This article deals with the effect of slurry composition and volumetric concentration on the flow behavior of slurries containing fine-grained and coarse-grained particles. Fluidic fly and bottom ash slurries and sand slurries were experimentally investigated. Kaolin slurries with and without a peptizing agent were used as the carrier liquid for the sand slurries to compare the effect of Newtonian and non-Newtonian carriers. The study revealed a time-dependent yield pseudo-plastic behavior of fluidic fly and fly/bottom ash slurries and the possibility of substantial reduction of the flow resistance by mechanical treatment or by the arrangement of particle size distribution. The flow behavior of fluidic ash slurries can be approximated by the Bulkley-Herschel model in the laminar region. In the turbulent region, the Wilson or Slatter models can be used. The effect of the size distribution of the sand slurry on the hydraulic gradient depends on the flow velocity. The coarse sand slurry reaches a higher hydraulic gradient than the fine sand slurry: the difference decreases with growing velocity. The highly concentrated sand-kaolin slurries show non-Newtonian behavior. When the carrier kaolin slurry is peptized, the hydraulic gradient in the laminar region becomes markedly lower, and the favorable effect vanishes in the transitional and turbulent regions. The addition of small amounts of kaolin favorably affects the flow behavior of the sand slurry.