Investigations into Roman Mortar from Sucidava, Romania.

This study presents an interdisciplinary investigation of mortars from the Roman fortress of Sucidava, constructed in the 3rd century AD along the Danube River, at the northern frontier of the Roman Empire. The mortars were characterized using a multi-analytical approach, including macroscopic examination, optical microscopy, cathodoluminescence microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), and qualitative phase analysis by X-ray diffraction (XRD). The results highlight the presence of two distinct mortar types: (i) a gray, coarse-grained masonry mortar consistent with opus caementicium, covered by a finishing layer, and (ii) a fine-grained red plaster mortar analogous to opus signinum, as described in ancient architectural treatises. The structural and compositional features of mortars sampled from the eastern wall of the Sucidava Fortress confirm the persistence of advanced Roman mortar technology in this northern frontier region, even after the division of the Roman Empire. The substitution of volcanic ash with bone ash led to the incorporation of phosphorus-bearing compounds, which, according to recent studies, contribute to enhanced mortar performance.