## 2 Literature Review
### Lignin Utilization for Road Subgrades Stabilisation
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**AASHTO Soil Classification** & \begin{tabular}{c} **Maximum Dry Unit** \\ **Weight (\(\mathbf{Y_{d, max}}\))** \\ **(kg/m3)** \\ \end{tabular} & **Main Properties of the Stabilizing Agent** & \begin{tabular}{c} **Additive** \\ **Content (\%)** \\ \end{tabular} & **References** \\ \hline \hline A-2 & 2260 & Lignosilonate; powder & 2.9, 5 (*), 8 & [53] \\ \cline{2-5}  & – & Lignin; by-product of a paper mill; powder & 3, 6 (*), 9, 12, 15 & [54] \\ \hline \hline \multirow{5}{*}{A-7} & 1631–1839 & \begin{tabular}{c} 5\% lignin + 50\% hemicellulose + 20\% \\ cellulose + 25\% other components; \\ by-product of ethanol production (corn) \\ \end{tabular} & 12 & [55] \\ \cline{2-5}  & 1720 & \begin{tabular}{c} 80\% lignin + 10\% cellulose + impurities; \\ by-product of the paper manufacturing \\ industry; powder \\ \end{tabular} & 2, 5, 8, 12 (*), 15 & [56] \\ \cline{2-5}  & 1720 & \begin{tabular}{c} 60–70\% lignin + water + other components; \\ by-product of commercial biomass \\ conversion; powder \\ \end{tabular} & 2, 5, 8, 12 (*), 15 & [57] \\ \cline{2-5}  & 1720 & Lignin; by-product from a paper mill; powder & 8, 12 (*) & [58] \\ \cline{2-5}  & 1631 & \begin{tabular}{c} 5\% lignin + 25\% water + 5–10\% gases + 4\% \\ char + 35–41\% aldehydes; by-product of \\ commercial biomass conversion; liquid \\ \end{tabular} & 12 & \\ \cline{2-5}  & 1631 & \begin{tabular}{c} 5\% lignin + 50\% hemicellulose + 20\% \\ cellulose + other components; by-product of \\ ethanol production (corn); powder \\ \end{tabular} & 12 & [59] \\ \cline{2-5}  & 1631 & \begin{tabular}{c} 40\% lignin-derived phenolic oligomers + 20\% \\ water + 40\% other components; liquid \\ \end{tabular} & 12 & \\ \cline{2-5}  & 1720 & \begin{tabular}{c} Lignin; by-product of the the paper \\ manufacturing industry; powder \\ \end{tabular} & 2, 5, 8, 12 (*), 15 & [60] \\ \cline{2-5}  & 1720 & \begin{tabular}{c} Lignin; by-product of the paper \\ manufacturing industry; powder \\ \end{tabular} & 2, 5, 8, 12 & [61] \\ \cline{2-5}  & 1720 & \begin{tabular}{c} Lignin; by-product of the paper \\ manufacturing industry; powder \\ \end{tabular} & 2, 5, 8, 12 & [62] \\ \cline{2-5}  & - & Lignosulfonate; co-product of the paper pulp \\ industry; liquid & 2.5 & [62] \\ \hline \hline \multirow{5}{*}{A-6} & 1691 & \begin{tabular}{c} 25\% lignin + 25\% water; by-product of \\ commercial biomass conversion (agricultural \\ residues); liquid \\ \end{tabular} & 12 & \\ \cline{2-5}  & 1691 & \
\begin{tabular}{c} 5\% lignin + 50\% hemicellulose + 20\% \\ cellulose + other components; by-product of \\ ethanol production (corn); powder \\ \end{tabular} & 12 & \\ \cline{2-5}  & 1693 &
\begin{tabular}{c} 25\% lignin + 25\% water + 5–10\% gases + 4\% \\ char + 35–41\% aldehydes; by-product of \\ commercial biomass conversion (forest and \\ agricultural residues); liquid \\ \end{tabular} & 1, 3, 6, 12 (*), 15 & [64] \\ \cline{2-5}  & 1858 & Lignosilonate; powder & 3.37, 5 (*), 8 & [65] \\ \hline \hline \multirow{5}{*}{A-7} & 1100–1400 & Lignin; by-product from a paper mill; liquid & 1, 2, 3, 4, 5 & [66] \\ \cline{2-5}  & 1420–1690 & Lignosilonate; powder & 0.5, 1, 2, 4 & [67] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A review of studies concerning the application of lignin as a stabilizing agent for soil subgrade in road construction. an increase in curing time. Therefore, the highest CBR value was recorded at 15 days of curing for the 12% lignin stabilized silt (CBR = 124.0%). The same authors [60] underlined that CBR values depend on the degree of compaction, too. In particular, higher CBR values can be observed increasing the degree of compaction from 94% to 96%. The analysis of the CBR values as a function of the additive content showed that the highest value can be recorded for a 12% stabilized soil at 28 days of curing and 96% degree of compaction, whereas lower values are measured for an additive dosage of 15%.