Phase evolution during the low temperature formation of stoichiometric hydroxyapatite-gypsum composites

Both plaster of Paris (CaSO₄·(1/2)H₂O, POP) and bone-like synthetic calcium phosphates (CaP) have been used as bone-like cements. The current study investigated the formation of composites involving POP with each of three types of stoichiometric hydroxyapatites (abbreviated as SHAp, S-SH, and C-SH, each with a Ca/P molar ratio of 1.67). The kinetics, variations in solution chemistry during the formation of these composites and phase compositions of the formed products were investigated over a course of 24 h. Although the presence of gypsum precursors was shown to decrease the alkalinity of the medium involving SHAp formation from its precursors, a delay in the growth kinetics of gypsum was observed. The same behavior was observed in the presence of commercial apatite (C-SH), whereas in the presence of synthetic apatite (S-SH), no delay was observed. A possibility of formation of a calcium sulfate phosphate double salt, as an intermediate, was investigated and confirmed by XRD analysis.