Upcycling of Sludge Gasification Slag in Sustainable Foamed Concrete: Fresh Properties, Thermal Conductivity, and Microstructure

Abstract Traditional foamed concrete (FC) with the absence of fine aggregate is susceptible to the occurrence of early cracking induced by drying shrinkage, which could compromise the thermal insulation performance. This study proposes a solution by introducing sludge gasification slag (SGS) into sustainable FC for recycling the waste sludge and enhancing the thermal insulation, thereby diminishing the environmental contamination and energy consumption. The influence of replacement level and particle size of SGS on the rheology behavior, flowability, drying shrinkage, strength development, and thermal conductivity of FC was explored. The pore structure and micromorphology were evaluated to determine the mechanism. The presence of SGS increased the yield stress and plastic viscosity of cement slurry, but decreased the flowability of FC. Furthermore, the inclusion of SGS in FC had a detrimental impact on the strength. In contrast, the existence of SGS was beneficial in alleviating the drying shrinkage and inhibiting the thermal conductivity, and these improvements increased as the replacement and particle size increased. Additionally, introducing SGS into FC tended to increase porosity, average pore size, average roundness, and coarse pore-size distribution, which contributed to form better mechanical locking between SGS and cement matrix. A quantitative relationship between thermal conductivity and pore parameters was established, which can provide a guidance for predicting the thermal conductivity.