Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa

Reclaimed High Calcium Fly Ash Use as a Highway Base Material

K.L. Bergeson, R.K. Lapke, and D.R. Overmohle

Geotechnical and Materials Division,
Department of Civil and Construction Engineering,
482B Town Engineering Building,
Iowa State University,
Ames, Iowa 50011.

Reclaimed fly ash for this research project was obtained from the sluice pond disposal site at the Unit #3 power plant in Council Bluffs, Iowa. The process of reclaiming the hardened ash involved using a single pass of a conventional Bomag MPH-100 scarifier-reclaiming unit. A six- to eight-inch thickness of the hardened ash was scarified and pulverized to a maximum size of less than two inches. The scarified material was picked up by an end loader and stockpiled at the site. Laboratory testing of samples from the stockpiled material indicated the reclaimed fly ash nearly meets the individual Iowa Department of Transportation (Iowa DOT) specifications for a low quality Class B crushed stone with respect to gradation, Los Angeles Abrasion, and freeze-thaw durability. The reclaimed ash exhibited bulk specific gravities in the range of 1.50, characteristic of a lightweight aggregate, and absorption values of about 25 percent. Reclaimed material was separated and graded to a 0.45 power grading curve with particle size ranging from one inch down to the #200 sieve size. Standard Proctor testing indicated an optimum moisture content of nine percent (free water) and a maximum dry unit weight of 82 pcf. ASTM C593 testing was conducted on standard proctor sized samples. Various percentages of cement kiln dust were used as a pozzolanic activator and cementing agent. The results of compressive strength, freeze-thaw, and volumetric shrinkage testing indicate that the utilization potential of reclaimed high calcium fly ash as a highway base material is high.

Previous research conducted at Iowa State University (1,2,3) has indicated that raw high calcium ashes can be economically agglomerated into aggregate sized particles and can be used as an artificial aggregate in concrete and used for base materials. This research was directed at evaluating the use of high calcium fly ash reclaimed from a sluice pond disposal site as a highway construction resource.


Fly ash for this research project was produced at the Council Bluffs Unit #3 plant located near Council Bluffs, Iowa. It is a Class C fly ash and has a calcium oxide content of about 29 percent.

The ash is disposed of at the utility site by end dumping from conventional dump trucks directly into the water of the sluice pond where it is allowed to harden without further manipulation. This process continues by advancing unloading operations out into the sluice pond area by using the hardened ash, previously placed and leveled, as a working platform. The depth of the hardened ash generally ranges from six to eight feet.


The process of reclaiming the hardened ash involved using a single pass of a Bomag MPH-100 scarifier-reclaiming unit. A six- to eight- inch thickness of the hardened ash was scarified and pulverized to a maximum size of less than two inches. The scarified material was picked up by an end loader and stockpiled at the site. Field observations indicated that this procedure using conventional equipment is economical and rapid.

The reclaimed material was bulk sampled from the stockpile and returned to the laboratory for testing. Table 1 summarizes the results of laboratory testing of the reclaimed ash in comparison with Iowa Department of Transportation specifications (4) for a Class B crushed stone subbase material. Sieve analysis results on a representative sample of the reclaimed ash indicate a relatively well graded material with a low amount of minus number 200 sieve size fines. The reclaimed ash would meet Class B crushed stone subbase gradation specifications if the plus-one-inch material were scalped. Los Angeles abrasion testing was conducted in accordance with AASHTO T96 testing procedures. The reclaimed ash exhibited a 54 percent loss which is marginally within specifications. Freeze-thaw soundness testing of the reclaimed ash was accomplished using Iowa DOT test method number 211-A, "Method of Test for Determining the Soundness of Coarse Aggregates by Freezing and Thawing" Method C. The reclaimed ash aggregates exhibited a loss of 28 percent which slightly exceeds the 25 percent maximum specified loss. The specifications, however, also require that the sum of abrasion and freeze-thaw loss not exceed 65 percent. The reclaimed ash aggregate summation was 82 percent which exceeds this. Bulk specific gravities averaged about 1.50 and

absorptions about 24 percent. Dry unit weights of about 90 pcf were exhibited by the plus 2 inch material. These values indicate properties of a lightweight aggregate. This in combination with the freeze-thaw data suggests that the reclaimed material might have a very open pore system, capable of withstanding freeze-thaw pressures, to exhibit a loss of only 28 percent in the freeze-thaw test.

