Lignin is an industrial by-product, stockpiles of which are rapidly increasing worldwide due to growing demand. Highway subgrade construction has been identified as one of the viable answers to consume huge quantities of lignin as an environmentally friendly, low-cost, and less energy intensive chemical additive for soil stabilization. This paper presents a systematical laboratory investigation on physical, geomechanical, and microstructural characteristics of lignin-stabilized silty soils. A traditional soil stabilizer, quicklime, was selected as a reference binder for comparison purposes. A series of macroscale laboratory tests were conducted to examine the Atterberg limits, particle size distribution, compaction behaviors, unconfined compressive strength, California Bearing Ratio, and resilient modulus of lignin-stabilized soils. In addition, soil pH, scanning electron microscopy, mercury intrusion porosimetry, X-ray diffraction, and Fourier transform infrared resonance analyses were carried out to explore the mechanisms controlling the changes in engineering properties of lignin-stabilized silty soil. The study reveals that the level of lignin content has a considerable influence on the mechanical properties, particle size distribution, and pore volume of the stabilized silty soil. The optimum lignin content for silty soil is approximately 12% in this study. With the same curing time and compaction conditions, 12% lignin-stabilized silty soil possesses superior mechanical performances compared with the 8% quicklime-stabilized one. The precipitated lignin-based cementing material, which bonds soil particles closely together and fills the pores to produce a more stable soil structure, is the controlling mechanism for the superior mechanical performance of lignin-stabilized soils. Theoretical simulation of the pore size distribution curves demonstrates that the lignin-stabilized silty soil exhibits bimodal type when the lignin content is less than 8%, whereas it displays unimodal type when the lignin content further increases. The reduction in crystalline size and the variation of functional groups for lignin-stabilized soils confirm that electrostatic reaction as well as ionic binding takes place during the process of stabilization.

This paper presents details of a study that deals with determination of engineering properties and microstructural characteristics of a foundation soil (silt) sedimented in Jiangsu Province of China when it is stabilized by lignin-based industrial by-product. A series of laboratory tests were carried out with respect to evaluate the effect of lignin content and curing time on the overall soil properties including Atterberg limits, pH, unconfined compressive strength, stress-strain characteristics, secant modulus, and California bearing ratio. In addition, scanning electron microscopy, X-ray diffraction, and mercury intrusion porosimetry studies were conducted to understand the microstructural characteristics and stabilization mechanism of the stabilized silt. The results reveal that lignin has a great potential to improve engineering properties of silt and shows a promising prospect as a new environmentally friendly soil stabilizer. Curing time and lignin content have significant influence on the basic engineering properties and microstructural characteristics of the lignin stabilized silt. The optimum content of lignin for foundation silt in Jiangsu Province of China is approximately 12%. The precipitated cementing material is formed after stabilization of lignin with a period of curing. The stabilized silt switches over its response from a brittle to ductile material in the presence of lignin. Peak analysis results of the pore-size distribution curves demonstrates that the lignin stabilized silt exhibits bimodal behavior when the lignin content less than 8%, whereas it displays unimodal type when the lignin content is more than or equal to 8%. These observations provide enhanced understanding of lignin-based industrial by-product as a soil stabilizer at the foundation construction.

Lignin is an organic industrial by-product, stockpiles of which are rapidly accumulating worldwide. A feasibility investigation was carried out with respect to the stabilization of silty soils using lignin. The fall cone test method was employed to determine the undrained shear strength ($s_u$ ) and Atterberg limits of the lignin-stabilized silty soil with 7 days of curing. In addition, scanning electron microscopy (SEM) was conducted to qualitatively evaluate the changes in the microstructure of silty soil after treatment and better understand the mechanisms controlling the improvement in $s_u$ of the stabilized silty soil. The study revealed that the $s_u$ of silty soil increased with an increase in lignin content. This was mainly attributed to the bonding effect of lignin-based cementing materials, which creates the aggregation of soil particles and fills the pores between detached particles. The liquid limit ($w_L$ ) increased remarkably with increasing lignin content and the maxium increment of 27% occurred at 12% lignin-stabilized soil. The chemical composition of added lignin had a considerable influence on the Atterberg limits of the stabilized silty soil. The $s_u$ was found to exhibit good exponential decreasing correlation with the moisture content ($w$ ) as well as the liquidity index ($I_L$ ). By comparing the $s_u$ values with the empirical models in the literature, the new relationships among $s_u$ , $w$ , and liquidity index ($I_L$ ) were finally proposed for estimating the $s_u$ value of the lignin-stabilized soils. It is concluded that the application of reclaimed lignin as a silty soil stabilizer can be one of many viable answers to the development of environmentally friendly soil stabilizer and the profitability of industrial by-products.