Effects of gypsum and sewage sludge amendment on physical properties of fine bauxite refining residue

The poor soil structure and low hydraulic conductivity of fine bauxite refining residue (red mud) from alumina production are the major factors hindering its revegetation. Red mud, disposed of by the dry stacking-method, was amended with gypsum (38.5 and 77 t/ha) and sewage sludge (38.5, 77, and 144 t/ha) to evaluate their effects on soil physical properties in a field experiment. Sewage sludge amendment significantly reduced soil bulk density (25%) and particle density (9%), and increased the total porosity of red mud (8%). Both sewage sludge and gypsum contributed to the increases in hydraulic conductivity of red mud (from 1.3 to 24 x 10⁻⁵m/s) after one growing season. Plant cover percentage and dry weight yield of Agropyron elongatum increased with an increase in gypsum and sewage sludge amendment. Plant growth did not significantly affect soil physical properties, but the enhanced growth was due to improved soil structure and hydraulic conductivity. An application of 77 t/ha gypsum and 144 t/ha sewage sludge exerted the maximum effect on soil physical properties of red mud and should ensure the initial establishment of plants.