Lignin is a natural copolymer found in plants that is a major waste from bioenergy and paper industries. In this study, comprehensive laboratory testing programs were carried out to evaluate the strength improvements and water resistance of soil after being stabilized by lignin and
polymerized lignin. The chemical polymerization of pre-treated lignin involves a process of linking short lignin molecules to form longer ones and consuming hydrophilic functional groups to improve its stability under wet conditions. Various testing combinations were considered,
including four lignin or polymerized lignin contents (0, 6, 8, and 10 %) and four lime contents (0,2, 4, and 6 %). The results show that lignin-stabilized soil will take at least 14 days to reach maximum strength, whereas the minimum curing period is 21 days for soil stabilized with
polymerized lignin. The optimum dosage is 8 % when lignin or polymerized lignin is used for soil stabilization. Polymerized lignin can provide approximately 10 to 20 % more strength improvement than lignin; however, lime seems work better with lignin than polymerized lignin when added as a strength booster. The water soaking tests showed that polymerized lignin has better moisture resistance than lignin, and additional lime can further improve the performance of stabilized soil with respect to water susceptibility. However, the strength of specimens cannot fully recover, decreasing slightly after each dry–wet cycle for both specimens treated by lignin and polymerized lignin. The total decrease is about 5∼10 % after two cycles. The temperature and pH value control and oxygen supply approach used in this study will be an important reference for future studies.