Influence of particle size distributions on the density and density gradients in uniaxial compacts

This study investigated the influence of particle size distributions on the density, density gradient and strength of uniaxially die-pressed alumina refractories. To design the particle size distributions with a maximum grain size of 1 mm, a modified Andreasen model was applied. It allowed the fine and coarse fractions to be adjusted separately. Cylinders were double-action or two-sided die-pressed. The samples' bulk densities were measured before and after firing (1600 °C). From the bulk densities, the shrinkage was calculated. The cold crushing strength (CCS) was determined after firing. Some fired samples were axially and radially cut to analyze the density distribution and gradients. Bulk densities before and after firing, shrinkage and strength were highest for increasing amounts of the finest fraction (0-0.02 mm) up to about 55 wt%. Shrinkage and fired bulk density correlated strongly with the CCS. The axial density gradients decreased for increasing fine fraction and decreasing coarse fraction amounts. The radial density gradients were diminished for increasing fine fraction amounts only. Summarized, densest compacts with lowest density gradients were achieved for particle size distributions with a reduced amount of medium sized grains. These batches contained ≈65wt% fine particles (0-0.2 mm), 10 wt% medium sized grains and ≈25wt% coarse particles (0.5-1 mm).