A methodological proposal for the identification of the carbonatic raw materials used for the binder of ancient mortars

The aim of this work has been mainly to set up a new methodology useful to recognise the kind of limestone utilised in the preparation of the lime used as binder in ancient mortars. On the occasion of the study of the static conditions of the bridge known as the “Ponte di Augusto” erected over the river Nera we carried out a research on the mortars used for the building. The Ponte di Augusto, built in the 27 b.C. during the works of enlargement of the consular way, is one of the widest ever erected by the Romans and is sited close to Narni, in Umbria not far from the Latium border. Narni was founded by the Sabin people and then turned in a Roman colony (Narnia) which had a strong military importance because of its strategic position on the "Flaminia way". At present only one arch is standing which should not be the widest. Its chord is 20 m long and its height at the intrados is 27 m. The arches, the piers and the abutments were made by a framework of well cut travertine ashlars filled with a mortar that shows a really strong cohesion with mechanical characteristics similar to that of the modern concrete. The Romans realised a high-quality mortar of relative low porosity (~ 30%) and strong mechanical characteristics. These good characteristics appear mainly to be ascribed to the use of a binder of high hydraulicity. The comparison between chemical and mineralogical analyses performed on binder and lumps evidenced that the high hydraulicity was not obtained by the addition of particular additives like pozzolana. Hence the research was directed towards the individuation of the possible local limestone whose chemical composition best fitted the chemical data of the mortar’s binder. Because of the presence of many limestones of different age that show a chemical composition similar to that of the binder, we thought about a micropaleontological approach. Micropaleontological analyses has been performed in order to detect both the presence of relicts of calcareous and/or siliceous fossils inside the binder resisting to burning and for the determination of the age of the original limestone. This kind of analysis revealed to be promising and an experimental study was set that consisted in the realisation of mortars from the calcination of different types of limestone coming from the outskirts of Narni. A micropaleontological study has been subsequently performed on the binder of these mortars.