Recycling reuse of municipal sewage sludge in sustainable structural materials: Preparation, properties, crystallization and microstructure analyses

Abstract

As a common solid waste in daily life, the disposal of municipal sewage sludge has always been global hot issues concerned by the world environmentalists. The present study reported the fabrication of lightweight high-strength bricks as sustainable structural materials by using kaolin and recycled municipal sewage sludge waste, and systematically discussed the effect mechanism of sintering temperature on product technical properties from both phase composition and microstructure. It showed that after sintering at 1250–1400℃, the porosity, sintering shrinkage, bulk density, compressive and bending strength of lightweight high-strength bricks were 11.30–37.18%, 23.81–38.86%, 1.57–2.22 g/cm3, 35–105 MPa and 23–84 MPa, respectively. And the variation trend of sintering shrinkage, bulk density and mechanical strength was positively correlated with sintering temperature, but completely opposite to that of porosity. For the phase composition, the change of sintering temperature would not affect the crystallization phase types in bricks, which were all composed of aluminum iron phosphate (Al0.67Fe0.33PO4), cristobalite (tetragonal SiO2), hematite (Fe2O3), sillimanite (Al2O3·SiO2) and mullite (3Al2O3·2SiO2). For the microstructure, the melting and consolidation degrees of bricks were higher as the sintering temperature rose, and the structure became more dense and compact under the action of liquid glass phase, leading to a remarkable improvement and optimization of technical properties. Besides, the developed lightweight high-strength bricks presented strong competitiveness in structural materials, advancing a win–win for environment and development.