Influence of texture in bauxite residues on void ratio, water holding characteristics, and penetration resistance

The low water holding capacity of the residue sand (RS) component of bauxite refining residue limits plant growth during the prolonged dry periods. Whilst an increase in fines-content would be expected to increase the water holding capacity, it may have a detrimental impact upon other soil physical properties. Using undisturbed (puddled) mixtures in which intra-aggregate voids dominated (i.e. voids resulting from the packing characteristics of the primary mineral particles rather than from soil structure), an experiment was conducted to investigate the effect of adding fines-material to RS on plant available water content (PAWC) and penetration resistance (Pr). As predicted by the particle packing theory of binary mixtures, an increase in the fines-content of RS resulted in, first a decrease, and then an increase in the void ratio. These changes in void ratio (and pore size distribution) influenced both the PAWC and Pr of the mixtures. Initially, an increase in the fines-content tended to decrease PAWC slightly due to a decrease in the total void space. However, beyond the minimum void ratio (emin), the addition of further fines-material resulted in an increase in PAWC due to an increase in the void ratio. Trends in Pr approximated the inverse of changes in the void ratio; the addition of fines-material causing an increase in Pr in the coarse matrix ( emin). This study suggests that in undisturbed mixtures, improvements in PAWC were identifiable only after addition of substantial quantities of fines-material. Indeed, the initial addition of fines-material to RS (i.e. < emin) not only resulted in a decrease in PAWC but an increase in Pr.