Influence of salt on the microstructure of loess under various drying conditions

This study explores the effects of drying modes, salt types and salinities on the microstructure of loess, a primary material for the earthen sites in northwest China that is susceptible to salt-induced deterioration. Based on typical salinity conditions at earthen sites, loess with negligible salt, 2% sodium chloride (NaCl), 3% sodium chloride, 2% sodium sulfate (Na2SO4) and 3% sodium sulfate were subjected to fast drying and slow drying. Microstructural changes and void ratio evolution were analysed using mercury intrusion porosimetry and volume displacement methods, respectively. Results indicated that during drying, the pore size distribution (PSD) of negligible salt and sodium sulfate samples underwent a ‘bimodal–trimodal–bimodal’ evolution, while sodium chloride samples kept a bimodal pattern. A new micro-pore peak emerged due to drying-induced shrinkage and disappeared as the micro-pores merged into the meso-pores or macro-pores. Fast drying resulted in greater surface shrinkage and a more pronounced micro-pore peak over a wider moisture range. Furthermore, sodium sulfate compressed the diffuse double layer (DDL) before crystallisation, reducing the micro-pore size. After crystallisation, it filled the pores, decreasing macro-pores while increasing structural damage. In contrast, the sodium chloride’s hygroscopic thickening of water films and DDL compression jointly controlled the evolution of the PSD.