Crystallisation of Tricalcium Aluminate (TCA) investigated by using different sources of slaked lime with sodium aluminate solution for the production of filter aid for polishing of aluminate liquor of Alumina Refinery. The reactivity of slaked lime depends on the formation of crystal size and those on the product are discussed. From the experimental results, it had observed that the reactivity and crystal size of calcium hydroxide is played a major role during formation of TCA. The TCA particle morphology indicated as bipyramidal was evident from the micrographs. The effect of particle size depends on operating parameters, the intensity of stirring and residence time assessed for the liquor as a filter aid. The medium reactivity of slaked lime, agitation intensity of 500 RPM with 30min residence time forms the crystalline solid with size ranging d50~10-15 micron as for better filterability and lower the solubility of Calcium ion in Sodium Aluminate liquor.