Effect of industrial sludge particle size on the performance of baking-free sludge bricks.

Cement solidification is essential to reduce heavy metal leaching from industrial sludge and make it recyclable. This paper studied the effect of aggregate grading optimized by sludge containing heavy metal of different particle sizes on the performance of baking-free sludge bricks, which was mixed with industrial sludge cured by both micro-silica fume and cement. First, the gradation of fine natural aggregates was adjusted according to the Fuller curve. Fine aggregates in baking-free bricks were replaced by artificially processed sludge with particle sizes of 0.15 mm ~0.3 mm and 2.36 mm ~4.75 mm. In this case, a mixed proportion scheme was designed. Then, the strength, water absorption, porosity, and heavy metal leaching were measured. With the help of nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), the phase and pore structures were observed. The test results showed that: (1) Sludge can replace part of the fine aggregates to make baking-free bricks, resulting in the reduction of the strength of baking-free bricks, but the strength still meets the standard strength requirements of GB/T 21,144-2007. (2) When fine aggregates are partly replaced by sludge with finer particle sizes, baking-free sludge bricks exhibit higher strength, lower water absorption, better pore structures and microphase structure, and stronger solidification of heavy metals, the leaching number of heavy metals meets the standard requirements of GB5085.3-2007, which will be provided support for actual industrial production.