## 4 Conclusions  
When a lime treatment of soil is carried out, it is necessary to verify if the roadworks comply with the procedures defined during design and laboratory tests. Therefore, the calcium content as consequence of the added CaO could be useful to monitor the treatment works. Three goals should be pursued in order to define a parameter and the method to calculate it: Rapid procedure of evaluation, method suitable for before/after analysis, and bi-univocal correspondence between the CaO content and the identified parameter. Due to their execution rapidity, XRD tests and SEM-EDS analyses have been conducted on a clayey natural and treated soil. X-ray diffraction (XRD) is a powerful nondestructive technique for characterizing crystalline materials. It provides information on structures, phases, preferred crystal orientations (texture), and other structural parameters, such as average grain size, crystallinity, strain, and crystal defects. XRD tests on TQ0 allowed the identification of its composition: Its mineralogical phases are calcite, illite, quartz, kaolinite, and chlorite-serpentine mixed layer. XRD tests at 7 and 28 d mellow periods on TQ3 and TQ5 were used to verify the successful addition of the stabilizing element, through intensity variation and the \"birth\" of new diffraction peaks (i.e., mineralogical phases). These tests allowed the development of chemical reactions in the treated soils to be monitored over time and to verify a qualitative difference between the addition of 3% and 5% of quicklime. Therefore, for the examined natural soil, the first percentage could be considered the percentage able to improve the mixture (ICL) and the second one the quantity able to stabilize it (LSO). EDS analyses allowed the hypothesis of the chemical composition of the samples studied through XRD tests to be validated, while SEM images allowed the maturation of the stabilized mixture to be investigated, identifying the formation of acicular structures responsible for the performance improvement. They confirmed 5% of CaO as the percentage of quicklime that stabilizes the natural soil. On the other hand, the EDS spectra allowed us to obtain the values of Ca content for different percentages of added quicklime: 16 specimens have been examined for each considered percentage of added quicklime between 0% and 8%. A linear regression curve between added CaO and after-treatment Ca content was obtained: It has a high coefficient of determination (i.e., 0.9805) that confirms the statistical reliability of the proposed approach to evaluate the increase in calcium content due to the added quicklime. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Ca content**} & \multicolumn{8}{c}{**Soil ID**} \\ \cline{2-9}  & **TQ0** & **TQ1** & **TQ2** & **TQ3** & **TQ5** & **TQ6.5** & **TQ8** \\ \hline _er_ & 0.76 & 0.16 & 0.60 & 1.51 & 0.73 & 0.41 & 0.96 \\ Average & 35.45 & 37.31 & 39.54 & 42.86 & 45.28 & 48.96 & 50.35 \\ Minimum & 34.68 & 37.14 & 38.94 & 41.35 & 44.55 & 48.55 & 49.39 \\ Maximum & 36.21 & 37.47 & 44.36 & 44.36 & 46.01 & 49.37 & 51.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: EDS analyses of Ca contents on natural soil and treated specimens. However, further studies with SEM-EDS analyses should be carried out on different treated soils, before and after the quicklime addition, to define a reliable method (e.g., number of specimens to analyze, procedure to prepare specimens, definition of range of application). Indeed, after a preliminary laboratory analysis to identify the linear regression curve between Ca and CaO, the EDS results on the in situ-treated soil could allow verification of the added CaO. Conceptualization, L.M. and A.D.; Data curation, L.M., A.T. and A.D.; Formal analysis, A.T.; Investigation, A.T.; Methodology, S.N. ; Project administration, A.D.; Supervision, L.M. ; Validation, S.N. ; Visualization, L.M. ; Writing--original draft, L.M. and A.T.; Writing--review & editing, L.M. All authors have read and agreed to the published version of the manuscript.