27    COMPACTION OF FLY AND
BOTTOM ASH MIXTURES
Ahmed Korayem "Karim," Doctoral Candidate
Rodrigo Salgado, Assistant Professor
C. W. Lovi'll. Professor
School of Civil Engineering, Purdue University
West Lafayette, Indiana 47906
INTRODUCTION
Fly ash and bottom ash constitute the major components of coal combustion by-products.
Nearly three million tons of these materials are produced annually by coal burning power plants
in Indiana. The major part of these materials is disposed of as a form of waste. Utilization of
these materials in embankment construction, besides reducing the disposal problem, can save
natural soils for more efficient utilization. This chapter describes a laboratory study that investigates the behavior of explicit mixtures of fly and bottom ash in compaction. Samples of bottom
ash and Class F fly ash were collected. Mixtures of fly ash and bottom ash in known proportions
were developed in the laboratory. The behaviors of these materials in compaction were studied
and the effects of increasing the fly ash contents in the mixtures were investigated. Increasing the
fly ash content from zero to an optimum value led to reducing the optimum moisture content as
well as increasing the maximum dry density of the compacted mixture. Increasing the fly ash
content further leads to reduction of the maximum dry density and increasing the optimum moisture contents. The results are especially important for controlling the compaction of ash mixtures
in highway embankments.
BACKGROUND
Nearly 23% of the United State's electricity is generated by coal burning power plants.1 Approximately 85 million tons of coal combustion by products (CCBPs) are produced annually.2
While parts of these CCBPs have found their way to utilization, the majority are still disposed of
as a form of waste. Fly ash and bottom ash represent the major part of the CCBPs. Fly ash and
bottom ash represent the noncombustible portion in addition to the unburned portion of the coal.
Utilization of compacted single types of ash, whether fly or bottom ash, in embankment construction was reported in the literature.3-4 However, the largest portions of the fly and bottom ash
are currently disposed of in the form of mixtures in disposal ponds.5 The utilization of these mixtures in embankment construction has been delayed because of concerns about the control of
their compaction on the site, and the behavior of the compacted mixtures in the embankment
body.
Fly/Bottom Ash Generation and Disposal
Class F fly ash is generated when bituminous or anthracite coal is burned. It consists typically
of spherical fine particles that are carried by the combustion hot gases through the top of the
combustion chamber. The ash is then separated from the gases by some form of collection system such as electrostatic precipitators. The ash is then collected and either stored in silos in a dry
condition or pumped hydraulically to a pond. Fly ash particles sizes can typically be found in the
range of 75 microns to a fraction of a micron. More than 90% by weight is typically smaller than
45 microns. This size range is comparable to the silt size range. Commonly, fly ash is either dis-
51st Purdue Industrial Waste Conference Proceedings. 1996. Ann Arbor Press, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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