## 3 Results and Discussion
### XPS Data Matrix
In fact, even considering the variability range that goes beyond the energy step (+ / -0.1 eV) only reported in Table 3, the BEs listed therein record the concomitant increase for the two constituents, up to their maximum values, 348.0 eV(Ca2+) and  
Figure 7: PCA applied to the XPS dataset of Table 3: (**a**) scree plot showing the percentage of variance explained by each principal component; (**b**) score plot of the samples in the PC1–PC2 space, highlighting clustering trends; (**c**) loading plot indicating the contributions of the original chemical variables to PC1 and PC2. 290.2 eV(CO\({}_{3}\)\"), right in the time interval of the two clusters and the subsequent decrease, although not always to the same extent, for the samples collected from the cubic walls aged 52 months and the old farmhouse. The variations in the binding energies of the carbonate constituents and in their ratio are clearly conditioned by the temporal impact of the abiotic and biotic agents on the calcarenitic walls, monitored in this work by their XPS characterization; therefore, they could be regarded as an integrative part of the set of degradation trend indicators reported in Figure 6, as hereafter summarized. To start with, the weathering susceptibility of carbonates to dissolve and form Ca(HCO\({}_{3}\))\({}_{2}\) is mainly responsible for the recurrent Ca\({}^{2+}\) < CO\({}_{3}\)\" ratio, most visible in our graphical percentages up to nine months of outdoor exposure. In subcutaneous succession, the reported attachment of autotroph microorganisms takes place and the calcarenite colonization proceeds with the cooperation of different heterotrophic lithotypes mutually interacting for the construction of a protective biofilm, as indirectly testified by the increase in carbonaceous components in the surface layers and directly by SEM, XRD and biological analysis, already mentioned. Among the metabolic routes of the bio-colonizing communities are those responsible for dissolution by Ca\({}^{2+}\) complexation and those of reprecipitation, the reprecipitate CaCO\({}_{3}\) clearly structurally different from the pristine calcite and thus presumably of different BEs [10; 11; 12; 27; 28]. From the graphical trend of Figure 6 and data matrix of Table 3, the time required for the adhering biofilm to consolidate onto calcarenite and then structurally evolve for the living of cohabitant microorganisms apparently ranges around two years or less, judging from the BE values of the samples collected at twelve and twenty-one months, further supported by the better equivalence of Ca\({}^{2+}\)CO\({}_{3}\)\" percentage of the last, likely due to the reprecipitation of calcium carbonate, contributing in the outermost layers. Considering all the points paid attention to so far, the main features that characterize long-exposure calcarenitic surfaces and that most capture attention in Figure 6 are the following:  
* Carbonaceous C-C added to functionalized carbons (COx/CN) are the prevalent components of the surface layers, closely followed by Al-silicate groups in second place, even with the inclusion of M-oxides placed at the lowest percentage, if considered as 'intricate' parts of the silicate framework [30]. * Contrary to the bulk composition of calcarenite and differently from the surface composition 0R, in the highly degraded surfaces, the carbonates are the minor components, confirming the negligible degradation of the silicates, which become the major components, most probably ensuring with their solidity the surface adhesion required for the formation of a structured biofilm (with trapped pollutants) for the protection of the living microorganisms [10; 11; 12]. * The non-equivalent intensity of the carbonate constituents, visible on long-exposed surfaces, may contribute carboxylic anions, belonging to extracellular polymeric substances (EPS) produced by microorganisms and bindingCa\({}^{2+}\)[27; 28].