Sustainable utilization of phosphorus-rich sludge in cement composites: Multiscale evaluation and mechanistic analysis

초록

The treatment of sludge poses challenges for resource utilization and environmental protection, and utilizing sludge as supplementary cementitious materials (SCMs) presents a viable solution. However, the phosphorus content significantly influences the cement hydration process. This study primarily investigates the feasibility of phosphorus-rich sludge (PRS) as SCMs and examines the effects of combining PRS with calcined clay on cement hydration. The research focuses on hydration heat, mechanical properties, microstructure, and durability of composites comprised of PRS, limestone, calcined clay, and ordinary Portland cement. The results indicate that PRS delays early hydration reactions, leading to a decrease in early compressive strength; after 7 days, the compressive strength is approximately 14.3 MPa (sample S20C10L15). However, over time, the strength increases to 34.1 MPa at 28 days, reflecting a 138.5 % increase compared to the 7-day compressive strength (with sample OPC exhibiting only 69.1 %). This correlates with the hydration exothermic peak occurring in the later stages and the transition of phosphorus's effect on cement hydration from inhibitory to non-inhibitory. The phosphorus in PRS partially replaces silicon atoms in C-(A)-S-H gel, forming C-S(A,P)-H gel. This process refines pore sizes within PRS cementitious materials, enhancing compressive strength over time. Early hydration inhibition and the subsequent pozzolanic reaction of PRS reduce portlandite content, which lowers carbonation durability and thermal expansion coefficient. However, the quaternary blend of PRS, limestone, calcined clay, and ordinary Portland cement demonstrates excellent mechanical properties. This study provides experimental evidence for reducing the environmental impact of PRS and exploring carbon reduction strategies in the cement industry.

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