## 2 Materials and Methods
### Methods  
Samples of the mixtures were manufactured, either unstabilized or stabilized with CEM at two different dosages of 2.5% or 5% by mass, as these are the two most commonly used dosages in SRE [23]. The combinations were identified as follows: first, the type of mixture used (M1, M2 or M3) was indicated, followed by the amount of CEM added in each case. \"C0\" was used when no CEM was added, \"C2.5\" when stabilized with 2.5% CEM, and \"C5\" when 5% CEM was used. Table 4 shows the identification of the combinations, their mixture compositions and their stabilizer dosages. The manufacturing, curing, and testing conditions were carried out following the new methodology to characterize SRE materials explained in [23]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Mixture (\% Dry Mass)**} & \multicolumn{2}{c}{**Stabilizer**} \\  & **Gravel** & **Sandy Gravel** & **Sand** & **Sludge** & **Clayey Soil** & **CEM (\%)** \\ \hline M1C0 & - & - & - & - & 100 & - \\ M1C2.5 & - & - & - & - & 100 & 2.5 \\ M1C5 & - & - & - & - & 100 & 5 \\ \hline M2C0 & 17.9 & 26.4 & 31.8 & - & 23.9 & - \\ M2C2.5 & 17.9 & 26.4 & 31.8 & - & 23.9 & 2.5 \\ M2C5 & 17.9 & 26.4 & 31.8 & - & 23.9 & 5 \\ \hline M3C0 & 17.9 & 26.4 & 31.8 & 23.9 & - & - \\ M3C2.5 & 17.9 & 26.4 & 31.8 & 23.9 & - & 2.5 \\ M3C5 & 17.9 & 26.4 & 31.8 & 23.9 & - & 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Identification of the combinations, mixture compositions and stabilizer dosages. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Sludge** & **Clayey Soil** \\ \hline LL (\%) & 25.0 & 38.4 \\ PL (\%) & 21.2 & 22.8 \\ PI (\%) & 3.8 & 15.6 \\ \hline USCS & ML & CL \\ \hline \hline \end{tabular}
\end{table} 
Table 2: LL, PL, PI and USCS of the sludge and the clayey soil. \begin{table}
\begin{tabular}{c c} \hline \hline  & **CEM** \\ \hline Bulk Density (g/cm\({}^{3}\)) & 1.08 \\ \hline Oxides (\%) & 14.1 \\ CaO & 70.8 \\ Fe\({}_{2}\)O\({}_{3}\) & 4.1 \\ Al\({}_{2}\)O\({}_{3}\) & 3.4 \\ SO\({}_{3}\) & 4.2 \\ MgO & 1.2 \\ Others & 2.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Bulk density and chemical properties of CEM. The samples were prepared as follows. First, impurities in the sand were removed by passing it through a 1 mm sieve, discarding any material retained. All materials were then oven-dried at 60 \({}^{\circ}\)C until they reached a constant weight (i.e., less than 0.1% mass variation). Next, the mixing process was carried out. This step was not necessary for mix M1, but for mixes M2 and M3, the required amounts of each material were weighed in their dry state and mixed for one minute until homogenized. To ensure M1 soil sample representativeness, strategic sampling points were selected to uniformly cover the entire study area, considering the natural variations in the terrain (such as slopes, land use types, and vegetation). Additionally, an adequate volume was collected to ensure homogeneity and avoid biases. Afterward, water was added according to the OMC of each combination. The OMC was determined by performing an SP test on each mixture following the UNE 103500:1994 Standard [30], and the results are presented in Section 3.1. Once water was added, the mix was blended for five minutes to ensure complete homogenization. The homogeneous mixture was then compacted into three layers using a 2.5 kg manual rammer with a 102 mm diameter in a 122 mm high mold (the same mold and rammer used in the SP test). After demolding, the specimens were wrapped in plastic to prevent dehydration and ensure proper cement hydration. Before each test, the specimens were dried in an oven at 60 \({}^{\circ}\)C until they reached a constant weight (i.e., less than 0.1% mass variation) to homogenize their moisture content. This temperature was selected to prevent the loss of non-evaporable water through chemical reactions (cation exchange, flocculation, and pozzolanic reactions). UCS tests were performed on three samples of all combinations at 7, 28, and 90 days, following the UNE-EN 13286-41:2022 standard [31].