Technological Evolution of Historic Mortars: From Lime-Based Mortars to Roman Opus Caementicium

Bulk samples of mortars were collected from archaeological sites dated from Hellenistic to Roman period in order to investigate the technological evolution of historic structures from simple lime based systems to advanced hydraulic materials. Roman opus caementicium enabled the construction of vaults with a mechanical behaviour of an inert monolith; the rapidly hardened mortar with its concrete-like strength provided a monolithic effect on lighter vaulted structures. The samples were characterized in terms of their isotopic, chemical and mineralogical composition. Stable isotope analysis (C-13 and O-18) provided information relative to the origin of CO2 and water during calcite formation making possible to distinguish different mortar technologies. The results of micro-morphological and petrographic examination elucidate the technological continuity of historic mortars. Hellenistic mortars are composed of lime enhanced with quartz aggregates. Roman mortars are composed of lime, pozzolan and various aggregates such as quartz, feldspar, ceramic and rock fragments. The isotopic values comprise a range of delta C-13 and delta O-18 values from -17.6% to 3.6% and -25.9% to 0.4% very different from that of local limestones used for mortar production. This study indicated that stable isotope analysis is an excellent tool to fingerprint the origin of carbonate and therefore indicate the variations in mortar's technology.