Synergistic utilization of pickling sludge and fly ash in cement-sand bricks: performance assessment and life cycle analysis

Stainless steel pickling wastewater neutralization leads to the generation of pickling sludge. The hazardous nature of the sludge further raises environmental concerns related to its improper disposal. Solidification/stabilization of pickling sludge in cement-based bricks emerges as an optimum solution for harmless disposal and valorization of this hazardous waste. This study investigates the solidification/stabilization of pickling sludge in bricks and the effect of co-utilization of fly ash on the environmental and mechanical properties of such bricks. Life cycle assessment (LCA) conducted using ReCiPe 2016 methodology to evaluate the environmental impacts of the bricks in 18 midpoint and 3 endpoint categories. Among the midpoint categories, impacts of Global Warming Potential (GWP), Terrestrial Ecotoxicity (TE), Human Non-Carcinogenic Toxicity (HNCT), and Fossil Resource Scarcity (FRS) were significant. In these damage categories, Pickling Sludge Brick-I (PSB-I) showed a significant reduction of 31, 52.13, 20.96, and 28.66% compared to Cement Brick (CB), respectively. Similarly, in endpoint categories, lower impacts for different PSBs (PSB-I-IV) in human health, ecosystems, and resource scarcity were observed compared to CB and Cement-Fly Ash Brick. Additionally, with a gradual increase in sludge content (2.5, 5, 7.5, and 10%), a consistent decrease in compressive strength of bricks was observed, accompanied by a decrease in environmental impacts. However, all developed PSBs complied with IS 16720:2018, with PSB-IV showing the highest compressive strength and optimal environmental balance. In addition, a microstructural study by scanning electron microscopy (SEM) demonstrated adequate packing in PSBs with a gradual decrease in voids resulting in a dense microstructure. Notably, the PSBs confirmed the safe immobilization of heavy metals, as observed by the absence of heavy metal leaching in toxicity tests. The findings demonstrate solidification/stabilization of sludge in bricks provides satisfactory mechanical performance while reducing environmental impacts, thereby establishing both technical and environmental benefits.