This article presents the results of characterization and rheological studies conducted on fly ash and fly ash-bottom ash mixture samples at high solids concentration collected from an Indian Thermal Power Plant. The rheological characteristics of the ash-water slurries in the concentration range of 60-65% by weight were best described by a pseudo-plastic power law model. Using the power law flow parameters obtained from the rheological data, a non-Newtonian head loss model was employed to evaluate the head loss of ash slurry pipelines for mine backfilling purpose.