Mineralogical clustering of the structural mortars from the Sarno Baths, Pompeii: A tool to interpret construction techniques and relative chronologies

Abstract Structural mortars constitute one of the most diffuse geomaterials, with stones and bricks, in ancient monuments and architectural complexes, especially related to the Roman civilization, which pushed the binder technology to technical levels unsurpassed until post-industrial revolution times. The archaeometric study of mortars is an essential tool to extrapolate great amounts of information concerning supply of raw materials, technological skills of the ancient civilizations and, finally, relative and absolute chronologies of diachronic construction phases, both related to ancient and modern architectural modifications of the buildings. In this contribution, a novel approach for the quantitative mineralogical analysis of ancient mortars is proposed. The analytical process is based on the integrated application of quantitative phase analysis (QPA) of mineral components by means of the Rietveld method applied to X-ray powder diffraction (XRPD) data and multivariate statistical treatment of the obtained results by means of the principal component analysis. The methodology has been applied on a wide set of binding materials sampled from different structural elements of the Sarno Baths, a five-storey building located in the Pompeii archaeological site. The building is characterized by a marked complexity both in terms of structural layout and constructive techniques, being the result of several modifications in ancient times from the Late Republican age up to the Vesuvius eruption in 79 AD. Furthermore, several poorly documented restorations have been performed between the 19th century and the first half of the 20th century AD. In this perspective, a quantitative characterization of the employed mortars resulted useful not only to define ancient constructive technologies and relative chronologies, but also to discriminate between the original and restored parts of the building for the execution of adequate restoration procedures. The statistical clustering of the quantitative XRPD data clearly defined two ancient constructive phases and allowed a precise definition of the structural elements rebuilt in recent times. Furthermore, the obtained results have been cross-checked with additional analyses, namely XRD analyses on the separated binder fractions, petrographic analyses and scanning electron microscopy-energy-dispersive microanalyses. Such multi-analytical approach allowed the detailed characterization of the employed raw materials, of the pozzolanic reactions between binder and aggregate and of the textural and microstructural characteristics of the mortars. The data interpretation yielded interesting insights both on the advanced optimization of mix designs of binding materials in Roman times, to improve the structural properties of the architectural elements according to their functions, and on the formulation of the restoration products during the historic excavations in Pompeii.