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 (1st c. CE) in the Northern 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), Quantitative Phase Analysis—X-Ray Powder Diffraction (QPA-XRPD) and Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive X-Ray Spectroscopy (EDS). Archaeometric analyses allowed the samples to be identified as lime-based mortars enriched with ceramic fragments and sand-sized particles compatible with local alluvial deposits. Moreover, pyroclastic aggregates, inconsistent with the local geology, were added to the mortars as natural pozzolans, strongly reacted with the lime binder. Their provenance was determined through geochemical analysis by using SEM-EDS (Scanning Electron Microscopy with Energy Dispersive Spectroscopy) and LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry). The analysis targeted the inner regions of certain coarse clasts (having a grain-size ranging from approximately 450 μm to 2–3 mm), where fresh volcanic glass, unaltered by reactions, was still preserved, allowing the original geochemistry of the clasts to be delineated. The resulting fingerprints were then compared with the geochemical distribution of the pyroclastic products of the major Italian Plio-Quaternary magmatic districts. The lithological source of the analysed tephra appears to be petrochemically congruent with the Phlegraean Fields volcanic district. However, most of the volcanic clasts, especially the finer ones (< 450 μm) and shards, showed significant alteration as a result of pozzolanic reactions with the binder. The strongly alkaline anoxic underwater environment of the Venetian lagoon likely fostered the reaction kinetics, as the matrices showed a relevant development of M-A-S-H hydrates replacing the pristine Ca-bearing phases of the binder. On the other hand, the carbonation of the lime was almost null. The uniform mixture of local sands, ceramic fragments, and imported volcanic rocks, combined with brackish water, appears to have fostered pozzolanic and para-pozzolanic reactions in underwater conditions. This evidence shows, once again, how Vitruvius’ recommendations on the use of Phlegraean pozzolans (Vitr. De Arch. 5.12.2) to enhance the physical and mechanical properties of seawater concretes were firmly rooted in the advanced engineering knowledge of the ancient world.