Mineralogical and technological characterization of pozzolan mortars from the Brick Amphitheater in Nola (NA)

The ‘Brick’ Amphitheater is an archaeological site located in Nola (NA), at Masseria d’Angerio, dating back to the 1st century BC. Part of this structure was excavated during essays conducted in 1992 by the Superintendency, revealing an elliptical plan, dimensions of 138 m on the major axis and 108 m on the minor axis, with a total capacity of about 20,000 spectators. Since the extended use and the several architectural interventions over time, the archaeological area is of particular interest. The present preliminary study is aimed at: i) characterize the different types of the used mortars (Miriello et al., 2015); ii) investigate the origin of the raw materials used for mortars production (Columbu et al., 2022); iii) identify similarities and/or differences between the various samples analyzed, in order to make hypotheses about the different construction stages (Lezzerini et al., 2018). Furthermore, this study will try to evaluate, with the help of historical sources, whether and how much the analyzed samples might be related to different socio-economic situations. Eighteen mortar samples, 12 taken from different areas of the amphitheater and 6 from ancient walls, were investigated (Rispoli et al., 2021). An initial macroscopic observation was conducted, followed by a more detailed thin- section examination through Polarized Optical Microscopy (POM), which enlightened the type and distribution of elements constituting the mortar and the background matrix. X-Ray Powder Diffraction (XRPD) technique was used for qualitative mineralogical characterization. The obtained results allowed to assess that the raw materials used for mortars production were locally sourced, as they were compatible with the geological features of the Vesuvian Area. In particular, preliminary results confirmed the occurrence of lime-based mortar, in which the binder shows a prevailing cryptocrystalline texture and subordinately micritic, with a volcanic aggregate fraction around 40-50% mainly composed by pumice, leucite slag, amphibole, biotite and garnet. The analyzed mortars, especially those related to the amphitheater structure, are hydraulic, as they show reaction rims at the volcanic aggregate-binder interface. Comparison between different mortar samples shows three different groups, which differ in the production technology and are presumably attributable to three different construction phases of the archaeological site. The first group (samples A1-A9) stands out for the accuracy of preparation and choice of aggregates, the second group (samples A11-A13) and the third group (M2-M6) have a higher binder fraction and more lime lumps at with respect to the aggregate fraction. Moreover, the aggregate fraction of groups 2 and 3 appear to be coarser in the samples taken from the walls. These preliminary data represent a good starting point for further investigation in order to develop a suitable pozzolan mortar for future conservative interventions of the monument, and for the re-using of waste geomaterials in a view of a more sustainable restoration.