Characterization of ancient hydraulic mortars from the Roman Aqueduct Aqua Traiana

Aqua Traiana is a still functioning Roman aqueduct, built in 109 AD during Trajan empire, that collects water from the springs located north-west of Lake Bracciano (north of Rome) and carries it for 57 km to Transtiber (Rome). The mortars employed in its construction show an impressive durability, as they have maintained their mechanical characteristics for nearly two thousand years. Despite their high quality, only few studies deal with the characterization of hydraulic mortars in ancient roman aqueducts.The purpose of this work is to characterize the materials employed for the mortars of Aqua Traiana and to evaluate the provenance of the raw material. Fifteen samples were collected inside the specus of the aqueduct and analysed by optical microscopy (OM) in thin section and X-ray powder diffraction (XRPD) to determine the mineralogical composition; scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) to obtain the chemical composition; electron microprobe analysis (EMPA) to define the mineralogical phases identified by SEM-EDS; thermogravimetric analysis (TGA) to quantify CaCO3 content in the binder. The results allowed classifying the samples as pozzolan-lime mortars, and distinguishing original from restoration mortars (probably dating back to pope Paulus V restauration). More specifically, in the original mortars the binder is composed only of amorphous gel-like C-A-S-H. The absence of calcite in Trajan age mortars constitutes a unique case due to the environmental conditions and the starting raw materials that influenced the reactions involved.