## 3 Results
### ATR-FTIR Analysis  
The samples were also analyzed using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR). Table 3 summarizes the characteristic bands observed in the different samples. For most of the samples (SG_1, SG_3, SG_4, SG_5; SG_6, SG_7 and SG_29), typical gypsum bands were observed, in agreement with the IC results. Indeed, these samples showed high concentrations of sulphates (above 100 ppm), whereas samples SG_2, SG_8, SG_37A and SG_41B were associated with sulphate concentrations below 30 ppm. Based on the combined results of both IC and ATR-FTIR techniques, it is possible to assume that most of the ionic sulphate concentration derives from gypsum formation. Moreover, the characteristic calcite bands were recognized in all samples, which explains the nature of the high calcium concentrations observed in IC analysis. Finally, samples SG_2, \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Sampling**}} & \multicolumn{5}{c}{**Cations**} & \multicolumn{5}{c}{**Anions**} \\ \cline{2-10}  & \multirow{2}{*}{**Samples ID**} & \multirow{2}{*}{**Sodium**} & \multirow{2}{*}{**Ammonium**} & \multirow{2}{*}{**Potassium**} & \multirow{2}{*}{**Magnesium**} & \multirow{2}{*}{**Calcium**} & \multirow{2}{*}{**Chloride**} & \multirow{2}{*}{**Nitrate**} & \multirow{2}{*}{**Sulphate**} \\ \hline \multirow{4}{*}{A} & SG\_6 & 5.98 & n.d. & 1.32 & 2.08 & 326.23 & 7.78 & 65.21 & 730.11 \\  & SG\_7 & 2.01 & 0.17 & 0.35 & 0.93 & 350.20 & 3.13 & 24.84 & 843.13 \\  & SG\_8 & 3.93 & 0.40 & 2.86 & 1.67 & 58.32 & 7.02 & 23.09 & 16.68 \\ \hline \multirow{4}{*}{C} & SG\_4 & 1.05 & 1.09 & n.d. & 0.67 & 369.36 & 1.36 & 14.15 & 922.52 \\  & SG\_5 & 1.83 & n.d. & 0.53 & 2.22 & 97.77 & 2.76 & 16.91 & 123.47 \\  & SG\_29 & 1.94 & n.d. & 1.01 & n.d. & 121.07 & 1.62 & 10.05 & 284.24 \\ \hline \multirow{2}{*}{D} & SG\_3 & 2.54 & n.d. & n.d. & 0.71 & 483.05 & 4.15 & 6.12 & 1256.41 \\ \hline \multirow{2}{*}{E} & SG\_1 & 9.02 & 0.48 & 6.34 & 6.11 & 601.09 & 9.12 & 198.78 & 1475.08 \\  & SG\_2 & 5.22 & 0.40 & 2.51 & 2.83 & 68.80 & 10.71 & 74.89 & 12.14 \\ \hline \multirow{2}{*}{Vault} & SG\_37A & 3.13 & 2.20 & 8.90 & n.d. & 3.86 & 4.01 & 14.20 & 27.56 \\ \cline{1-1}  & SG\_41B & 2.37 & 1.11 & 0.44 & 2.58 & 60.84 & 5.06 & 11.86 & 26.72 \\ \hline \end{tabular} n.d. = not detected. Standard error = 5%.  
\end{table} 
Table 2: Concentrations of cations (sodium, ammonium, potassium, magnesium and calcium) and anions (chloride, nitrate and sulphate) in ppm (ug g\({}^{-1}\)) in the analyzed samples. In fact, sample SG_1 shows a good concentration trend between calcium and sulphate ions and lower concentrations of the other ions (Figure 2). This observation allowed us to hypothesize that the area under investigation is affected by the sulphation process, which leads to the formation of newly formed crystalline gypsum in the form of efflorescence. This hypothesis is confirmed when observing the ATR-FTIR spectrum acquired for this sample (Figure 3, Table 3). The characteristic peaks of the gypsum centered at 3522, 3400, 1683, 1620, 1108, 667, 597 and 418 cm\({}^{-1}\) can all be observed [18,19]. In addition, O-H stretching and bending vibrations of calcium carbonate were found, with peaks at 1412, 872 and 712 cm\({}^{-1}\)[19,20]. These peaks can also be attributable to newly formed carbonate salts and not to the substrate, since the samples were taken at salt efflorescence. In contrast, sample SG_2 shows low concentrations of these ions; in particular, sulphate ions (Figure 2). In fact, the typical bands of gypsum were not observed (Figure 3, Table 3). Sample SG_2, on the other hand, shows the classic carbonate peaks (Figure 3, Table 3), which can be attributable to efflorescence produced by carbonate salts.