Ancient building practices in the Phlegraean Fields: material characterization of the Tempio di Apollo (Italy)

This study investigates the building materials and techniques used in the Tempio di Apollo, located in the Phlegraean Fields (southern Italy), an area of active volcanoes with caldera-forming eruptions in the past and also characterized by present-day bradyseism. The availability of pyroclastic deposits strongly addressed the local building practices. Mortar and brick samples were collected from selected structural sectors of the monument, adopting a minimally invasive sampling approach. Analytical investigations were carried out through Polarized Light Microscopy (PLM), X-ray Powder Diffraction (XRPD), and Scanning Electron Microscopy (SEM). The mortars are lime-based and show variable degrees of cohesion. The binder appears micritic to cryptocrystalline, with frequent lime lumps. The aggregates are mainly composed of volcanic fragments such as pumice, with occasional ceramic fragments. XRPD data confirm calcite as the dominant binder phase, reflecting the original choices in raw material; the presence of gypsum and halite reflect subsequent alteration processes. These results underline the deliberate use of local volcanic products and illustrate the technological strategies adopted by Roman builders to ensure durability and structural performance. The study contributes to a broader understanding of ancient construction practices in dynamic volcanic landscapes.