Multi-analytical characterization of construction materials from the ‘Winter Baths’ of Thuburbo Majus (Tunisia): Implications for architectural chronology and conservation

Abstract Archaeological construction materials constitute key indicators of ancient building technologies and long-term durability. Nevertheless, the compositional variability and hydraulic behavior of building materials from North African baths remain under-documented. This study presents the first comprehensive petrological, mineralogical, and geochemical characterization of stones and mortars from the Winter Baths of Thuburbo Majus (Tunisia), with the aim of elucidating raw material selection, binder technology, and construction sequences. A multi-analytical approach combining Optical Microscopy (OM), X-ray Diffraction (XRD), X-ray Fluorescence (XRF), and Thermal Analysis (TGA-DTA) was implemented. The results demonstrate a strict correlation between the petrographic properties of the rocks and their architectural functions. Local materials were favored to minimize transport costs, whereas extra-regional stones were a minority, used to fulfill specific technical or aesthetic requirements. The mortar assemblage was classified into lime-based and gypsum-based systems. Lime mortars were further subdivided into: (i) moderately hydraulic mortars containing siliceous–carbonate aggregates; (ii) non-hydraulic air-lime mortars with crushed marble and vein calcite aggregates; and (iii) highly hydraulic mortars incorporating ceramic fragments (cocciopesto), characterized by secondary hydraulic reaction products indicative of pozzolanic activity. Variations in mineralogical composition, aggregate selection, and hydraulicity reflect deliberate technological adaptation according to structural and environmental requirements, particularly in humid bathing contexts. By integrating analytical data with established historical building phases, this research enables a precise chronological reconstruction of the building’s evolution as well as its construction sequence. In addition, this study establishes a rigorous scientific framework for creating compatible materials by leveraging the original mortar technologies identified in this work.