Thermal Stabilization of Iron-Rich Sludge for High Strength Aggregates

A key task in wastewater sludge management is preventing sludge from polluting the environment. Sludge invariably poses risks to both public health and the environment whether incinerated or landfilled. Alternative reuses need to be explored in order to solve the disposal problems of sludge in an environmentally sound manner. This investigation examines the potential of using an industrial sludge and marine clay to produce aggregates for replacement of regular coarse aggregate in concrete. The waste mixes were pressed into layers and sintered at elevated temperatures resulting in a fused hard solid mass. During the sintering process, the peak rates of weight loss occurred at temperatures of 80–90, 280–520, and 900°C, indicating mass loss through evaporation and volatilization of organic and metallic substances, respectively. The sintered materials were crushed into required sizes for a range of construction aggregates exhibiting varying characteristics. Laboratory test results indicated that sludge-clay aggregates of up to 20% clay content displayed better aggregate impact resistance of 23.1–28.8% Aggregate impact value (AIV) compared with 28.3–38.9% AIV for the control granite aggregate. Sodium salt and sintering temperature have significant influence on the product density. Concrete cast with the sludge-clay aggregates yielded compressive strengths ranging from 34.0 to 39.0 N/mm², while the sludge aggregates of 0 and 20% clay content produced concrete stronger than those cast with conventional granite aggregate. Leaching test results showed that the concentrations of the toxic elements leached from the aggregates were within acceptable levels, suggesting that the sludge-clay materials could possibly be used as concrete aggregates without detrimental effects to the environment. The experimental study indicated that conversion of the sludge and clay into construction aggregates could offer a feasible technical solution for waste management.