## 4 Results and Discussion
### Mortar Characterization
Samples MV_SS_02 and MV_SS_10 refer to surface layers of internal plaster, and optical microscopy images reveal low-interference colors typical of magnesium mortars and amorphous phases. Inert fractions are not evident, which is consistent with XRPD mineralogical investigations detecting small fractions of aggregate-related phases. Areas with low-interference colors may be attributed to hydromagnesite (a partially carbonated phase of magnesium hydroxide) [2; 33]. The magnesium phase may be separate from the calcite phase due to magnesium hydroxide's slower carbonation rate compared to calcium hydroxide. In sample MV_SS_10, hydromagnesite is visible in subspherical \"ghost\" forms devoid of internal structure (see Figure 4c,d). Observations using electron microscopy (Figure 5) highlighted the nature of the binders used in these mortars. Samples MV_SS_01 and MV_SS_07 are very similar to each other in both microstructure and the chemical composition of the binder matrix and lime nodules. Microchemical analyses using EDS on the matrices reveal high levels of calcium (Ca) associated with magnesium (Mg), silicon (Si), and aluminum (Al). This composition may be related to complex LDH phases formed through hydraulic reactions between lime and reactive clay phases in a highly basic and magnesium-rich environment (both the clay and the lime used may have released magnesium into the system). In particular, the high magnesium content in the matrix may be linked to the use of a slightly magnesian  
Figure 4: Optical microscopy micrographs of representative samples in polarized transmitted light (TL) and crossed nicols. (**a,b**) are MV_SS_01 and MV_SS_02, respectively. (**c,d**) Visible magnesium-enriched areas in the dolomitic lime mortar MV_SS_10 sample: (**c**) thin-section photomicrograph of diffuse brownish patches in the binder, (**d**) photomicrograph of the same sample using a backscattering electron image (BSI). time, but is mainly due to the high content of phyllosilicates, which are visible in the SEM micro-images and recognizable by their lamellar appearance. Microchemical analyses of the lime nodules reveal a calcic chemical composition, indicating the use of aerial lime binder. Moreover, accumulations of M-S-H, magnesium-silicate-hydrates, are evident (Figure 5a, point 1), and are properly recognized in ancient mortars [12; 34; 35; 36; 37; 38; 39]. The formation of M-S-H phases is promoted under certain conditions in high-pH reactive environments characterized by high magnesium activity, through the interaction of magnesium with silica, or, sometimes, also with alumina (M-A-S-H). Although they are not easily detectable by XRPD due to their disordered and paracrystalline structure, they can be diagnosed based on the chemical components present and their relative ratios [40; 41; 42]. Figure 5: SEM-EDS analyses of representative mortar samples. Backscattered electron images (BSI) and chemical analyses (EDS). (**a**) MV_SS_01: (1) lump of M-S-H, magnesium-silicate-hydrate; (2) and (3), microanalyses of the matrix. (**b**) MV_SS_02: (1) and (2), microanalyses of the matrix of the plaster layer. (**c**) MV_SS_10: EDS microanalyses of (1) hydromagnesite “ghost”, (2) lump, (3) carbonate aggregate, (4) calcic carbonate precipitation, (5) magnesite, and (6) matrix. Sample MV_SS_02 (Figure 5b) appears to be a homogeneous layer of fatty mortar with a chemical composition rich in Ca associated with varying proportions of Mg and Si, which is consistent with a mineral composition predominantly comprising 70% calcite, 10% magnesite, and 1.8% hydrotalcite (LDH). The fresco sample (MV_SS_08) exhibits a matrix microstructure that is less compact and a chemical composition mainly composed of Ca associated with Mg, which is attributable to the presence of calcite and hydromagnetic, as identified in the XRPD analysis. Finally, sample MV_SS_10 prominently displays hydromagnesite \"ghosts\" characterized by high Mg content and a typical subspherical appearance devoid of internal structure (Figure 5c) [3].