Influence of Ca/P ratios of starting solutions on the crystallization of amorphous calcium phosphate to hydroxyapatite

Amorphous calcium phosphate was precipitated at 20°C from highly supersaturated solutions having compositions of 1.5≤Ca/P≤2.0 at a pH=11. The phase evolution and morphological changes in precipitates as a function of aging time were investigated using XRD and TEM analyses. Higher Ca/P molar ratio of the starting solution resulted in faster crystallization to hydroxyapatite. Shorter induction times for crystallization were attributed to smaller particle sizes of the initially precipitated ACP as well as to the Ca-rich environment. Nucleation of the hydroxyapatite phase was suggested to be a major mechanism of crystallization, which was supported by the observation that no considerable growth of hydroxyapatite occurred during further aging. The increase in the number of HAp nuclei was accompanied by a size reduction of particles of amorphous phase.