To evaluate the reclaimed aggregate compaction characteristics, standard proctor tests were conducted using samples prepared by combining separate sieve fractions to form a gradation approximating a Federal Highway Administration 0.45 power straight line gradation for a nominal one inch maximum size aggregate. Results of standard proctor tests indicated an optimum moisture content of 34.0 percent and a maximum dry unit weight of 81.7 pounds per cubic foot. Considering an average absorption value of about 25 percent this indicates a free water need of about nine percent at optimum. The low dry unit weight is again indicative of a lightweight aggregate material.


In order to evaluate the strength characteristics of the reclaimed fly ash aggregates as a subbase material, proctor size samples were used with and without portland cement kiln dust as a pozzolanic activator. Samples were prepared and tested using procedures adapted from ASTM Method C593, section 8 "Compressive Strength Development and Freeze-Thaw Resistance of Nonplastic Mixtures." The kiln dust was obtained from Lehigh Portland Cement Company in Mason City, Iowa. The kiln dust exhibited a calcium oxide content of 52.2 percent.

For strength development characteristics proctor sized test samples were used. For sample preparation, air dry aggregates were soaked in water for 24 hours and brought to a saturated surface dry condition. For the two percent kiln dust treated samples, the kiln dust and six percent free water were mixed with the reclaimed aggregate and immediately compacted. For each additional two percent increase in kiln dust, the free water added was increased by one percent to maintain compactibility. Bulk dry densities of samples ranged from 80 to 85 pounds per cubic foot. Samples were cured and tested in accordance with ASTM C593 procedures, and were capped with a standard portland cement capping compound before testing. Figure 1 presents the results of the average strength of at least three specimens for each test condition. Untreated reclaimed ash aggregate samples disintegrated, after seven days of oven curing at 1003 F, both during the water bath and during the vacuum saturation process as per ASTM C593. Kiln dust treated samples exhibited significant strength development with increasing addition levels, and developed strengths above 600 psi at six percent kiln dust and in excess of 1500 psi at the 10 percent kiln dust level. These results indicate that the kiln dust may be functioning as a strong pozzolanic activator for the unreacted glassy phase of the fly ash present on the surface of the reclaimed ash aggregates, and may be producing additional cementitious reaction products. If this is true, long term strength gain would be expected to improve and these materials may exhibit "autogenous healing" properties provided there is available moisture. The mechanism of reaction, the cementitious reaction products being developed, and the nature of the aggregate/matrix bond are subjects of current research.

Dempsey and Thompson (5,6) have shown that the vacuum saturated strength correlates strongly with freeze-thaw strength testing results. For adequate freeze-thaw durability, ASTM C593 specifies a minimum 400 psi strength development under both water bath and vacuum saturation test conditions. Kiln dust treatment levels above six percent by dry weight easily meet these requirements.

Research conducted by Collins and Emery (7) on kiln dust/fly ash systems for base materials, using conventional aggregates, indicated strong compressive strength development and resistance to freeze-thaw degradation using class C fly ashes. Although not directly applicable to materials used in this project, indications are that kiln dust and class C ashes interact to produce cementitious reaction products.

