Stabilization of fly ash from medical waste incineration as filler material in flexible pavement

Abstract

Fly ash from medical waste incineration contains toxic heavy metals that act as groundwater contaminants due to their leaching characteristics when exposed to the environment. This problem becomes acute due to limited safe disposal options, and therefore it can become a severe environmental burden in Bangladesh. This study examines the effects of using fly ash from medical waste incineration (MWIFA) in the bituminous mixture as mineral filler as an alternative to conventional stone dust (SD) filler. Marshall samples were prepared with varying filler ratios of 0%, 2%, 4%, 6%, 8%, and 10% to determine optimum bitumen ratios and investigate engineering properties. MWIFA filler and SD filler were used in the hot bituminous mixes by adding varying percentages of bitumen from 4.0% to 6.0%. Several parameters such as Marshall stability, flow, unit weight, air voids (Va), voids filled with asphalt (VFA), and voids in the mineral aggregate (VMA) were determined for all mixes. The optimum bitumen ratios for each filler percentage are selected based on the respective results of these parameters. The optimum bitumen contents of all filler ratios were within 4.9 % to 6.5%, satisfying the standard range of Bangladesh Roads and Highway. All the parameter values for both fillers at respective optimum bitumen contents satisfy the Marshall method mix design criteria. The highest stability values for MWIFA and SD fillers are 25.80 kN and 27.82 kN, respectively, found at 10% filler. The lowest stability loss was found from the mixture prepared with 8% MWIFA filler among all the hot bituminous mixes, as determined by mechanical Marshall immersion tests. The optimum filler contents for MWIFA and SD fillers are found at 5.5% and 9% filler percentages, respectively. The bituminous mixes with 5.5% MWIFA as mineral filler would give better performance in wearing actions, whereas mixes with 9% SD filler will exhibit the same performance. The leaching behaviour of the specified heavy metals (As, Pb, Cu, Cr, Ni, Cd, Hg and Zn) has been analyzed using the Toxicity Characteristics Leaching Procedure (TCLP) and found to be insignificant. The heavy metal concentrations obtained from the TCLP test results are far below the USEPA regulatory limits. The cumulative concentrations of heavy metals (Cd, Ni, Zn, Cu and Pb) were found far below the Dutch regulatory limit (U1) from NEN 7345 Dutch tank leaching tests. The recommended proportion of medical waste incineration fly ash can be 5.5% (by total aggregate weight) for producing Marshall samples of optimum quality. The test results show that fly ash from medical waste incineration can be used efficiently as a filler material in asphalt paving mix replacing conventional fillers, especially in areas where MWIFA is abundantly available and easily accessible. MWIFA can be considered an eco-friendly mineral filler replacement as it has proven to be successfully recycled in pavement construction.