An Evaluation of the Reduction of Toxin Leaching From Synthetic Lightweight Aggregates Made from Coal Fly Ash and Mixed Plastic.
Neil, Chelsea Wren.
- Aggregates are particulate materials used in construction; e.g., sand and gravel that are mixed with cement to form concrete. For some applications, a lighter concrete may be needed; therefore, use of a lightweight aggregate is an often-chosen replacement for traditional aggregates. A potential lightweight aggregate is synthetic lightweight aggregates (SLAs) produced by commingling two waste strea... read morems - coal fly ash and mixed plastics - to create a granular material similar in size to natural sand and gravel. However, the presence of trace contaminants in the fly ash presents an obstacle to increased fly ash reuse. The goal of this study was to demonstrate the reduction in leachability for arsenic, lead, and selenium; all common trace contaminants in fly ash, via use of the fly ash in SLAs. It was hypothesized that the waste plastic in SLAs could serve to mitigate contaminant leaching from fly ash - as found in previous research on the use of plastics to reduce leaching of contaminants from municipal solid waste combustion products (Massardier etal, 1997). Two varieties of fly ash were tested: a high carbon "Raw" ash and a low carbon "Processed" ash, created from a carbon burn-out unit. Aggregates were created with ash-to-plastic ratios (by weight) of 0:100, 20:80, 50:50, and 80:20. These aggregates and the ashes were tested for toxin leaching using the Toxicity Characteristic Leaching Procedure. Arsenic levels were measured using a graphite furnace atomic absorption (GFAA) device and lead and selenium levels were measured using a Leeman Labs PS-1000 inductively coupled plasma (ICP) unit. Additional exploration into the mechanism of toxin leaching involved segregating the fly ash and creating aggregate with ash specimens with a known range of particle sizes. This process allowed the effect on unburned carbon on toxin leachability to be observed. Leaching was carried out on these aggregates and levels of lead and selenium were measured. The structure of various sizes of these aggregates was also observed using scanning electron microscopy (SEM). Results from these tests supported the hypothesis that inclusion of the ash into synthetic lightweight aggregates reduced its leachability. In most cases, and particularly those with a low pH, this reduction was significant; reductions of up to 96.69% were achieved for arsenic, 96.94% for selenium, and 80.58% for lead. Therefore, in cases where the leaching of toxins is a large concern, utilization of fly ash in synthetic lightweight aggregates, rather than as ash alone, provides a beneficial ash reuse strategy.read less