Use of Sulfur-Free Lignin as a novel soil additive: A multi-scale experimental investigation

With the rapid process of urbanization, more and more infrastructures are inevitably built on problematic soils, which require improvement and reinforcement. In this study, a novel material Sulfur-Free Lignin (SFL), which is a by-product of bioethanol industry, was used to improve the soil's geotechnical behavior as a sustainable, nontoxic and eco-friendly stabilizer. To systematically investigate the geotechnical properties of SFL-stabilized soil, the natural soil was modified by adding five different contents (3, 7, 10, 12 and 15%) and then a series of geotechnical experiments including Unconfined Compressive Strength (UCS), Atterberg limits tests, electrical resistivity and pH were carried out after 1, 7, 28, 60-days curing. In addition, to investigate the stabilization mechanism, the mineral composition, function group and microstructure characteristic were also studied through X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Mercury Intrusion Porosity (MIP) and Scanning Electron Microscopy (SEM) tests. The test results demonstrate that with increasing SFL content, the electrical resistivity gradually decreases, on the contrary, the Atterberg limits show a slight increase. However, the pH values are unchanged indicating the SFL would not lead to pH contamination risk. The 10% SFL-stabilized soil after 60 days of curing shows the greatest strength, which increases about 600% compared to the natural soil. Furthermore, the relationship between stiffness and UCS is generally proportional. According to the results of MIP and SEM, the strength improvement can be attributed to a smaller total volume of pores, reduction in the mean size of the pores and stronger bonds formed by SFL between isolated grains. The results of XRD and FTIR tests reveal there is no new mineral nor function group generated after adding SFL. Above all, the natural soil stabilized by SFL shows a satisfactory engineering performance and it would be a win-win solution to utilize SFL as a soil stabilizer for soil improvement in civil engineering and for waste elimination in bioethanol industry.