Measurement of Yield Stress and Flow Potential of Fly Ash Slurries Using a Rheometer

Failure of coal ash slurry impoundments generally results in an expeditious flow of toxic waste materials for several miles with detrimental impacts. To date, many studies have been undertaken to analyze failure causes and the failed ash materials. Yet, the flow and rheological behavior of fly ash slurries when an impoundment fails is still not yet well understood. In this study, rheological test results of six fly ash slurries at different solids content were used to investigate fly ash slurry flow behavior and yield stresses. In addition, anionic polyacrylamide was used to investigate the influence of a flocculant on the yield stress of fly ash slurries. It was found that flow behavior and yield stress of fly ash slurries mostly depend on the chemical properties of fly ash, the state and degree of hydration (when received), and solids content of fly ash slurries. The inter-connection between these influencing parameters is complex, and the influence of a single parameter was difficult to establish. Some hydrated fly ash slurries showed similar rheological behavior as non-hydrated fly ash slurries, whereas other hydrated fly ashes showed diagenesis behavior, turning into a hard, solid-like material that did not flow. The measured yield stresses showed an increasing trend with increasing solids content of all fly ash slurries. The use of the anionic polyacrylamide noticeably increased the yield stress of hydrated fly ash slurries by 10–50 times and 190 times for non-hydrated fly ash slurry. This is a very promising result, and this approach can be used to minimize the flow of fly ash slurries if an impoundment fails.