrix and 2) aggregates, to point out a) mortars’ mix-design and provenance of raw materials, along with b) the study of secondary minerogenetic processes and c) comparing the obtained results with the modern mortars.

The studied samples were taken from Piscina Mirabilis, Terme di Baia, Villa del Capo and Villa del Pezzolo, which represent four of the most important archaeological sites placed in two important geological, archaeological, and historical areas of Campania Region in Italy: Campi Flegrei and Sorrento Peninsula.

The Soprintendenza Archeologica della Campania, allowed to perform non-invasive, but representative, sampling of mortars that were characterised by multianalytical methodologies: POM, XRPD, SEM-EDS, TGA, and MIP.

Results confirmed the expertise of Roman workers, who skilfully combined volcanic tuff aggregate, hydrated lime, and ceramic fragments. The "ingredients" used in recipe mix of mortars had a local provenance and are very well consistent with the surrounding geological setting.

The most relevant characteristic detected in all studied samples is the mortar hydraulicity testified by evidences such as reaction rims between pozzolana and binder, Hydraulicity Index (HI), and thermal analyses investigation. Also, composition of secondary mineralogical phases in the cementitious matrix is particularly relevant. Contemporary presence of C-A-S-H gels, calcite, gypsum, tobermorite (Piscina Mirabilis), hydrocalumite (Villa del Capo) are distinguished features. These two late secondary minerogenetic products fill pore spaces and thus enhancing bonding in pumice clasts. The difference between ancient Roman mortars and modern hydraulic mortar was mainly related to porosity.