The effect of Different Particle Sizes of Bottom Ash on the Properties of Pervious Concrete and the Quality of Infiltrated Water

Abstract

Amounts of generated municipal solid wastes (MSW) are augmented by the fast population growth and development, making MSW management a global challenge. Although landfilling is a convenient waste disposal approach, it has hazardous environmental, health, social, and economic effects directing countries to shift towards more sustainable waste management approaches. With the advantage of reducing the volume of waste and recovering energy, MSW incineration process is being adopted by various countries. Given that Lebanon, a small country, is experiencing waste management challenges, municipal solid waste incineration (MSWI) might offer a practical solution, provided the byproducts of the process are controlled. Among the incineration byproducts, bottom ash (BA) constitutes 80-90% of the total ash content. Thus, sustainably dealing with the produced BA is a priority. The construction industry is facing challenges stemming from overexploitation, hence, depletion of natural resources. Therefore, current global trends focus on exploring new sustainable recycled materials as alternatives for construction natural resources. This is applicable for all types of concrete structures (conventional concrete, pervious concrete, etc.) in various applications (buildings, bridges, roads, etc.). Therefore, incorporating MSWI BA into pervious concrete offers environmentally friendly solution for the disposal of BA and depletion for natural resources. Pervious concrete samples containing 20% replacement of natural aggregates (NA) with BA, collected from a local incinerator in Lebanon, were prepared using different BA particles (4.75, 9.5 & 12.5 mm). Prepared samples were tested for their mechanical properties and metal leachability. The potential of using BA as heavy metal adsorbents was also investigated for a selection of heavy metals (Pb, Cd and Hg). The physical and chemical characterization tests of BA concluded that BA were seven times more water absorptive and have around 15% lower specific gravities compared to natural aggregates. The surface of BA was found to be rougher, less uniform, and more porous than natural aggregates. BA and NA were found to have similar chemical compositions; both containing CaCO3, SiO2, MgO and FeO indicating that BA can be used to substitute natural aggregates. Acid digestion, of different BA particle sizes, followed by ICP-MS analysis detected concentrations of heavy metals (Pb, Cd, Cr, Hg, Zn, Cu) ranging between 0.002 and 3 0.05 mg/g BA with the concentration of metal content increasing with decreasing BA particle size. Heavy metal adsorption tests, using 4.75 mm BA as a sorbent, resulted in adsorption percentages of 99% (Pb), 70% (Cd) and 48% (Hg) when exposed to contaminated water containing all three metals. Three different BA particle sizes, 4.75, 9.5 & 12.5 mm, were used as 20% replacement to NA in pervious concrete samples. Four control samples were also prepared using NA with the same aggregate grading to assess the mechanical properties and metal leachability of the PC samples containing BA. Upon incorporating BA into PC, the decrease in compressive strength ranged between 15% to 33%, compared to the control samples. The highest compressive strength value being for the sample with 4.75 mm BA aggregate replacement, with only 15% decrease relative to the control sample. No drastic variation between the control and test samples in terms of air void contents was observed. The compressive strength and air void values were all found to be in the range set by the ACI for pervious concrete applications. When analyzing the leachability of heavy metals into curing water, distilled, and acidic infiltrated water, chromium was detected in all PC samples in concentrations ranging between 4 and 46 ppb. Traces of Hg, Zn, Cu and Pb (less than 38 ppb) in some samples. The leached heavy metals were all found to be below the acceptable limit set by the EPA for all water quality standards.