Methods and effectiveness of collaborative implementation in flowable sludge of drying and solidification

In response to the challenge posed by the lack of coordinated linkage between the drying and solidification stages in the treatment process of existing flowable sludge, which results in excessively high costs, this study proposes a collaborative implementation method utilizing phosphogypsum-based cementitious materials. This approach seamlessly integrates drying and solidification through the development of phosphogypsum-based powders and aggregates characterized by rapid water absorption and slow-setting hydration. Consequently, a “drying-first, solidification-later” mechanism is achieved, supported by systematic evaluations of mechanical properties, durability, environmental impacts, and both economic and ecological benefits. The results indicate that when 15% phosphogypsum-based powder and 20% phosphogypsum-based aggregates are incorporated into flowable sludge (with a water content of 110%) and cured for 28 days, the unconfined compressive strength (UCS) reaches 4.84 MPa, the cohesion is 355.7 kPa, and the internal friction angle is 30.8°. After undergoing 10 cycles of wetting-drying or freezing-thawing, the UCS remains at 3.6 MPa and 3.3 MPa, with loss rates of 26.3% and 32.5%, respectively, thereby meeting the bearing capacity requirements for solidified soil foundations (≥1.0 MPa). Additionally, the concentrations of soluble phosphorus and soluble fluorine comply with Class II surface water standards, ensuring environmental safety. Mechanistic analysis reveals that phosphogypsum-based aggregates, containing 80% phosphogypsum, form an aggregate-like structure through rapid local water absorption and slow hydration, optimizing particle gradation and facilitating skeleton construction. Meanwhile, the phosphogypsum-based powder, containing 15% phosphogypsum, generates needle-rod-shaped ettringite (AFt) and network-shaped calcium silicate hydrate (C-S-H) gel via the same rapid water adsorption and slow hydration mechanism, playing a crucial role in cementation. The synergy between these two components achieves temporal separation and efficiency integration in the “drying-solidification” process, ultimately realizing the technical goal of “one-time implementation, two-stage completion.” Economic and environmental analyses reveal that the unit strength cost of this method is only 63% of that of traditional cement-based solidification, while carbon emissions are reduced by 94%. This innovative approach demonstrates significant economic and ecological advantages, making it highly valuable for widespread application.