To investigate the formation of HA during the mechanochemical process, Fe3O4-free HA was first synthesized using a suspension of CaHPO4·2H2O (dicalcium phosphate dihydrate; denoted hereafter as DCPD) and CaCO3 as the starting material. The pH of this suspension was varied between 12.0 and 13.5 by adjusting the amount of NaOH solution added. The XRD patterns for several samples obtained by a 6-h milling process at different pH values are shown in Figure 2. When the pH was lower than 13.0, CaCO3 remained in the products even after 6 h. At a pH of 13.5, however, the diffraction lines corresponding to CaCO3 disappeared, implying that the formation reaction of HA (expressed by Equation (1) [35–37]) was complete after milling.