## 4 Conclusions  
This work reports the five-year XPS monitoring of two calcarenite buildings, a newly installed cubic block and an adjacent old farmhouse, exposed to the outside, with the aim of clarifying the chemical indicators and the degradation pathways of calcarenite from the initial to the advanced stages. The interpretation of the large dataset obtained with XPS temporal monitoring was aided by the comparison of the complementary results provided in the first two years of the project using combined techniques, some reported in Supplementary Figure S3, and by referring to the most recent acquisitions of the literature dedicated to the monumental heritage cited in this work. Another important remark regards the support of chemometric methods--in this case, the unsupervised PCA exploration of the large XPS dataset. The trend of calcarenite degradation fully reported in Figure 6, starting from the reference calcarenite and projected through the sampling sequence up to 52 months, towards the surface state of the ancient farmhouse finds exactly the same temporal correspondence in both PCA graphs, score and loading plot, displayed together and jointly interpretable in Figure 7.  
The decay indicators identified by XPS, using the well-established curve-fitting procedure, can therefore validly represent the surface composition of the external calcarenite monuments and their variations over time under the given conditions. In particular, in our rural site, having limited the sampling to the upper areas of the two buildings not affected by soil interference, spontaneous vegetation growth and other large-scale invaders, the close similarities in their surface compositions may signify that the calcarenite monumental heritages, located outdoors in restricted and controlled areas, locally affected by unavoidable meteorological factors, pollutants and biological attacks, similarly deteriorate and can be monitored by surface analysis using the percentage variation of the derived indicators, referring to unexposed calcarenite, to trace the state of deterioration and plan subsequent interventions. The surface percentage of indicators could also be indirectly indicative of concomitant processes occurring below the surface, to be directly clearly confirmed by other suitable techniques with proper analytical depths. For example, the comparison of EDS composition in deeper layers (see in Figure S3 the combined microanalysis of the samples from the new building walls after one year of exposure and the bulk calcarenite) with the XPS composition reported in Table 3 for the same walls at 12 months,12M, even considering the different lateral resolutions of the two techniques, may provide further clues on the degradation processes. In fact, taking into account the presence of microorganisms at safer internal depths, shown by their hyphae propagation with SEM images and of the segregation of minerals revealed by EDS microanalysis, it looks like the increased content of whatever components below the surfaces correspond to their depletion on the surfaces, as, for example, encountered with 'extra cations/anions' no longer visible at the surface after the first months of exposure and/or mixed oxides strongly reduced over time. In summary, despite the proximity of the industrial area to the north west with the 'Fenice' incinerator (perhaps not fully operational at the time, given the microscopic evidence of lichen proliferation, sensitive to high pollution) from the observation of the graphic trend of Figure 6, two years seems to be the critical time period after which the interactive abiotic and biotic phases and the adaptation of the pioneer colonizing microorganisms converge towards the completion of well-structured biofilms and are strongly adherent to the calcarenitic surfaces and interfaces, necessary for the survival and growth of the bio-community with the arrival of new microorganisms, all cooperating against the adverse factors affecting the monumental heritage exposed outdoors.