Strength and Microstructure Characteristics of Basalt Fiber and Metakaolin-Stabilized Cement Clay under Coupled Effect of Sulfate Erosion and Freeze–Thaw Cycles

Cement stabilized clay is a type of composite cement-based material widely used in soft clay improvement engineering. It is inevitably influenced by the coupling effects of sulfate erosion (SE) and freeze–thaw cycles (FTC) in cold and coastal regions. Therefore, it is of great importance to study the deterioration mechanisms of cement stabilized clay under the combination of the two coupling effects. In this study, metakaolin-stabilized cement clay (MCC) and basalt fiber metakaolin-stabilized cement clay (BMCC) were prepared. In addition, unconfined compressive strength (UCS), pore structure, nuclear magnetic resonance, and scanning electron microscopy tests were conducted to study the effects of FTC number and freeze–thaw environment (pure water and 0.1 mol/L sodium sulfate) on the UCS and microstructure characteristics of cement stabilized clay. Test results indicated that the peak strain increased with the increase of FTC number, while the UCS was reduced continuously, and the first FTC resulted in the greatest rate of UCS loss. The UCS of cement-stabilized clay significantly decreased under the combined environment of SE and FTC. However, the incorporation of basalt fiber enhanced the freeze–thaw resistance of specimens. Moreover, the increase in the FTC number induced the micropores, and small pores transformed into mesopores and large pores in the present research. Specifically, the air content, average pore chord length, and pore spacing coefficient monotonically increased with increasing FTC number, while the specific surface area demonstrated an opposing normality. In the 0.1 mol/L sodium sulfate erosion environment, the porosity and proportion of mesopores and large pores were higher than those under pure water freeze–thaw environment after experimenting with the same FTC number. The combination of FTC and SE increased the size and number of pores and cracks of basalt fiber and MCC, and the basalt fiber exhibited obvious debonding and sliding phenomenon after 10 FTC numbers.