Abstract: To reveal the stabilization mechanism of lignin based industrial by-product treated soil, the lignin is utilized to improve the silt of subgrade. The unconfined compressive strength, pH value, microstructure, chemical elements, mineral composition and functional groups of lignin and lignin treated silt are tested to evaluate the mechanical properties and pH values of silt after treatment. The difference of microstructure between silt and lignin treated silt is compared. The interactions between lignin and soil are also discussed based on the results of chemical analysis. A possible stabilization mechanism of lignin treated silt is proposed. The experimental results show that the unconfined compressive strength of lignin treated silt increases with the increase of lignin content, while the lignin content exceeds a certain range, the strength of soil exhibits a decreasing trend. The optimum percentage of lignin for silt is approximately 12% and curing time has an important influence on the strength of treated silt. The relationship between unconfined compressive strength and pH value of treated silt is approximately linear. Compared with natural silt, the microstructure of treated silt is more compact and stabler. Soil particles are bonded with cementing materials and no new clay minerals are formed after treatment. The engineering properties of silt are improved by the addition of lignin through hydrolysis, cation exchange, protonation and electrostatic reaction, which leads to the reduction of thickness for double-layer and filling the pore. The proposed model of microcosmic mechanism can provide a theoretical reference for engineering application of lignin based by-products.

Key words: industrial by-product, silt, microstructue, pH value, functional groups analysis