ANCIENT ROMAN MORTARS: MIX DESIGN, MINERALOGICAL COMPOSITION AND MINEROGENETIC SECONDARY PROCESSES

Aim of this study is the characterization of ancient Roman mortars collected in a) Piscina Mirabilis, b) Terme di Baia, c) Villa del Capo (commonly called Bagni della Regina Giovanna) and d) Villa del Pezzolo sites. All investigated location are placed in two important geological, archaeological and historical areas of Campania Region: Campi Flegrei and Sorrento Peninsula. Goals of this research project were: a) to improve the knowledge of Roman construction techniques by means of detailed microstructural and compositional examination of 1) cementitious binding matrix and 2) aggregates, to point out both mortars' mix-design and provenance of raw materials, along with b) the study of secondary minerogenetic processes and c) comparing the obtained results with the modern mortars. Thanks to the permission by Superintendence of Archeological Heritage of Campania, it was possible to take out small, non-invasive, and representative samples of mortars. The collected samples were used for an integrated analytical program using multiple methodologies such as: optical microscopy (OM) on thin sections, X-ray powder diffraction (XRPD), scanning electron microscopy analysis (SEM), energy-dispersion X-ray spectroscopy (EDS), X-ray fluorescence (XRF), thermal analyses (TGA-DTA) and mercury intrusion porosimetry (MIP). This research activity was performed in collaboration with CTG (Group Technical Center) of Italcementi Hidelberg Group at Bergamo (Italy). Results have shown that building materials used in studied archaeological sites had a local provenance, and are very well consistent with the surrounding geological setting. A relevant characteristic detected in all samples is the evident hydraulicity of these mortars, as shown by the reaction rims of pozzolanic materials; this feature is supported by the Hydraulicity Index and thermal analyses investigations. Extremely interesting is composition of cementitious binding matrix characterized by various products of reaction, including amorphous gel-C-A-S-H, calcite, tobermorite (Piscina Mirabilis samples) and hydrocalumite (Villa del Capo samples). In particular, these two late secondary minerogenetic products refine fill pore space and thus enhancing bonding in pumice clasts. The difference between ancient Roman mortars and modern hydraulic mortar was mainly related to porosity.