Influence of Particle Morphology on the Hydraulic Behavior of Coal Ash and Sand

Flexible-wall hydraulic conductivity tests were carried out on bottom ash, fly ash and compacted specimens of sand with additions of 0, 3, 6, 9 and 18% of bentonite. In order to study the effect of bentonite inclusion and particle morphology on the hydraulic conductivity of the admixtures, an investigation was undertaken based on thin section micrographs. It was found that, for both bottom and fly ash admixtures, bentonite addition reduced only one order of magnitude the hydraulic conductivity, from 1.78 × 10⁻⁶ m/s to 1.39 × 10⁻⁷ m/s. On the other hand, the sand hydraulic conductivity was reduced five orders of magnitude, from 3.17 × 10⁻⁵ m/s to 5.15 × 10⁻¹⁰ m/s. Among several factors that can be responsible for the difficulty in reducing hydraulic conductivity, such as ash grain size distribution and elevated cation concentration (leached from the ash) in pore water, it can also be recalled the high particle voids observed in the ash by means of microscopic analysis. The same is not true with the sand, which has solid particles, without inner voids.