# Investigations on an Ancient Mortar from Ulpia Traiana Sarmizegetusa Archaeological Site, Romania
## 1 Introduction  
As of 2024, the ancient civil settlement of Ulpia Traiana Sarmizegetusa was on the list of UNESCO World Cultural Heritage monuments, which includes the Roman settlements on the borders (_Limes_) of the Dacia province. In this context, the remaining plinth of the central statue in Forum Vetus deserves reconsideration from an archaeological point of view. From the archaeometrical perspective, there is an opportunity to improve the knowledge on construction materials using modern analytical methods and to assess the reliability and challenges of radiocarbon dating applied to ancient mortars. Mortars are a wide class of building materials used for more than 8500 years [1,2]. Regardless of their formulation, all mortars are made of binder and aggregates. Materials close to our current understanding of mortars are the pozzolanic mortars that originated in Greece around 500 BC [3]. Romans later improved their use and methods of preparation [4]. Pozzolanic mortars are made of a pozzolanic material (for example, volcanic ash), slaked lime, water, and aggregates. Pozzolanic reaction takes place between a siliceous or siliceous-aluminous pozzolanic material and slaked lime/calcium hydroxide Ca(OH)\({}_{2}\) in the presence of water, at ambient temperature, leading to the formation of calcium silicate (C-S-H) and calcium aluminate (C-S-A-H) hydrates, which harden over time [5]. Along with the binder matrix made of hydrated compounds, calcium carbonate CaCO\({}_{3}\) can also be found in mortars [6]. CaCO\({}_{3}\) in a mortar's binder phase can originate, in significant amounts, from five sources:  
(S1) Non-decomposed CaCO\({}_{3}\) from the original limestone (geogenic calcite) [7]. This carbonate depends on the processing conditions for obtaining slaked lime and should be minimized, although, in some recipes, it is added intentionally to provide a secondary source of calcite/calcium carbonate, with the idea of regenerating genuine source of carbonates formed by hardening [4]. (S2) CaCO\({}_{3}\) from carbonation over time of Ca(OH)\({}_{2}\) in excess (pyrogenic calcite); this process may need, for aerial mortars, a carbonation rate of 80-92% after 100 years [8]. It can occur under ambient carbon dioxide concentrations (with different carbon-14/C-14 contents, depending on the historical period), being thermodynamically favored (exothermal reaction). It is optimal for dating construction segments because it reflects the \({}^{14}\)CO\({}_{2}\) level for the construction or restoration time [9]. (S3) CaCO\({}_{3}\) from carbonation of C-S-H hydrates areas of formation (C-S-H carbonation) [10]; this specific carbonation can take a variable number of years, depending on microstructural features, weather conditions, microstructure evolution due to micro-cracking, and weathering [11,12]. The degree of carbonation of the cementitious materials is, usually, estimated by the measurement of carbonation depth [12]. Various polymorphs of calcium carbonate, along with S-H/S-A-H and water, are the products of this carbonation process. (S4) CaCO\({}_{3}\) can be also obtained via recarbonation of Ca(OH)\({}_{2}\) (Ca(OH)\({}_{2}\) recarbonation); over time, a fraction of S1-S3 calcite may eliminate carbon dioxide by dissolution, leading to calcium hydroxide. CaCO\({}_{3}\) regeneration will take place by precipitation using a proximal CO\({}_{2}\) source with a different C-14 signature (older or younger than genuine calcium carbonate formed by hardening) [13,14]. Calcium carbonate/calcite, obtained from the S1-S4 sources, will have different levels of carbon-14. Due to its origin and mechanisms of formation, it presents distinct physical and chemical properties and can be separated under certain conditions as fractions. Differentiation between fractions can be achieved using Accelerator Mass Spectrometry, highlighting the formation pathways [13]. From the geomorphological point of view, calcium carbonate is distributed in nature in three mineral forms: calcite, aragonite, vaterite.