Because of the high absorption values exhibited by the reclaimed ash, additional testing was conducted to further evaluate freeze-thaw durability characteristics. Proctor sized samples were prepared using six and 10 percent kiln dust. At least eight samples were prepared at each addition level and oven cured for seven days. Samples were tested by placing them on a saturated one inch bed of sand (to simulate a saturated subgrade) and tested in a Logan freeze-thaw cabinet. The sand was maintained in a saturated condition with free water available to the proctor size samples at all times during the freeze-thaw process. The Logan cabinet was cycled from 0F to 70F with one freeze-thaw cycle being completed in about six to eight hours. Samples were considered to have failed when the majority of the specimens in each group disintegrated and its cylindrical form could not be observed. The six percent kiln dust treated sample disintegrated in less than 20 cycles. The eight percent treated samples survived about 130 cycles and the 10 percent treated samples survived about 270 cycles before failure indicating strong resistance to freeze-thaw degradation. This is attributed partially to the eight and 10 percent treated samples exhibiting a significant increase in strength over the six percent treated samples. In addition, there may be pore filling taking place from pozzolanic reaction products thereby reducing permeability and increasing resistance to freeze-thaw degradation.


  1. Results of this study indicate that high calcium ashes have the potential for being reclaimed from sluice pond disposal sites using a scarifier-reclaimer to produce an aggregate sized material in an economical, single-pass operation.
  2. Reclaimed hardened ash material, used in this research, exhibited a well graded characteristic with a small amount of non-plastic minus number 200 sieve size material.
  3. Abrasion and freeze-thaw testing for aggregate quality indicate that the reclaimed high calcium fly ash nearly meets Iowa DOT specifications for a low quality crushed stone base material on an individual test basis.
  4. Reclaimed fly ash material characteristics indicated properties similar to lightweight aggregates with high absorption values, low specific gravities, and low in-place unit weights.
  5. The portland cement kiln dust, used in this study at relatively low addition levels, was an effective activator of pozzolanic reactions to enhance compressive strength development and provide for good freeze-thaw resistance.
  6. The results of this study indicate that reclaimed high calcium ashes have a strong potential for utilization in highway base material applications.


This study has been supported by a research grant from Midwest Fly Ash and Materials Company, Sioux City, Iowa. Special thanks is extended to its president, Mr. Lonnie Zimmerman for the research support, for arrangements for the field reclaiming equipment and process, and for assistance in obtaining reclaimed ash samples. Iowa Power and Light Company personnel are also acknowledged and thanked for their cooperation and assistance.


  1. K.L. Bergeson, C.L. Kilgour, and S. Schlorholtz. Agglomeration of High Calcium Fly Ash for Utilization: I. Physical Properties. Fly Ash and Coal Conversion By Products: Characterization, Utilization and Disposal VI, Mat. Res. Soc. Proc., Vol. 178, 1990, pp. 197–205.
  2. C.L. Kilgour, K.L. Bergeson, and S. Schlorholtz. Agglomeration of High Calcium Fly Ash for Utilization: II. Binding Mechanisms and Leaching Behavior. Fly Ash and Coal Conversion By-Products: Characterization, Utilization and Disposal VI, Mat. Res. Soc. Proc., Vol. 178, 1990, pp. 207–216.
  3. C.L. Kilgour, K.L. Bergeson, and S. Schlorholtz. Storage Alternatives for High Calcium Fly Ashes. Fly Ash and Coal Conversion By-Products Characterization, Utilization and Disposal V, Mat. Res. Soc. Proc., Vol. 136, 1989, pp. 161–168.
  4. Standard Specifications for Highway and Bridge Construction. Iowa Department of Transportation, Ames, Iowa, 50010, 1984.
  5. B.J. Dempsey and M.R. Thompson. Vacuum Saturation Method for Predicting the Freeze-Thaw Durability of Stabilized Materials. In Highway Research Record No. 442, TRB, National Research Council, Washington D.C., 1973, pp. 44–57.
  6. Lime-Fly Ash-Stabilized Bases and Subbases. National Cooperative Highway Research Program. In Synthesis of Highway Practice 37, TRB, National Research Council, Washington D.C., 1976.
  7. R.J. Collins and J.J. Emery. Kiln Dust/Fly Ash Systems for Highway Bases and Subbases. U.S. Department of Transportation - Department of Energy, FHWA/RD-82/167, October 1982.

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