Effect of coating treatment approaches on the properties of sludge gasification slags used for lightweight aggregate of concrete

The sludge from urban sewage treatment plants can be converted into sludge gasification slags (SGS), which demonstrate its potential as lightweight aggregates. The different coating methods, coating material ratios, FA substitution ratios and curing conditions were evaluated in this study to enhance the SGS properties. The underlying mechanism of the coated cementitious paste on SGS was investigated by phase composition, interface micromorphology, and pore structure. Macroscopic results indicated that the two layers of coated cementitious pastes with different water-binder ratios (w/b) resulted in superior enhancement by effectively mitigating issues of agglomeration and insufficient hydration. By using coating materials at 40% of the mass of SGS and replacing 25% of cement with FA, combined with w/b ratios of 0.3 and 0.15 for the inner and outer layers, the cylinder compressive strength SGS was enhanced from 2.3 MPa to 6.06 MPa. The results of optical microscopy, SEM and X-CT analysis showed that the coated SGS presented a core-shell structure with a porous core and a dense shell. This method provided a cost-effective, low-carbon solution for lightweight aggregates with a production cost of 66.53 RMB/t and carbon emissions of 120.12 kg CO 2 /t. And the optimized coated-SGS exhibited a loose bulk density of 855.1 kg/m³, which satisfied the requirements for 800-grade lightweight aggregates.