Shear strength, water permeability and microstructure of modified municipal sludge based on industrial solid waste containing calcium used as landfill cover materials

In order to prepare a new type of landfill covering material for closure, we used industrial calcium-containing waste (slag, desulfurised gypsum and fly ash) to modify the municipal dewatered sludge. Shear, infiltration and rainfall infiltration model tests were performed to obtain the shear strength parameters of the modified sludge, the hydraulic conductivity during the wet-dry cycle, and the service performance against rainfall breakdown to evaluate the service performance of the modified sludge cover (MSC). Comprehensive characterisation of the modified sludge was analysed by XRD, FTIR and SEM-EDS to revealed the mineral structure, microstructure, and modification mechanism of the sludge. The MSC samples had high shear strength and shown the characteristics of evolving from plasticity to brittleness. After curing for 14 d, the values of cohesion c and internal friction angle φ reached 150.75–384.69 kPa and 37.60–57.29°, respectively. The MSC exhibited excellent anti-seepage service performance under dry and wet cycle conditions. Compared with traditional compacted clay, its hydraulic conductivity dropped by an order of magnitude, and after six wet and dry cycles, the hydraulic conductivity of the modified sludge reached stability at 1.4–7.2 × 10 −7 cm/s. The 60-cm-thick MSC layer can completely withstand the impact of long-term rainfall during the rainy season in the middle reaches of the Yangtze River in China. Analysis results also show that the modification mechanism of the sludge could be ascribed to the generation of dense blocks and clusters of C-S-H and C-A-S-H gels originated from SiO 2, Al 2 O 3, and CaO phases in industrial calcium-containing waste and sludge by the activation of the alkali.