Permeability of Fly Ash and Fly Ash-Sand Mixtures

Abstract

In an experimental program aimed at evaluating the potential of fly ash for use in liners, permeability tests were performed on two pozzolanic fly ashes from western U.S. coal sources compacted at varying densities and mixed in varying percentages with a quartz sand. Permeability testing was performed in a falling head, flexible membrane, triaxial cell permeameter. A back pressure of 380 kPa and a hydraulic gradient of 10 to 20 were applied to the permeability specimens. The two pozzolanic fly ashes exhibit different degrees of “self-cementation,” which causes a dramatic influence on the resulting permeability values. The highly self-cementing fly ash exhibits nearly a full three orders of magnitude lower permeability than the less self-cementing fly ash. Increasing density, while not as important as the coal source, decreases permeability in both types of fly ash on the order of about half a magnitude. Moisture content is also an important factor since it controls the extent of cementation as well as compaction density. Increasing percentages of fly ash in the mixtures decreases the permeability up to a limiting value when the volume of fly ash and water added exceeds the available pore space between the sand grains; for instance, the mixture of 40% self-cementing fly ash and 60% sand exhibits essentially the same permeability as the 100% fly ash specimen. The study indicates that the addition of pozzolanic fly ash to an otherwise highly permeable soil results in a dramatic reduction in permeability. The “self-cementing” fly ash exhibits 103 to 104 folds of permeability reduction with final permeabilities of less than 10−6 mm/s and seems quite suitable for liner applications.