Promoting the sustainable fabrication of bricks from municipal sewage sludge through modifying calcination: Microstructure and performance characterization

Abstract

The disposal of municipal sewage sludge (MSS) has long been a huge burden for metropolis. It has been proved that the MSS can be recycled in fired brick manufacture, but the influence mechanism of calcination procedure is still worth to be investigated deeply. And the shift from natural clay resources to rich shale and sludge resources is urgently needed in brick industry. In this study, 15% of MSS was used as pore-forming agent and mixed with 85% of shale for preparing the green body. The influence of calcination procedure (firing temperature and heating rate) on the overall performance, involving the volume stability, compressive strength and durability, was analyzed systematically. Results showed that 900 °C was the optimal firing temperature at which the compressive strength of bricks reached up to 34.4 MPa, the thermal conductivity was lower than 0.50 W/(m·K), and the water absorption and strength loss after freezing-thawing attack were both below 20%. Rising the temperature too quickly tended to form a highly heterogeneous structure characterized by the appearance and phase gradient distribution outside in. The quantitative microstructure determination and mechanism analysis further confirmed that amorphous phase content and porosity were the crucial factors affecting the brick performance. The leaching analysis confirmed that incorporating MSS in brick preparation achieved a safe disposal of heavy metals. The fundamental study would lay a solid foundation for the firing optimization and recycling sludge more efficient, and provide date support for complete usage of shale and MSS in fire brick manufacture.