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.

The reinforcement technology of carbonation based on reactive magnesia (MgO) and carbon dioxide (CO₂) is a low-carbon and high-efficiency foundation treatment method. This paper investigates the compaction, mechanical and microstructural characteristics of carbonated reactive MgO-stabilized silt with varying MgO-soil ratios, carbonation time and water-soil ratios. The results indicate that the maximum dry density of uncarbonated reactive MgO-stabilized silt increases while the optimum moisture content reduces compared to the parent soil. The unconfined compressive strength of reactive MgO-stabilized soil was found to have increased after CO2 carbonation for several hours. With increasing MgO-soil ratio and carbonation time, the failure mode changes from elasticplastic to brittleness, and the failure strain of carbonated specimens mainly ranges between 0.8% and 1.6% and the ratio of the deformation modulus to unconfined compressive strength is about 30 to 200. The water-soil ratio has a slight influence on the evolution of strength. Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analyses have indicated that the carbonation products facilitate the strength growth of reactive MgO-stabilized silt. Thermogravimetric analysis (TGA) shows that CO₂ uptake increases with increasing carbonation time and achieves the highest under the MgO-soil ratio of 20%, carbonation time of 6 h, and water-soil ratio of 25%. The stabilization mechanism of carbonated reactive MgO-stabilized silt is proposed according to microstructure analyses, providing a deeper understanding of the application of the reactive MgO carbonation technology in the ground reinforcement.

In this study, electrical resistivity was related to the fundamental geotechnical properties of cement solidified/stabilized heavy metal–contaminated soils. Lead-contaminated soils were artificially prepared with various concentrations and treated by portland cement Type I at three content levels. The compacted cylindrical specimen was prepared with a diameter of 50 mm and height of 100 mm. The apparent electrical resistivity, electrical conductivity of pore fluid, unconfined compressive strength, and water contents were measured at different curing times. The result shows that the apparent electrical resistivity increases with the increase of curing time and the decrease of porosity and saturation degree as a result of cement hydration development. The effects of lead concentration, cement content, and curing time on the apparent electrical resistivity were discussed. The cementation index m was used to evaluate the cementation degree of cement-treated lead-contaminated soils. Lead pollutant at high conc...