Calcium phosphate-based bioactive materials have recently attracted considerable attention. In particular, poorly crystalline apatites (PCA) are successfully used for bone repair and regeneration, enhancing the osteointegration and osteoconduction properties. Our study aims to better understand the mechanism of apatite formation on the poorly crystallized apatite powders. For this purpose, two PCA samples were synthesized in the same conditions while varying only the maturation time.The most noticeable difference between both samples is that the immature specimen (PCA2h) was richer in both water and HPO42− labile species than the mature specimen (PCA2m). Besides, it has a higher surface area and more negatively charged surface. Both PCA samples were soaked in a simulated body fluid (SBF) for different periods of time at 37 °C, leading to the neoformation of bone-like apatite layer on their surfaces due to their bioactivity. The surface of all samples was investigated using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDX) spectroscopy, zeta-potential measurements and BET analysis. FTIR spectroscopy, XRD, and TG-DTA were used to characterize bulk samples. Thermodynamic calculations of CaP precipitation were also performed. Experimental and theoretical data showed that the bone-like apatite formation mechanism on both samples undergoes by three stages. Only on the PCA2h, did a fast precipitation of positively Ca-rich amorphous phase (ACP) occur at a first time (prenucleation stage). It is converted into Ca-poor ACP by uptaking anions from surrounding media, to promote the formation of apatite nuclei during the second stage (nucleation stage). Then, nascent apatite crystals grew into bone like-apatite coating by poorly crystallized BCO3-apatite which is the most thermodynamically stable CaP phase in SBF (growing and maturation stage). While the bone like-apatite formation process on the mature PCA particles include similar events without Ca-rich ACP deposit. Thus, the formation of the Ca-poor ACP starts immediately at the beginning of soaking (at the 1st stage), followed by apatite nucleation (2nd stage), promoting the growth and maturation of BCO3-apatite crystals (3rd stage). Our results highlight the fact that in the waterlogged conditions of the hydrated shells surrounding the immature PCA nanocrystals, a high density of labile orthophosphate anions leads enriches the PCA-liquid solution interface in Ca2+ ions, momentarily stabilizing them into a positively charged amorphous complex likely to be strongly solvated. Then, this interface becomes more attractive for free orthophosphate anions, enhancing the bioactivity to improve osteointegration. The composition of the hydrated shells coating of the biomimetic apatite crystal in both labile species and hydration water seems to govern their bioactivity via surface charge potential and hydrophilic capacity. These data allow a better understanding of the bioactive behaviour of biomaterials involving biomimetic PCA phases for tissue engineering.