The influence of cyclic humidification and dehumidification on the migration of colloidal clay particles in loess

To investigate the influence of colloidal clay particle migration on the physical and mechanical properties of loess during cyclic humidification and dehumidification, tests such as apparent morphology, shear strength, scanning electron microscopy, mapping and X-ray fluorescence (XRF) were carried out. They compared and analysed the variations in sample mass parameters, surface crack rates, shear parameters, microstructure characteristics, and particle occurrence and content under different colloidal clay contents and different dry‒wet conditions. The aim was to reveal the internal relationships between cyclic humidification and dehumidification in different environments and structural mechanics, and structural causes. The results show that the mass change parameters and apparent fracture rates of the samples in the open environment increase with increasing dry‒wet frequency and colloidal clay particle content. The overall change parameters and the total change parameters of the closed environment are smaller, fewer cracks, and cracks are only produced after a large amount of accumulation of humidification and dehumidification. Under the same positive pressure conditions, the shear strength of the open environment decreases with an increasing number of dry‒wet cycles, whereas the shear strength of the closed environment increases with an increasing number of cycles. The microscopy images revealed that the image brightness of sodium (Na) in the open environment gradually darkened, whereas the image brightness of high-frequency dry and wet loess in the closed environment was brighter and the element distribution was more uniform. Further analysis of SEM-EDX mapping and XRF images revealed that the difference in particle transport in different environments is the key factor leading to the difference in the structural and strength characteristics of loess. In the open environment, cyclic humidification and dehumidification greatly reduced the effective cementation among the skeleton particles, and the structure of the skeleton was significantly reduced. As a result of particle migration in a closed environment, on the one hand, the ‘viscous bridge’ effect of cementation between the skeleton particles of loess is strengthened; on the other hand, the surface of the skeleton mineral crystals that constitute the main part of the pore wall is smoother, and the viscous force of seepage through the pores is weakened, thus resulting in a strong structure.