Effect of mechanochemical treatment on the synthesis of calcium dialuminate

Calcium dialuminate (CaAl4O7) powders were synthesised from mechanochemical ly treated mixtures of aluminium hydroxide + calcium hydroxide and aluminiu m hydroxide + calcium carbonate. On grinding, both mixtures produce X-ray a morphous precursor phases which show Al-27 MAS NMR resonances characteristi c of Al in octahedral and tetrahedral sites, and a site identified by a res onance at 37-39 ppm (possibly pentahedral Al). Although grinding does not c ompletely destroy the carbonate XRD reflections in the carbonate mixture, b oth precursors show a high degree of homogeneity and behave similarly on he ating, forming monophase CaAl4O7 at < 1050 degrees C. By contrast, the same unground compositions form mixtures of alpha-alumina and various calcium a luminates (but not CaAl4O7) on heating as high as 1250 degrees C. Calcium d ialuminate synthesised from the carbonate-containing mechanochemical precur sor had a smaller particle size which may be advantageous for subsequent fa brication and sintering.