Assessment of Heavy-metal Leaching from Recycled Concrete Debris and Bottom Ash as Sustainable Alternatives for Railway Ballast in Tropical Environments

Railway track ballast comprises natural rocks such as granite and basalt. Substituting various waste materials instead of natural aggregate will considerably reduce environmental damage, promote sustainable development, and reduce cost. However, harmful substances may be leached that might change the quality of rivers. This study is conducted to monitor the heavy metal leaching activity under the influence of tropical weather change. Various track design samples were mixed in 6 different compositions and placed in a leaching tank for 270 days under room and hot temperatures. Samples were continuously heated at 40°C for 10 minutes at random intervals during day time. Some 2 ml of leachates were tested using the ICP-OES method to monitor the changes in the sample. The leaching of heavy metals was analyzed and compared with the specification of the National Water Quality Standards (NWQS) set by the Department of Environment (DOE) Malaysia. The mixture of 50% conventional ballast, 40–45% inert concrete debris, and 5–10% bottom ash is highly recommended. It was concluded that the leaching parameters were within the threshold permitted. Hence, with minimal pre-treatment, bottom ash, and concrete waste can be used as an alternative material in railway tracks.