PETROGRAPHIC AND CHEMICAL MAPS OF POZZOLANIC CEMENT MICROSTRUCTURES IN ANCIENT ROMAN SEAWATER CONCRETES

The hydraulic concretes of ancient Roman harbor constructions along the central Italian coast remain compact and coherent despite immersion in seawater for 2000 years, yet the processes of pozzolanic cement development in the volcanic ash-hydrated lime-seawater systems remain unclear. Analysis of drill core specimens indicate that the concrete is formed of alkali rich, vitric tuff coarse aggregate bonded by a pozzolanic mortar composed of sand- to gravel-sized vitric tuff and pumice clasts, occasional volcaniclastic sands, opaque relicts of lime clasts, and a binding matrix with diverse crystalline cements identified through X-ray diffraction. The Zr/Y and Nb/Y of pumice in the mortars of the Cosa (~60 BCE) port and the Trajanic (~115 CE) port at Ostia near Rome, 2.2 – 10.3 and 1.7 – 13.3, respectively, indicate a Campanian origin, most likely from Campi Flegrei, as described by Vitruvius in De Architectura (31–27 BCE). Petrographic maps of high-resolution photomicrographs overlain with SEM-EDS elemental maps describe pozzolanic cements in mortar fabrics of the Claudius (~50 CE) and Neronis (~65 CE) harbor concretes from Ostia and Anzio. These reveal perimetral dissolution of lime and tuff clasts; elevated Ca and Si in residual clast nuclei associated with precipitation of tobermorite (C-S-H); and discrete, bead-like microstructures with concentrations of Al and Cl or Al and S, associated with calcium-chloroaluminate and ettringite. These reactions most likely took place at pH>12. In contrast, phillipsite cements apparently precipitated in response to dissolution of volcanic glass at lower pH, ~9.1–10. These mineral phases have analogies to natural rock cements that develop in volcanic ash submerged in seawater (pH 7.5–8.4), as in the authigenic alteration of the 1963-1964 Surtsey phreatic eruptions. They represent stable pozzolanic cement systems that enhanced the chemical durability of the ancient maritime concretes._