THE LOW-TEMPERATURE FORMATION OF OCTACALCIUM PHOSPHATE

The low temperature formation of octacalcium phosphate (Ca8(HPO4)2(PO4)4.5H2O)was investigated. Octacalcium phosphate (OCP) was formed by the hydrolysis of alpha-tricalcium phosphate (alpha-TCP). and by reaction between monocalcium phosphate monohydrate (MCPM) and tetracalcium phosphate (TetCP). Relationships between phase formation, microstructural evolution, and variations in solution chemistry were examined. Hydrolysis of alpha-TCP to form OCP occurs more rapidly at elevated temperatures. At the highest temperature studied, 70-degrees-C, initial precipitation of OCP occurs in about one hour, but its inevitable hydrolysis to the more stable HAp phase takes place over several days. At room temperature, nearly three days are required to initiate OCP formation, yet remains a final product phase for well over a period of months. When the initial solution pH is less than 7, pure phase OCP is the final product, while HAp forms when initial pH values are higher than this. Furthermore, OCP precipitates faster at the highest initial pH values where its formation is observed. OCP formation by reaction between MCPM and TetCP is also dependent on temperature and time of reaction. For temperatures between 40 and 55-degrees-C and at reasonable times (less than four days) the product is phase pure OCP. After that time, inevitable hydrolysis of the OCP product to HAp occurs. Between 30 and 40-degrees-C, DCPD (CaHPO4.2H2O) is simultaneously present with OCP, and below 30-degrees-C, HAp and DCPD coexist within this aforementioned time span. Conversely, between 55 and 60-degrees-C, DCP (CaHPO4) and OCP are the product phases (in the allowed time of reaction, before the eventual degradation of OCP), and above 60-degrees-C HAp and DCP are the final products.