Recycling of heavy metal contaminated river sludge into unfired green bricks: Strength, water resistance, and heavy metals leaching behavior – A laboratory simulation study

Recycling the sludge in building materials is a promising approach to reduce construction costs and environmental pollution. However, when the sludge is used as a raw material the mechanical properties and environmental risks of sludge should be taken into consideration. In this study, heavy metals contaminated river sludge (CRS) incorporated into the production of unfired green bricks with the combination of hemihydrate phosphogypsum (HPG), ground granulated blast furnace slag (GGBFS), and Portland cement (PC). The mechanical properties of brick specimens were determined by conducting unconfined compressive strength (UCS) and water resistance tests, while the hydration products and strength formation mechanism were detected by XRD, TGA, and SEM-EDS investigation. Furthermore, the TCLP and SPLP tests were used to assess the heavy metals releasing behavior. The explored results show that the UCS of samples increases with the rise of the replacement level of PC by GGBFS. The highest UCS and softening coefficient of bricks were 15.24 MPa and 0.94 using the combinations of 60% CRS, 20% HPG along with 17% GGBFS and 3% PC. The microstructural analysis revealed that HPG could react with GGBFS and PC to generate massive ettringite (AFt). These AFt may bridge gypsum crystals to form a dense network microstructure. Additionally, specimens prepared in this study exhibited an excellent fixing capacity for heavy metals. The heavy metal immobilization rates of the optimal mixture were all higher than 99.41%.