Vitruvian binders in Venice: first evidence of Phlegraean pozzolans in an underwater Roman construction in the Venice Lagoon

Four mortar samples were collected from a submerged Roman well-cistern in the norther part of the Lagoon of Venice, recently investigated during underwater surveys promoted by the team of maritime archaeology of the University Ca’ Foscari of Venice. Samples were preliminary described following a standardized protocol of analytical techniques, including Polarized Light Optical Microscopy (PLM), X-Ray Powder Diffraction (XRPD) and Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive X-Ray Spectroscopy (EDS). Analyses were aimed at performing a petro-mineralogical characterization of the materials and parametrizing the complex reaction chemistry of the hydraulic processes developed in the mortars in underwater conditions. Archaeometric analyses allowed the mortars to be identified as cocciopesto enriched with local sand in variable concentrations. Moreover, pyroclastic aggregates from the Phlegraean fields (pulvis puteolana) were added to the mortars to enhance the hydraulic capabilities of the compounds. Provenance determination of the volcanic pozzolans was determined geochemically by analysing via SEM-EDS and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) unreacted cores of volcanic glass and by comparing the results with the geochemical profiles of pyroclastic products of the Italian Plio-Quaternary magmatic districts. Around 950 geochemical profiles of juveniles, relating to differentiated eruptive events within the Phlegraean Fields, Somma-Vesuvius, Ischia and Procida-Vivara, constituted the reference database. An updated pattern of trace elements was selected among REE and HFSE to discriminate the origin within the Neapolitan magmatic activities, and descriptive chemical elements plotted one-by-one in explorative scatterplots. Provenance determination was further refined by discriminant analysis and spider-diagrams to cross-verify the scatterplot distributions. This operation was complicated by the extraordinarily advanced reaction processes of the analysed pozzolans, seriously affecting their geogenic chemical and petro-mineralogical features. The strongly alkaline anoxic underwater environment of the Venetian lagoon probably fostered the kinetics, as the matrices showed a relevant development of M-S-H hydrates replacing the pristine Ca-bearing phases of the binder, suggesting that the aerial carbonation of the lime was almost null. The perfect blend of local raw materials with imported volcanic rocks and brackish water promoted the development of pozzolanic and para-pozzolanic processes that influenced the durability and the perfect preservation of the analysed mortars over time to the present day. This evidence shows, once again, as much as the Vitruvian mention to the prodigious effects of pulvis puteolana used in seawater concrete (De Architectura, 5.12.2-3) was thoroughly grounded in the outcomes of ancient engineering knowledge