Application of Lignin-Stabilized Silty Soil in Highway Subgrade: A Macroscale Laboratory Study

Abstract

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.