The ancient pozzolanic mortars and raw materials of the Heliocaminus baths in Hadrian's Villa (Italy)

In this work were studied ancient mortars and materials used to construct the personal baths of Emperor Hadrian, located in its Villa of Tivoli (province of Rome), a luxurious residence from the Roman Imperial Age (II century AD; Mac Donald and Pinto 2006). These baths is one of the most innovative and interesting building inside the Villa for the presence of the Heliocaminus room characterized by a dome-shaped coverage, various technical solutions and precious marbles (Columbu et al., 2014). The construction of these original baths is probably due to the Hadrian interest about architecture and for the dome structures. Overall approximately 85 samples (including the stones) are taken from the baths, of which about 40 mortar samples divided in different groups according to their function, i.e.: bedding mortars of bricks and cubilia ashlars (opus reticulatum), mortars of marble coatings of the floors and walls, cocciopesto conglomerates (opus signinum) of floors (rudus) and walls (trullisatio), concretes of vaults (opus caemeticium), plasters of interior walls, volcanic rocks and lateritious fragments used as aggregate in the mortars, pyroclastites of cubilia ashlars and outcrops. To determine the compositional characteristics the samples were analyzed by polarized microscope, microprobe (SEM-EDS) and X-ray diffraction (XRD, XRPD). The minero-petrographic characterization has allowed to define the geological provenance of materials used as aggregate in the mortars (e.g., volcanic scoria, leucitite, etc.) or for cubilia of the walls. The study of binder / aggregate ratio, obtained with two different analytical methods (with image analysis on thin sections and on specimen faces of mortar bulk sample; Columbu et al., 2015), has shown a discrepancy in some groups between the method of mixing dictated by Vitruvius and those used in the analyzed materials. Through the thermo-differential analysis (TGA, DSC) the hydraulic degree of the mortars has been studied and compared with data of the physical properties (resistance index to puncturing, porosity, real and bulk density, etc.) in order to define the relationship between pozzolanic characteristics and mechanical strength. The results, together with the analysis of the aggregate particle size and morphology, have shown a clear relationship between the physical characteristics of the mortars and their function within the structure of the baths, explaining the choice and selection of raw materials. This research have provided significant data about the preparation of Roman mortars and also on the different construction phases of the baths linked often, as well as for the Villa, to the stops due to the travels of the Emperor along the Empire boundaries.