Recycling of sewage sludge in clay-free thermal insulation brick: assessment of microstructure, performance, and environment impact.

Recycling municipal sewage sludge in fired bricks not only contributes to environmental protection, but is also an alternative to natural clay resource. The complex compositions of sludge have a great influence on the brick property. This work presents a systematical investigation on fired bricks made only with sludge and shale. The physicochemical properties of the raw mixtures, macroscopic performance, microstructure, and its evolution were quantitatively determined. The coordination between shale and sludge enables the blend of raw materials to achieve the desired gradation and plasticity. Although a reduction in compressive strength was observed, the open porosity was increased to 31.6% and thermal conductivity was reduced to 0.51 W·(m·K), indicating the benefit to the performance of lightweight thermal insulation bricks. The fitting results confirm the pore-forming effect induced by the organic matter in the sludge. The obtained bricks possess good performance, especially in thermal insulation properties, and environmental and economic benefits.