Abstract: The type of alkali activator is crucial for the strength development of phosphogypsum-based cementitious materials (PPSC), yet its role in soil solidification remains unclear due to the soil interference. Through unconfined compressive strength (UCS) tests, pH measurements, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM), this study analyzed the effects of activator type (cement, quicklime, NaOH, NaOH+sodium silicate) and PPSC content on the mechanical properties and microstructure of solidified soil, revealing the regulatory mechanism of the alkaline environment on the performance of PPSC-solidified soil. The results indicate that the alkaline environment generated by the activator significantly influences the early-age strength development of the solidified soil. Notably, the alkaline environment induced by the cement activator exhibits the least potency, failing to effectively activate the reactivity of PPSC within the soil during the early hydration stage, resulting in the lowest early-age (7 d) UCS (0.46 MPa), which is substantially inferior to those observed in the quicklime-treated group (1.11 MPa), the NaOH+sodium silicate-treated group (0.84 MPa), and NaOH-treated group (0.98 MPa). However, with the continued hydration of the cement activator, the alkalinity in the solidified soil intensifies, thereby diminishing its impact on the later strength development. After 28 d of curing, the strength rankings of the solidified soil are as follows: cement-treated group (1.61 MPa)＜quicklime-treated group (1.74 MPa)＜NaOH+sodium silicate-treated group (1.95 MPa)＜NaOH-treated group (2.08 MPa). Furthermore, the alkalinity of the solidified soil demonstrates a discernible pH threshold effect. When pH＜10.08, the PPSC in the soil cannot fully hydrate, and the strength of the solidified soil is not significantly affected by the PPSC content. When pH≥10.08, the strength increases markedly with the increase of PPSC content. This study holds significant implications for the efficient resource utilization of phosphogypsum and soil solidification.

Key words: phosphogypsum, slag, alkaline activators, solidified soil, mechanical properties, microstructure