Geoenvironmental impacts of using high carbon fly ash in structural fill applications.
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Geoenvironmental impacts of using high carbon fly ash in structural fill applications.

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  • English

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      Final report.
    • Abstract:
      Fly ash produced by power plants in the United States occasionally contains significant amounts of unburned carbon due to the

      use of the increased prevalence of low nitrogen-oxide and sulphur-oxide burners in recent years. This ash cannot be reused in

      concrete production due to its reactivity with air entrainment admixtures, so it is largely placed in landfills. Highway structures

      have high potential for large volume use of high carbon fly ashes (HCFAs). However, in such applications, even though

      mechanical properties of the fly ash-amended highway base layers and embankments are deemed satisfactory, a key issue that

      precludes such highway embankments built with fly ash is the potential for negative groundwater effects caused by metals in the

      fly ash. This study evaluated the leaching potential of metals from highway base layers stabilized with HCFA and highway

      embankment structures amended with HCFA. Three different laboratory tests were conducted: (1) batch water leach tests, (2)

      toxicity characteristics leaching procedure tests, (3) column leach test. Additionally, two numerical modeling analyses were

      conducted: (1) WiscLEACH and (2) MINTEQA2. Analyses were conducted on eight fly ashes and two locally available sandy

      soil materials that are mainly used in highway base layer and highway embankment structures. Laboratory test results indicated

      that an increase in fly ash content in the soil-fly ash mixtures yielded an increase in leached metal concentrations with the

      exception of zinc (Zn). The pHs had significant and different effects on the leaching of metals. The leaching of chromium (Cr),

      zinc, (Zn), aluminum (Al), arsenic (As) and selenium (Se) increased with increased pH levels, while leaching of barium (Ba),

      boron (B), cooper (Cu), iron (Fe), magnesium (Mn), silver (Sb), vanadium (V) decreased with increased pH levels.. Numerical

      model WiscLEACH was used to simulate the leaching behavior of leached metals from HCFA-stabilized highway base layers

      and HCFA-amended highway embankment structures. WiscLEACH predicted field metal concentrations were significantly

      lower than the metal concentrations obtained in laboratory leaching tests, and field concentrations decreased with time and

      distance due to dispersion in soil vadose zone. Numerical model MINTEQA2 predicted that leaching of metals were solubility

      controlled except As, Se and Sb. Speciation analyses indicated that leached metals were present at their less or non-toxic forms.

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