13.3 Adsorption tests

PAC added for mercury control presents different challenges for ash use in concrete. Activated carbon is much more adsorptive than unburned coal with a high affinity to adsorb AEAs that are used in concrete production. These characteristics are the result of activated carbon's complex pore structure (Fig. 13.7). Mercury is sequestered in very small portions of the carbon structure, but there remain ample pores and surface area available to adsorb other compounds, including AEAs from fresh concrete mixtures.

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Fig. 13.7. Micrograph of activated carbon particle.

Additionally, PACs used for mercury control can have different effects on ash quality for use in concrete. Fig. 13.8 shows high variability in the impact of two different PAC samples (identified by red and blue markers) on potential air entrainment demand as measured by FI testing.

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Fig. 13.8. Number of drops of AEA needed to reach stable foam in ash/water slurry containing two different PACs.

The increased variability of content, type, and adsorption capacity of carbon in fly ash has raised the need to develop more accurate adsorption test methods for determining the impact of activated carbon on ash. There are several test methods working toward utilizing existing activated carbon adsorption tests to measure the adsorption capacity of ash and its impact on concrete. For example, IN, which has been traditionally used to determine the adsorption capacity of carbon black, is now being considered as a potential test method to determine the adsorption capacity of fly ash. In this test method, carbon black/fly ash is first boiled in a 5% HCl solution to remove any sulfur that may interfere with the results. After drying and filtering (and crushing, if necessary), a known amount of carbon black (or fly ash, as may be the case) is mixed with a standard iodine solution. Subsequently, the solution is filtered to separate the solids. Lastly, the concentration of the remaining iodine in the filtrate is measured by titration. The results are expressed in grams of iodine adsorbed per kilogram of carbon black/fly ash (Ahmed, David, Sutter, & Watkins, 2014). Although the IN test provides a relative indication of surface area, it may not measure the capacity of fly ash to adsorbed other chemical species.