On the tracks of Pulvis Puteolana: provenance and selection of Roman hydraulic materials

Abstract

Phlegraean pozzolana played an important role in the production of Roman hydraulic mortars, capable of enhancing mechanical strength even in contact with water. For this reason, this geomaterial was widely exported along the Mediterranean coasts (ROMACONS Project).

Nowadays, the term pozzolana is strictly referred as the unconsolidated facies of the Neapolitan Yellow Tuff (NYT; ca. 15 ka), while the term pozzolan suits any fine-grained, mainly glassy volcanic ash as well as commercial products exhibiting pozzolanic activity.

This study characterizes pozzolana stricto sensu and two incoherent deposits belonging to post-NYT eruption collected from quarries from Phlegraean Fields area, historically used by the Romans and compares them with pozzolanic aggregates found in hydraulic mortars from both Campanian and extra-regional archaeological sites.

A multi-analytical approach was employed to obtain detailed minero-petrographic and chemical information with Polarized Optical Microscopy (POM), X-Ray Powder Diffraction (XRPD), X-Ray Fluorescence (XRF), Field Emission Scanning Electron Microscopy coupled with Energy-Dispersive X-Ray Spectroscopy (FESEM-EDS). In addition, technological and physical properties were assessed through pozzolanic activity test and laser particle size analyses.

Experimental data showed that post-NYT pyroclastic deposits also exhibit pozzolanic activity and can develop hydraulic properties. These results are consistent with the hypothesis that both NYT and selected post-NYT deposits were recognized empirically by Romans as suitable raw materials for hydraulic mortars preparation for both local and distal constructions. The Fratini test is therefore used here as a modern assessment of pozzolanic reactivity, while provenance and selection hypotheses are primarily based on the combined petrographic, mineralogical and geochemical evidence. In fact, the compositional match between pozzolana samples analyses and literature glass data from Roman mortars, in relation to the compositional fields of known Phlegraean Fields eruptions, suggests that post-NYT deposits were also employed as pulvis puteolana.

This research establishes a first reference database to further the knowledge of ancient Roman construction technology.