Implications of new mineral phases in the isotopic composition of Roman lime mortars at the Kom el-Dikka archaeological site in Egypt

Over time, the carbonated components of lime-based mortars dissolve and recrystallise, inducing calcite cementation in the resulting micro- cracks and pores, favouring self-healing capacity. In such types of mortars, the binder may originally contain gypsum, or, on the other hand, dissolution and precipitation processes of compounds from different sources (soil, construction materials or airborne particles) may derive in the formation of new saline minerals. The uptake of such components may interfere with chemical and physical analysis, hindering the interpretation of the data needed to determine the identification, provenance of raw materials, formulation, and production processes of mortars, carried out in archaeometric investigations. This article discusses the presence of such mineral phases in the mortars sampled at the Kom el-Dikka archaeological site located in Alexandria, Egypt, and their effect on the findings. A multi-technique approach was undertaken to characterize the ancient mortars and determine their behaviour and evolution, by using the following analytical techniques; polarised optical microscopy (POM), scanning electron microscopy (SEM–EDS), X-ray diffraction (XRD), thermogravimetry (TG), X-ray fluorescence (XRF) and ion chromatography (IC). The results achieved through archaeometric and geochemical investigations show the presence of different mortars at the Kom el-Dikka site. Also, petrological techniques successfully explain the variation in isotopic composition (¹³C and ¹⁸O) patterns mediated by the presence of the saline and carbonate phases.