Mechanism of precipitation and phase composition of CaO–SiO2–P2O5 systems synthesized by sol–gel method

• Synthesis of CaO–SiO2–P2O5 powder systems using water soluble silica precursor
• Final pH value of suspension controlled chemical composition of precipitates.
• The calcium content and Ca/P ratio affected phase composition of systems.
• Si-Hap and Si-TCP were created after annealing of precipitates at 950 °C.

CaO–SiO2–P2O5 powder systems were synthesized by the aqueous precipitation method utilizing sodium metasilicate as a precursor. Products of the synthesis were hydrated calcium silicate phosphate systems with different chemical composition, which was controlled by the change in pH value of the final suspension during the precipitation. Both the crystallinity and phase composition depended on calcium content in the final powder system. It was revealed that the amorphous phases were created at a higher calcium content and more basic precipitation conditions whereas the nanocrystalline hydroxyapatite was found at a lower calcium content and even at more acidic precipitation conditions. After annealing of dry precipitates at 950 °C, Si saturated calcium phosphates (hydroxyapatite and α-TCP) were formed in a large amount of the amorphous glassy silica phase and the amount of major calcium phosphate phase was influenced by the calcium content or Ca/P ratio in precipitates. The strong calcium phosphate particle refinement was observed with the rise in silicon content in systems.