Fine slag and coarse slag are the byproducts of the entrained-flow gasifier, which may contain a considerable amount of carbonaceous residue. In this study, the characteristics of slags (LHA, LHB, and GT) from three commercial gasifiers were collected and studied. The coal-water slurry (CWS) blended with these different slags (5 wt %) was investigated in detail, including the apparent viscosity, contact angle, and zeta potential. The experimental results showed that the fine slag contained the residue carbon at normally higher than 15 wt % and the combustible content in coarse slag was mainly concentrated in the particle size of 20-60 mesh. According to the reactivity index, the residue carbon in slags from LHB and GT showed higher reactivity compared to that of raw coal at 1300 degrees C. The addition of slag would cause little increase of the apparent viscosity. The addition of fine and coarse slag in CWS was better than the addition of fine slag alone. CWS blended with slags would decrease the contact angle of the samples. All the results showed the feasibility of the high-carbon-content slag for coal water slurry preparation to achieve residual carbon utilization.