Durability of solidified sludge with composite rapid soil stabilizer under wetting–drying cycles

Composite rapid soil stabilizer (CRSS) comprising calcium sulfoaluminate cement, gypsum, lithium carbonate, and ionic soil stabilizer is characterized by rapid hardening and high strength. In this study, it was used to solidify sludge rapidly, and how the number of wetting–drying (WD) cycles and the CRSS dosage influenced the durability of the solidified sludge was studied. The results show that for a given CRSS dosage but an increasing number of WD cycles, the dry mass and density of the solidified sludge decreased, the water content and pH of the soaking solution increased, the unconfined compressive strength increased slightly, then decreased slowly, finally stabilized. The ettringite in the solidified sludge was transformed from columnar to needle-like. the diffraction peak intensity decreased, and the microscopic pores increased in both size and quantity. With increasing CRSS dosage, the apparent structure was more intact, the strength attenuation was smaller. After 12 WD cycles, with a 20% content of solidified sludge, the dry mass loss was only 2%, the strength loss was only 12%, and the cumulative pore volume did not increase obviously, which was maintained at about 0.20 ml/g, showing the high resistance to WD erosion. Based on the chemical reaction and combined with the macroscopic and microscopic test results, the microscopic evolution mechanism of the durability of solidified sludge under WD cycles—especially the strength evolution mechanism—is revealed clearly. The solidified sludge with CRSS had good durability against WD cycles, thereby laying a theoretical foundation for applications of CRSS.