Conversion of recycled sludge waste into high value-added ceramics as engineering construction materials: Fabrication, characterization and performance evaluation

Sludge from municipal wastewater treatment plants usually exhibits extremely homologous traits with clay in both mineralogical component and chemical composition, and accordingly it can be utilized as an alternative feedstock for clay to fabricate engineering construction materials and advance sludge valorization. In this work, different proportions of wastewater treatment plant sludge (WTPS) were incorporated into kaolin to develop value-added construction ceramic materials. The thermal behavior, crystalline composition, physicochemical performance, microscopic characteristics and sintering mechanism of value-added ceramics were investigated. The results showed that WTPS not only contained a large amount of inorganic components such as SiO2, Al2O3 and Fe2O3, but also rich organic compounds, which contributed to the formation of pore structure inside the ceramics during sintering. Different WTPS addition did not affect the types of crystalline components in sintered ceramics, but markedly impacted the sintering behavior. Only when the WTPS dosage reached 40 wt%, the original mixtures would be fully sintered into construction ceramics with the apparent porosity of 11.30 %, thermal conductivity of 0.50 W/(m & sdot;K), bulk density of 2.22 g/cm3, linear shrinkage and volume shrinkage of 14.95 % and 38.86 % respectively, as well as vastly low water absorption (0.58 %) and excellent flexural strength (84 MPa). However, with the WTPS content up to 50 wt%, the flexural strength of sintered ceramics was reduced to 50 MPa, despite the elevated sintering degree, which was related to the high formation of liquid phase and the violent expansion of several closed pores. Therefore, converting WTPS into high value-added construction ceramic products is an operable and beneficial way to achieve sludge waste recycling, reduction and stability.