Q.: Our specifications for some projects prohibit the use of fly ash. We have reason to believe, however, that fly ash has sometimes been used in violation of the specs. Is there any fly ash test that can be made quickly enough on fresh concrete to permit rejection of a delivery?
A.: If there is unauthorized fly ash in the mix it doesn't always mean it was put there purposely. Sometimes fly ash gets into a mix accidentally through a leaky wall between adjacent cement and fly ash bins or silos. At other times a small amount of fly ash may have got hung up in a chute that was used for batching fly ash for the previous mix and then this may break loose in a batch where it isn't wanted.
Whether fly ash has been purposefully added or not, there is sometimes a need to test for its presence and the Iowa Department of Transportation has worked out a successful test that can be used at the jobsite. It requires a microscope of about 60X to 100X, glass slides, U.S. standard sieves of Number 200 and Number 325 sizes, a plastic squeeze bottle containing water and one containing alcohol.
The procedure begins by nesting the sieves with the Number 325 sieve on the bottom. The operator places about one tablespoonful of the mortar portion of the concrete on the Number 200 sieve and washes it thoroughly, using the squeeze bottle containing water. This causes most of the sand to remain on that sieve while a portion of the cement and fly ash collect on the Number 325 sieve below it. The latter sieve is drained while being tapped lightly. Then the residue on this sieve is washed, using the squeeze bottle containing alcohol, until the liquid coming through is clear. After the residue has been allowed about 15 minutes to dry, a small portion is tapped from the sieve onto a microscope slide and examined, preferably with a dark background. If any glassy spheres can be seen it indicates that there is fly ash in the sample.
Further details and complete procedures are given in a publication of the Iowa Department of Transportation. One method permits taking a sample at the site, placing it in a jar of water and shaking it, and then transferring it to the laboratory for testing at some later time.
Copies of the procedure, "Method of Test for Determining if Fly Ash Is Present in Plastic Portland Cement Concrete or Portland Cement," Materials Instructional Memorandum 371, are available from the Office of Materials, Iowa Department of Transportation, 800 Lincoln Way, Ames, Iowa 50010.