New insights about the consolidation of archaeological mortars located in underwater environment: The case study of the apsidal fishpond of Castrum Novum (Santa Marinella, Rome, Italy)

This work is part of a research project titled MaTaCoS (Advanced materials and technologies applied to the conservation of underwater cultural heritage) funded by the Italian Ministry of Economic Development (MISE), concerning development of innovative tools and methods for the protection of Underwater Cultural Heritage, with particular regard to cleaning and consolidating procedures to be carry out directly in situ. The fishpond of the archaeological site of Castrum Novum (Santa Marinella, Rome, Italy) was chosen as a pilot site for experimentation. Castrum Novum was a Roman colony whose ruins are located between Torre Chiaruccia and Casale Alibrandi. The archaeological site lies on a wide area facing the sea, at the 64.4 km of the Aurelia State Road, in the Province of Rome, in a territory corresponding to today's Santa Marinella, which, during the Roman ages, belonged to Caere, now Cerveteri [1]. During the first half of the third century BC, it was one of the most important cities found along the ancient Etruscan coast as Alsium (now Palo Laziale) and Pyrgi (Santa Severa). Other significant remains, concerning the ancient city and the ancient harbour, lie close to the beach where now some modern stilts stand. The apsidal fishpond is one of these structures on the coastline. It is composed of only one tank, with an average immersion of 0.37 m below the sea level, and it develops with an NE/SW orientation. The masonry structures reach the maximum thickness at the apex of the fishpond (4.70 m) and consist of a concrete conglomerate composed of slightly rough stones of medium size bound with non-hydraulic mortar. After sampling, for a complete characterization of selected archaeological fragments, different and complementary techniques (stereomicroscopy, polarising optical microscopy and X-ray powder diffraction analysis) were carried out in order to: a) define the minero-petrographic features; b) investigate their state of conservation. The obtained data allow defining the main constituents of mortars from a compositional point of view. The raw materials, in fact, are quite homogeneous, as well as the ratio in which they were mixed, confirming the typical "recipe" used in Roman times to manufacture hydraulic-type mortars by adding pozzolana. At the same time, it was possible to identify the various degradation processes they are interested in, mainly, biological colonization (bio-fouling) that develops differently according to environmental conditions. From the applicative point of view, the textural, mineralogical and chemical information might represent the first step both for the definition of restoration interventions and for the planning of maintenance protocols