At higher pH values, the formation reaction and crystallization of HA tended to progress more rapidly even at room temperature because NaOH may prompt CO2 gas (generated in the process) to dissolve into the solution and allow for the stabilization of the HA phase at higher pH values [38–40].
We also studied the effect of the milling time on the formation of HA while maintaining the pH at 13.5. As shown in Figure 3, DCPD (showing an intense diffraction line at 2θ = 20.9°) had already disappeared after 30 min. On the other hand, the diffraction lines indicating the presence of CaCO3 decreased with increasing milling time, while the characteristic reflections of HA were clearly observed. When the milling time was longer than 1 h, we obtained a single phase of HA. In addition, we confirmed that the sample obtained by vigorously stirring the starting suspension for 1 h (without milling) contained large amounts of DCPD and CaCO3 (data not shown). From these results, we can conclude that milling and high-pH conditions effectively contribute to the rapid formation of HA.