The crack characteristics and microscopic mechanism of composite solidified soil under alternating wet-dry and freeze-thaw cycles

In order to solve the problem of cracking and deterioration of saline soil in semi-arid and seasonal frozen areas, sulfur-free lignin, basalt fiber and hydrophobic polymer were used to synergistically solidify saline soil. In this study, a series of indoor tests including wet-dry and freeze-thaw (WDFT) cycles were conducted to simulate the cyclic changes in water and temperature, and to further investigate the development pattern of crack morphology in solidified soil. Digital image processing techniques were employed to quantify both macroscopic cracks and microscopic pores in the samples. The results show that the untreated soil shrinks and produces micro- cracks due to tensile stress under the action of WDFT cycles, and the main cracks are formed after expansion and penetration. Sulfur-free lignin aggregates particles and fills the pores. Basalt fibers and hydrophobic polymers reduce crack expansion by resisting the tensile stress generated by shrinkage, while preventing water from entering the structural unit. The geometric and statistical parameters of the samples are related to the number of WDFT cycles. The crack expansion and failure mechanism are reflected in changes in microstructure characteristics such as crack ratio, length and width, fractal dimension and pore direction. The findings contribute to understanding the cracking pattern and expansion mechanism of solidified saline soil and provide a scientific basis for the use of solidified materials to reduce soil cracking.