Effect of recycled phosphogypsum and calcium aluminate cement on the strength behavior optimization of cement-treated dredged soil: A co-utilization of solid wastes

Dredged soil and phosphogypsum (PG) are waste materials that must be treated to reduce their negative environmental effects. Guided by the concept of waste treatment, this study proposed the use of PG as a supplementary cementitious material to stabilize waste-dredged soil, and calcium aluminate cement (CAC) was selected to further improve the strength of the cement-treated dredged soil. Several laboratory tests were conducted to investigate the pH, unconfined compressive strength (UCS), and failure strain of the cement-treated soils in different proportions. Microstructural and mineralogical tests were performed to reveal the mechanisms underlying the strength improvement of PG and CAC. The results showed that both PG and CAC enhanced the strength of cement-treated dredged soil. PG provided SO2- 4 to promote the formation of ettringite (aluminum ferrite trisulfate (AFt)), whereas CAC neutralized the acidity of PG and provided reactants to the reaction system, leading to an increase in the pH and strength with an increase in the relative CAC content. Meanwhile, an exponential relationship was obtained between pH and qu. Mineralogical changes demonstrated that the major hydration products of cementitious materials, such as calcium silicate (aluminate) hydrate (C-(A)-S-H), AFt, and calcium aluminate hydrate (C-A-H), enhanced the strength by filling pores between particles and bridging soil particles. However, excess CAC content may not be favorable for the later strength formation, the relative CAC content is recommended to be in the range of 40%-60%. Compared to using sand, the construction of a square kilometer of reclamation consumed 3.5 million tons of PG, and saved 1.54 billion USD by using dredged soil as raw material. Hence, the use of PG to treat dredged soils will have great environmental sustainability, economic benefits, and engineering value.