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Ask an Expert
To Ask an Expert an ash related question, e-mail your questions to Debra Pflughoeft-Hassett. Please include your name, company and telephone number. All questions will be distributed to the appropriate expert. Examples of previously asked questions are listed below.
Is coal ash hazardous?
Is coal ash radioactive?
Does disposed ash present problems in the environment?
How much fly ash can I add to my concrete?
How can I obtain samples of concrete containing coal ash?
What causes the color of ash?
What do other countries do with their coal ash?Coal ash is generally nonhazardous although, as with most materials of natural origin, some coal fly ash can contain elevated concentrations of potentially hazardous trace elements. Since the total mass of coal is reduced by approximately a factor of 10 during combustion, the concentration that can occur can be as high as around a factor of 10, depending on the concentration of ash in the particular coal being combusted. Some ash is highly alkaline and should be treated as one would treat portland cement.
This issue has been around for some time now (at least since 1960), and there has been considerable research done to determine the correct answer. There is radioactivity in everything because of naturally occurring radioactive materials (NORM). In living organisms, a large proportion of the NORM is from potassium-40 which occurs naturally and is radioactive. NORM is actually unavoidable as all potassium contains a small fraction of K-40. As far as other types of radioactivity, coal could be a source but not with the coals currently burned in the United States. The small amount of radioactivity in coal that is transferred to coal ash after burning is from NORM sources and is considered to be very stable in the ash matrix. Although the concentrations are low, they remain in the ash and are not very susceptible to leaching or other release.
Does disposed ash present problems in the environment?The general answer to this question is that most ash disposal has not caused environmental problems. Several environmental groups, however, have identified what they term "damage sites." Some of the identified damage sites are true environmental problems but most are misidentified. Research to date has shown that most CCBs are benign and do not present environmental problems either through beneficial use or disposal.
How much fly ash can I add to my concrete?Higher additions (40%-60%) of fly ash are generally achieved with high-calcium (Class C, or cementitious) fly ash such as those from Powder River Basin (PRB) coals. PRB coals are subbituminous and are found primarily in Wyoming and Montana. It is important to note that many state Departments of Transportation limit the addition of fly ash to 20% without addressing issues of optimum performance. Increasing the addition of Class C fly ash from 20% to 40% will generally result in a better performing concrete. This is not true of Class F (nonreactive, pozzolanic) fly ash. Generally, the 20% addition of Class F is close to optimum.
How can I obtain samples of concrete containing coal ash?The EERC's samples are associated with specific projects and cannot be released for reasons of confidentiality. Your best option for obtaining samples is your local ready mix company. If you need to conduct tests on your samples, the EERC does have the capabilities to create mix designs for specific applications and can perform a number of strength tests, permeability tests, freeze-thaw tests, etc. Additionally, the EERC can provide information that can aid in environmental impact evaluation.
For general information on the use of fly ash in concrete, we recommend the Fly Ash in Cement and Concrete book by Richard Helmuth, published by the Portland Cement Association, or Fly Ash in Concrete by E.E. Berry and V.M. Malhotra, published by CANMET, Energy, Mines and Resources, Canada.
Ash color is caused primarily by the presence of two components. Carbon from the incomplete combustion of coal can lend a gray to black color to fly ash, depending on concentration. Carbon-free ash can range from blue-gray to brown, and the color is generally due to the presence of iron oxides in the ash matrix. Iron in the +2 oxidation state is bluish grey, while iron in the +3 oxidation state is generally brown. The difference in oxidation state of this element is generally responsible for the characteristic color of carbon-free ash. If blue gray ash is ignited in the presence of air at 750° C, the iron will oxidize to give an ash with the characteristic light brown color.
What do other countries do with their coal ash?
Most industrialized countries utilize much more of their coal ash than the United States. Norway and Hong Kong, for instance, utilize all of the coal ash they produce to meet their needs. However, the United States utilizes only one-third of all coal ash produced.
