# Investigations on an Ancient Mortar from Ulpia Traiana Sarmizegetusa Archaeological Site, Romania
## 1 Introduction
Although vaterite can also form in anthropogenic materials such as mortar, it is unstable. Aragonite, on the other hand, can be identified [15]. However, if it is highlighted by methods of analysis prior to radiocarbon dating, it can be removed [16]. Mortar maturation means that all these processes have been completed, and an equilibrium reached, long before analyses. The analyses can establish the occurrence of ripening, and also the presence or absence of recarbonation phenomena [13]. Aggregates constitute a major part of mortars/concretes; ancient pozzolanic mortars were made with natural aggregates, such as sand, limestone, and mussel shells [17], or artificial aggregates, such as crushed ceramics, which contain aluminosilicates as a pozzolanic material [18; 19]. When using quarry sand/beach sand or even that from riverbeds, a fifth category of carbonates may appear, which can seriously affect the radiocarbon data these are carbonate deposits/calcareous concretions (S5) (calcareous concretions) [20]. During formation, porosity plays an important role in carbonation. Pores have a wide range of sizes and shapes in mortars/concretes. Coarse pores greater than 100 \(\upmu\)m in diameter are usually formed by the following [17]: (i) _entrapped air_, formed by entrapment of air during mixing. They present both irregular shapes and distributions (and can serve as the primary source of carbon dioxide, at a C-14 level very close to the time of manufacture); (ii) _entrained air pores_ are round voids formed by organic materials, some of them containing proteins, presenting bubble-like shapes (carbon dioxide will result from the decomposition of organic matter usually close in time to the manufacturing moment). Coarse pores contribute negatively to the strength of the mortar, along with cracks that can occur due to weathering, mechanical actions, and/or chemical reactions, which lead to the formation of higher-molar-volume compounds (developing internal stress that can lead to cracking). By these means, carbon dioxide of a much younger age than the building can appear in a proportion determined by the depth profile toward the building surface. _Interconnected capillary pores_ contribute to water and air/CO\({}_{2}\) transfer, with their specific formation making a positive contribution to the strengthening of the mortar over time and to its self-regeneration [4]. In lime-based mortars, pores smaller than 0.1 \(\upmu\)m are present in limited amounts and can be identified in hydrated phases. Intermediate-sized pores, of around 50 \(\upmu\)m, can be formed between the binder and the aggregate, though decreasing mortar strength. The carbonation reaction of Ca(OH)\({}_{2}\) in mortars is possible if a free pathway exists for water and CO\({}_{2}\) to coexist at the carbonation site. The consequence for ancient lime mortars is that the radiocarbon dating result may be highly dependent on the carbonation depth, the weather conditions, and the air/water/CO\({}_{2}\) mass transfer within a given mortar's system of pores. The age of carbonation can vary with depth and position (due to the porosity features of a given volume). Also, low amounts of contaminating carbon-bearing compounds (older/younger \({}^{14}\)C in organic or inorganic compounds with different radiocarbon ages) can be attributed to complex interactions over time: chemical and biochemical degradation, meteoric waters, human impact, etc. [21; 22]. With increasing age, the microstructure become denser due to calcium carbonate particles increasing in number and size, thus occupying some of the pores [23]. The aim of the present article is to reveal all these interactions and C-14-bearing sources by investigating a mortar collected from the archaeological site Ulpia Traiana, Romania. This was achieved by testing the limitations of analytical methods used in the material characterization of whole mineral fractions and of lump fractions of well-documented archaeological remains.