## 2 Materials and Methods Considered in the Research
### Methods of Experimental Research  
In the first stage of the conducted experimental research, each of the considered stabilisers was added to the natural clayey soil with three different percentages of share in relation to the dry mass of the soil. Such laboratory-prepared samples were in accordance with the relevant national codes harmonised with the European Norms (EN) with respect to the uniaxial compressive strength (UCS) test (the code [33] ensuring optimal moisture content and compaction using the Proctor test with 600 kNm/m\({}^{3}\) energy). The samples were stored in plastic foil until the corresponding test day (3, 7, and 28 days after the treatment) to evaluate the long-term effectiveness of chemical stabilisation of the clayey soil. Along with the modification of the physical-mechanical properties, this study also examined the changes in the chemical properties of the treated soil, specifically the variations in pH value with analyses conducted 24 h, 3 days, and 28 days after the treatment using a soil-to-water ratio of 1:2.5. The testing procedure involved mixing 10 g of dry soil with 25 mL of distilled water for 10 min. After achieving a clear solution around 30 min later, the pH value was measured by immersing electrodes into the solution. Within this stage of the investigation, research control was ensured in such a way that each result (UCS and pH value) represents the mean value of three tested samples for each of the seven stabilisers used, conventional (lime and cement), waste (fly ash and rock flour), and alternative (ferric chloride, calcium chloride, and potassium chloride), for each of the three considered contents of stabilisers and for each of the three considered time intervals after the chemical treatment. This means that a total of 378 tests were performed in order to obtain 126 reliable results. Based on the initial tests, for each of the considered reagents, the optimal content of stabilisers in the mixture with clayey soil was determined. Within the second stage of experimental research, subsequent tests were then conducted on samples with such determined optimal stabiliser content to assess changes in the Arterberg's limits, modulus of compressibility (M\({}_{v}\), E\({}_{\text{ood}}\)), California bearing ratio (CBR), and soil swelling (s). The Arterberg's limits were determined according to the code [34]. In the oedometer test (in accordance with the code [35]), the modulus of compressibility was evaluated using samples of 20 mm in height and 70 mm in diameter, subjected to a maximum load of 400 kPa after 24 h of saturation. The CBR value, obtained by compaction of samples with optimal moisture content determined by the Proctor test, was also analysed (based on the code [36]). Prior to CBR testing, the samples were immersed in water for 96 h, and soil swelling was measured. Oedometer tests were conducted after 3, 7, and 28 days of chemical stabilisation, whereas CBR tests were carried out 7 and 28 days after the addition of the stabiliser. As it was the case in the first stage, here again the control of the research was provided in such a way that each result (Aterberg's limits, E\({}_{\text{ood}}\), CBR, and swelling) represents the mean value of three tested samples for each of the seven stabilisers studied with their optimal content and for each of the considered time intervals after the chemical treatment, indicating that a total of 210 tests were performed to obtain 70 reliable results. The above-described experimental research and corresponding tests were conducted at the Laboratory for Geotechnics of the Faculty of Civil Engineering and Architecture of the University of Nis. The specified mix design table (Table 2) summarises the mixture specimens with the proposed stabilising materials and the corresponding percentage shares considered in the study. In this research, the examined specimens, treated with the selected chemical stabilisers, were prepared with an optimal moisture content of 18.5%, which was obtained by Proctor test for untreated soil.