Insufficient soil strength is the primary cause of damage to engineering structures in saline frozen soil regions. To better utilize and address the problems arising from saline frozen soil, a combined solidification method using fly ash, quicklime, and sodium silicate (FA–L–S) was employed to solidify sulfate saline soil. Unconfined compressive strength (UCS) tests were conducted under different salt contents and temperature conditions, and a series of microscopic tests were also performed to explore the relationships among hydration product formation, pore structure evolution, and macroscopic mechanical behavior. The results revealed that the sulfate radical markedly promoted the formation of ettringite (AFt) and C–(A)–S–H gel with increasing salt content from 1% to 3%. Needlelike AFt crystals and a continuous gel filled the pores and cemented the soil particles, forming a dense skeleton, and the UCS increased accordingly. This densification is evidenced by the enhancement of the AFt peaks in X-ray diffraction/thermogravimetric patterns and reduced porosity in scanning electron microscopy/nuclear magnetic resonance analyses. When the salt content exceeded 3%, the negative effects of AFt and mirabilite began to emerge, and microcracks formed within the solidified saline soil owing to the excessive expansion of AFt and the crystallization of mirabilite; the soil structure became loose, and the UCS decreased. In addition, as the pore water froze at negative temperature, an additional bonding effect was induced, resulting in a higher UCS compared with the samples at a positive temperature. This work analyzed the synergistic effect of ice and hydration products on the UCS of sulfate saline soil, which is helpful for the treatment of sulfate saline soils in saline frozen regions.
Xiao et al. (Tue,) studied this question.