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To improve the engineering performance of high-liquid-limit soil, lignin, a by-product of the papermaking industry, was used. First, the influence of lignin content and curing age on the physicochemical and mechanical properties of the improved soil was determined by carrying out pH, Atterberg limits, heavy compaction, unconfined compressive strength (UCS), California bearing ratio (CBR), and resilience modulus tests. Secondly, microscopic images obtained by scanning electron microscopy (SEM) were analyzed. The characteristic and evolution rules of the microstructure were expounded for the improved soil. Finally, combining SEM with energy-dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) analysis, the mechanism of lignin improvement on high-liquid-limit clay is discussed from the perspective of molecular structure. The results showed that the pH value of lignin-improved soil was much lower than that of quicklime-improved soil, which were 7.0 and 11.7, respectively. When the lignin content was 3%, the dry density and mechanical indexes (UCS, CBR, and resilience modulus) of the improved soil all showed the maximum values. From the perspective of microstructure, the connection between soil particles was strengthened through the wrapping and filling of the flocculent cementing materials produced by lignin. The improvement mechanism of lignin on soil was the combined result of ion exchange, hydrogen bonding, covalent bonding, and electrostatic attraction. [ABSTRACT FROM AUTHOR]