Hydro-mechanical behaviour of phosphogypsum-based water-absorbent polymers in wastewater sludge

Ultra-high water content is generally presented in wastewater sludge, and rapid water removal during disposal is the key to recycling. This study aims to explore a novel approach for rapidly reducing the ultra-high water content of sludge using a phosphogypsum-based water-absorbent polymer (PG-WAP), synthesized from phosphogypsum (PG), ordinary Portland cement (OPC), and ground granulated blast furnace slag (GGBS). Firstly, bentonite was selected as an additive to enhance the degree of pozzolanic reaction in PG-WAP. Then, the effectiveness of PG-WAP for drying sludge is evaluated using water absorption measurements and unconfined compressive strength (USC) tests, and the water absorption mechanisms are revealed by X-ray diffraction (XRD), thermal analysis (TGA), and scanning electron microscopy (SEM). Finally, the economic benefits of PG-WAP for drying sludge were assessed and compared with conventional drying methods. The results indicate that PG-WAP demonstrates an excellent drying effect, with an average water absorption rate of 53%. Microstructure characterization shows that the synergy between bentonite and PG leads to the formation of non-expansive AFt, which enhances structural integrity and mitigates the strength reduction associated with high PG content. Moreover, PG-WAP combined with PG-based solidification agent, enables simultaneous drying and solidification of sludge in a single mixing process. • This study explores a novel approach to utilizing PG for drying sludge. • A PG-WAP was proposed to improve sludge drying and optimize grading. • One-step and two-stage theories for PG-based drying and solidification were proposed. • The synergy of PG and bentonite enables localized sludge-water separation.