ETL 1110-3-503
15 Sept 99
are physically similar to cement kiln dusts, but differ in chemical make up and are by-
products of the manufacture of lime. As of 1994, about 2 to 4 million metric tons of lime
kiln dust is generated in commercial lime plants (Collins and Ciesielski 1994).
Kiln dusts have had some usage as mineral fillers in hot-mix asphalt, stabilizers for base
courses, and as stabilizers for sewage sludge. Only a few states have used kiln dusts fo r
pavement applications and the results of these have not always been successful (Collins
and Ciesielski 1994). One study using cement kiln dust found that the material could be
used to reduce the incidence of wetting-induced failure or collapse of highway
embankments (Miller et al. 1997). In one application involving the stabilization of a
roadway test section base course, a cement kiln dust and a fly-ash were successfully used
as a replacement for either lime or portland cement. During construc tion it was found
that delaying placement of the mixture for 8 hours after wetting resulted in substantial
increases in strength (Lin and Zhang 1992).
d. Coal ash by-products. Coal ash is a by-product of the burning of coal for power
generation. The coal ash generated consists of fly ash captured from the exhaust gases
and bottom ash and boiler slag remaining after combustion is complete (Ahmed 1991).
Coal burning plants are located in at least 44 states throughout the United States. In
1992, these plants generated 66 million tons of coal ash, consisting of 48 million tons of
fly ash, 14 million tons of bottom ash, and 4 million tons of boiler slag. About 12 million
tons of the fly ash was used for a variety of applications (Collins and Ciesielski 1994 and
American Coal Ash Association 1992). In 1998, 90 tons of fly ash were produced and
12 million tons were utilized in portland cement concrete (ASCE 1998 and NaQuin
1998).
Fly ash is a pozzolan, meaning that when combined with calcium and water, it reacts to
form cementitious materials. Fly ash can be classified as either Class F or Class C,
depending on the type of coal burned. While both classes are considered to be
pozzolanic, Class C ashes are usually self hardening (Halstead 1986). Fly ash is one of
the few waste materials which has an ASTM standard for procedures of sampling and
testing. The use of fly ash in portland cement concrete mixtures is the most widely used
and accepted application by state and federal agencies. Depending on the use and
requirements, fly ash can be used to replace some of the cement in the mixture. When fly
ash is to be used as a replacement for cement, it requires considerably more testing and
monitoring of quality compared to other uses such as fill materials or as an additive to
hot-mix asphalt (Halstead 1986). Fly ash can not only reduce the overall cost of the
concrete, but it can make the concrete more dense and less permeable (ASCE 1998).
Many producers of fly ash are taking actions to assure or improve the quality of their
product for the users. The Corps of Engineers (CE) and many other Federal and state
agencies use fly ash. Fly ash has been used in roller compacted concrete mixtures
(Shelburne and Degroot 1998). The one common limitation to fly ash replacement of
portland cement is the increased cure time. The necessary cure time can vary depending
upon the particular fly ash used and local climatic conditions. Cement replacements of
up to 50 percent have been used in mass foundations of bridge piers; however,
replacements of 25 percent are more common (Collins and Ciesielski 1994 and NaQuin
1998).
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