Decoding Roman construction techniques through a multiproxy study of pozzolanic mortars from the Brick Amphitheater of Nola (Campania, Southern Italy)

The long-lasting performance of ancient pozzolanic mortars highlights the advanced expertise of Roman builders. By intentionally mixing lime with specific volcanic materials, like ash or sand, the Romans developed hydraulic mortars and concretes that could harden even underwater and exhibit high mechanical strength. Furthermore, these pozzolanic additives accelerated the setting process via hydraulic reactions, offering a faster alternative to the slow carbonation of pure slaked lime. This study provides the first integrated reconstruction of the technological choices and historical evolution of the Brick Amphitheater of Nola by linking the provenance and compositional variability of volcanic aggregates to its different building phases. Due to the site's complex historical stratigraphy and multiple construction phases, a targeted, non-invasive sampling strategy was adopted. Eighteen bedding mortar samples from the Amphitheater structures and adjoining Late Antique walls were analyzed using an integrated analytical including Polarized Optical Microscopy, X-Ray Powder Diffraction, Field Emission Scanning Electron Microscopy with Energy-Dispersive X-Ray Spectroscopy, Mercury Intrusion Porosimetry, Differential Thermal Analysis and Thermogravimetric Analysis. The research aims to distinguish construction phases and trace the evolution of building techniques by i) identifying the geological provenance of raw materials, ii) analyzing the mortar mix design, and iii) reconstructing the site's chronology. Results confirmed that raw materials were locally sourced, employing aggregates from Somma-Vesuvius district, as well as recycled Neapolitan Yellow Tuff, highlighting the Roman builders' expertise in selecting and combining local volcanic aggregates with hydrated lime to produce natural hydraulic mortars. The observed hydraulicity is attributed to the binder-aggregate interaction, evidenced by reaction rims and the formation of calcium-aluminum-silicate-hydrate (C-A-S-H) gels. Mortars from the Amphitheater structures display optimized binder-aggregate ratios and well-compacted pore networks, contrasting with the greater heterogeneity, increased porosity and mixed aggregate origins of mortars from Late Antique walls. These findings reflect intentional and consistent material selection and formulation, also reflecting chronological and functional evolution of the site.