Microstructural change of improved dispersive soil based on scanning electron microscope and nuclear magnetic resonance technology

The dispersive clay is a special clay that is easy to disperse and disintegrate in low salt water. This characteristic leads to the instability and failure of many water conservancy and geotechnical projects. The dispersive clay is widely distributed in Northeast China. In order to improve the erosion resistance of the dispersive clay, explore the evolution characteristics of pore structure of the dispersive soil and analyze the action mechanism of the lignin-improved dispersive soil, the dispersive soil of Nenjiang River Diversion Project in the south of Heilongjiang Province is taken as the research object, and the dispersity, element composition and microstructure of different lignin contents (0%~10%) are tested and observed through the dispersity identification tests, scanning electron microscope (SEM) tests and nuclear magnetic resonance (NMR) tests. The test results show that: (1) The lignin can effectively improve the dispersibility of the dispersive soil. The anti-erosion capability of the soil can be effectively strengthened if the lignin content is≥3% and the curing age is≥7 days. (2) The SEM image analysis and the NMR tests have good consistency. With the increase of the lignin content, the micro-pores in the soil gradually develop into large pores, and the porosity first decreases and then increases. Under the action of freeze-thaw cycle, the large pores in the soil show an increasing trend, which has a more significant impact on the modified soil when the lignin content ≤ 5%. (3) Ca²⁺ in lignin reacts with Na⁺ in the soil by ion exchange, which can be adsorbed on the surface of the soil and form a hydrophobic layer to improve their erosion resistance. However, due to the excessive lignin content and mutual adsorption of its own sulfonic acid groups, the viscosity of the soil increases, and it is difficult to compact and form macroporous defects. The appropriate amount of lignin can produce a good modification effect on the dispersive soil, fill the soil pores and improve the erosion resistance of the soil, which may provide a theoretical reference for the pore structure and practical engineering performance of the dispersive soil.