Minero-petrographic characterization of historical mortars from Aquileia

Mosaics are complex systems composed of several layers (Pasqualucci, 2004) of finer and coarser mortar. This stratification makes them interesting case studies for evaluating the properties of different mixtures. Maintaining the stability of these layers is a fundamental aspect of conserving a mosaic. The durability of mortars is considered one of the fascinating mysteries that has spanned centuries of research in construction engineering. The mechanical characteristics and strength of mortars are due to the pozzolanic reaction, a chemical process that is strictly related to its composition. Given these premises, a clear understanding of the nature of ancient mixtures is essential to preserving the layered mortar systems that compose mosaics and developing compatible conservation materials. In this research, 40 samples of mortars collected from fragments of mosaics were analyzed. The archaeological material studied was gathered during a field walking survey of the peripheral areas surrounding ancient Aquileia, Italy, which is well known for its famous mosaics. There are several works in literature that focus on the study of the mortars used to produce them (Secco et al., 2018; Dilaria et al., 2022). A mineral-petrographic characterization was carried out using a multi-analytical approach that allowed for a thorough description of the investigated material. Specimens were observed under a Polarized Light Microscope (PLM) to explore the main textural and compositional features of both binder and aggregate fractions. Among the selected materials, five remarkably well-conserved specimens were analyzed with Scanning Electron Microscopy coupled with X-ray probe (SEM-EDX). These specimens served as reference materials for comparison with samples taken from other less-conserved stratifications. Mortars detached from the specimens were analyzed using X-ray diffraction (XRD), enabling the identification of the crystalline phase and mineralogical content. Furthermore, Thermogravimetric analysis coupled with an Infrared spectrometer (TGA-IR) was used to identify specific regions of decomposition indicators of the presence of different materials. Moreover, one fragment with a complete stratification of mortars still attached to tesserae was analyzed with X-ray Fluorescence (XRF), allowing for the elemental mapping of the material. Thus, it was possible to obtain a complete characterization of the mortar according to its stratifications (Boschetti et al., 2021). The extensive dataset acquired through the analysis of a significant number of samples in this research has facilitated a better understanding of the mortar technology used in the production of Aquileian mosaics.