## 1 Introduction  
The presence of silty and clayey soils is a critical factor not to underestimate during the construction of roads and transport infrastructures in general. This is because their behavior is vulnerable to changes in moistness. Specifically, clays are characterized by a certain activity, which is their capacity of modifying their physical properties and their bearing capacity, depending on the variable water content as a consequence of seasonal water fluctuations [1]. When clayey materials are necessary, lime treatment stabilization allows us to use them. Lime is the stabilizing agent able to improve the strength and stiffness properties of sharply soils, reducing their swell-shrink potential. It causes finely divided clay particles to agglomerate into coarser particles that improve load-bearing properties, and the lime-treated soil hardens by chemical reaction. Lime can be added as calcium oxide (quicklime, CaO) or hydrate lime Ca(OH)\({}_{2}\); however, stabilization with quicklime appears more effective than hydrated lime [2]. Its addition to natural soils allows:  
* the reduction of the moisture content [3];* the cation exchange between clay particles and calcium ions of the lime and, thus, the start of the flocculation process, which allows a size distribution change [4];
* the reduction in the plasticity index (PI) through a modification of Atterberg's limits (i.e., measure of the key values of moisture content of a fine-grained soil: Its shrinkage limit, plastic limit, and liquid limit) [5];
* the development of pozzolanic, cementing, and carbonation reactions [6]. Concerning compaction, there is an increase in the optimum moisture content (OMC) for compaction in situ: Clays are generally moist, and the increased OMC shifts close to the natural moisture content (NMC) [7; 8; 9]. It is important to know the correct percentage of lime to add, distinguishing between the initial consumption of lime (ICL) and lime stabilization optimum (LSO). These are two specific percentages able to modify and stabilize natural clayey soils, respectively. ICL is the minimum percentage necessary to satisfy the cation exchange reactions [10]: According to the standard ASTM C977-18 [11], it should be at least 1.5% in order to lead the mixture at pH 12.4, when soils are completely modified [12; 13]. Percentages of lime above ICL result in pozzolanic reactions: At LSO, the soil reaches the maximum value of unconfined compressive strength. This is due to the formation of hydrate substances that stabilize the material and guarantee the improvement in the mixture's performance. They are the results of long-term pozzolanic reactions [14]. During this process, strong alkaline conditions (pH = 12.45) induce the silica and alumina of clay minerals to combine together with calcium of the added lime, forming new cementitious products: Calcium silicate hydrates (CSHs), calcium aluminate hydrates (CAHs), and calcium alumino-silicate hydrates (CASHs) [15; 16; 17]. Laboratory studies of lime-treated soils end with some appraisals about Atterberg's limits [5], linear swelling, and unconfined compressive strength [18; 19]. Finally, appropriate tests during the in situ roadworks need, e.g., the correct value of lime released by the lime-spreader, the water content, the plate loading test, and the density in situ, for the best compaction [20] in order to confirm the laboratory results [21; 22]. This paper focuses on the results obtained from X-ray diffraction (XRD) tests and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS) analyses used to investigate physical, chemical, and cementitious changes on lime-soil mixtures. Both methodologies are nondestructive techniques to characterize materials [23; 24; 25]: XRD provides information on structures, phases, preferred crystal orientations (texture), and other structural parameters, while energy-dispersive X-ray spectroscopy (EDS) works together with SEM to provide qualitative and semi-quantitative results.