Assessment of magnesium potassium phosphate cement for waste sludge solidification: Macro- and micro-analysis

The feasibility and performance of eco-friendly and sustainable magnesium potassium phosphate cement (MKPC) to solidify urban river sludge are innovatively evaluated. To probe exhaustively into the mechanical performance and micromechanism of MKPC solidified dredged sludge, the effect of binder content, Mg/P molar ratio, retarder (borax) content and curing age are analyzed through unconfined compressive strength (UCS), mercury intrusion porosimetry (MIP), X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests. The experimental findings indicate the compressive strength is raised owing to an increase in MKPC amount and curing age. The isolation effect of sludge matrix weakens the retarding effect of borax, but reasonable borax content is beneficial to the strength improvement at longer curing age. The UCS of solidified samples with lower molar ratio (Mg/P = 3) increases rapidly at the early age while limitedly in later stage. The UCS reaches the maximum at the molar ratio of Mg/P = 4–5. The XRD and MIP analysis demonstrates that MgKPO₄·6H₂O (struvite-k) is the major hydration phase produced along with unreacted dead burned magnesia. Struvite-k can effectively fill the large pores (1–10 μm) and cause the transformation of large pores into small pores (0–0.1 μm), which has a good refinement performance for pore structure. The SEM images show that the prismatic or layered struvite-k forms preferentially and its final morphology is intimately connected with the Mg/P molar ratio. When the MKPC dosage and curing age increase, the struvite-k crystals are significantly densified and some microcracks appear due to the surface tension effect during the hydration heat-associated dehydration in crystal growth process. The synthesized crystal is fully developed and the crystal grows more completely under appropriate reaction conditions and reasonable relative concentration of reactants, which produces stronger interconnection between sludge particles and denser microstructure within matrix, contributing to the strength development of solidified sludge. In summary, the mechanical, eco-friendly and cost-efficient benefits could be highly anticipated from substituting MKPC for Portland cement (PC) in sludge solidification.