# Investigations on an Ancient Mortar from Ulpia Traiana Sarmizegetusa Archaeological Site, Romania
## 3 Results
### Microstructural Analysis  
The microstructure was characterized by SEM analysis. Figure 5 shows a highly inhomogeneous microstructure, with both large and small--and, probably, interconnected--pores and cracks, grains of various sizes, and entrapped air bubbles. Figure 6 shows a large collection (a nest) of calcite grains with sizes less than 10 microns, which most probably resulted from the carbonation of a relatively big Ca(OH)\({}_{2}\)/(CH) site. Figure 5: Inhomogeneous morphology of the sample surface captured by SEM (magnification 100\(\times\)). Although small, calcite grains are, most of them, well developed with well-defined grain boundaries. The nest is surrounded by a dense and compact surface layer that could be a biofilm. These observations reinforce the conclusions from Section 2.1, excluding the presence of calcareous concretions in the mortar and even more so in the lumps (S5 calcareous concretions), as defined in the Section 1. A very small (less than 150 \(\upmu\)m in size) silica grain was identified in the fracture, partially connected to the binder matrix, which contains mostly Ca and Si (as Al accounts for only 2.8%), distributed homogeneously--see Figure 7.  
Figure 8 presents an albite grain with partially filled channels on its surface; channels seem to be oriented, almost completely, in diagonal directions. EDAX maps confirm the existence of Na, Al, and Si oxides and spectral analysis confirms the existence of feldspar, identified in XRD (Figure 4). Figure 6: Nest of well-faceted CaCO\({}_{3}\) grains, typically less than 10 \(\upmu\)m, captured by SEM (magnification 2400\(\times\)). Figure 7: Silica grain, magnification 1200\(\times\) (confirmed by EDAX maps). The existence of a calcium and phosphorus compound (Ca-P compound) is evidenced in Figure 9 along with variously sized pores, CaCO\({}_{3}\) lumps, quartz grains, and large areas of binder matrix. The Ca-P compound presents interconnected pores and is well embedded in the binder matrix. Its surface is covered with very small, probably calcium carbonate nodules and may represent a microfragment of bone, as suggested by literature data [32]. The submicronic, spherical, and uniformly sized CaCO\({}_{3}\) grains identifiable in Figure 10 are most probably formed on a C-S(-H) bed (in atomic percentages: Ca--14.55, Si--2.09, Al--0.94). Their shape and size strongly contrast with the rest of the identified compacted, faceted-shaped, carbonate crystals, shown in Figure 6; a possible explanation for the different morphology is that their source may not be the carbonation of Ca(OH)\({}_{2}\). Judging by the small size, a credible hypothesis could be that they were formed by the carbonation of hydrated compounds, such as in S3 C-S-H carbonation, thus providing a proof of carbonation depth [12]. Figure 8: Eroded albite grain (magnification 4000\(\times\)) confirmed by EDAX maps. Figure 9: Ca–P compound showing interconnected channels (confirmed by EDAX maps as inset images)—magnification 500\(\times\). The same submicronic CaCO\({}_{3}\) lumps can also be found on the surface of the gneiss aggregate--Figure 11.  
From SEM-EDAX analysis, it appeared that the microstructure of the mortar is inhomogeneous and crossed by a developed network of cracks, channels, and interconnected pores. It shows areas developed in the entire mass of the sample of calcite, as a result of the mortar hardening in the masonry, but also areas of lumps, clearly defined, which can be  
Figure 11: Submicronic CaCO\({}_{3}\) grains grown on the surface of the aggregate, captured by SEM (magnification 10,000\(\times\)). Figure 10: Layer of submicronic CaCO\({}_{3}\) grains, captured by SEM (magnification 10,000\(\times\)). isolated for further investigations. The latter were most likely formed by the carbonation over time of some 'nests' rich in slaked lime adherent to initial clusters of C-S-H/C-S-A-H hydrates.