The presence of calcium hydroxide (Ca(OH)(2)) in Bayer residue slurry inhibits the effectiveness of the seawater neutralization process to reduce the pH and aluminum concentration in the residue. An increase in the slurry pH (reversion), after seawater neutralization, is caused by the dissolution of calcium hydroxide and hydrocalumite (solid components found in bauxite refinery residue). Reversion was not observed when the final solution pH was greater than 10.5, due to hydrocalumite being in a state of equilibrium at high pH. Hydrocalumite has been found to form during the neutralization process when high concentrations of calcium hydroxide are present in the residue liquor. The dissolution of hydrocalumite releases hydroxyl (OH-) and aluminum ions back into solution after the seawater neutralization (SWN) process, which causes pH and aluminum reversion to occur. This investigation looks at the effect of Ca(OH)(2) and subsequently hydrocalumite on the pH and aluminum concentration in bauxite refinery residue liquors after the SWN process.