Influence of temperature and polyphosphate contents on hydroxyapatite crystallization in calcium aluminate cement modified with sodium polyphosphate

Calcium aluminate cement modified with sodium polyphosphate (CAP) contains precursors to hydroxyapatite (HAp) within its matrix. This study explored HAp crystallization in the CAP system by varying the concentrations of the sodium polyphosphate solution (ratios of sodium polyphosphate to water at 0.46, 0.80, and 1.14) and temperatures (60, 90, 120, and 150 degrees C). By curing for 7 days at 25 degrees C, the CAP system was able to solidify and avoid the conventional hydration reaction of calcium aluminate cement. When the ratio of sodium polyphosphate to water was 0.46, the hydrated minerals of calcium aluminate cement (hydrogarnet, gibbsite, and boehmite) and HAp crystallized together through thermal treatment. With a ratio of 0.80, monocalcium aluminate was entirely consumed by the thermal treatment, facilitating the formation of HAp and boehmite. However, with a ratio of 1.14, only a small amount of HAp crystallized, leaving many of the clinker phases unreacted. Furthermore, the FTIR results showed an increase in the peak corresponding to P-O-P or P-O-H (P-O-Na) as the sodium polyphosphate increased, suggesting that the formation of sodium calcium orthophosphate gel is less likely at higher levels of polyphosphate due to the presence of linear metaphosphate chains. These results indicate that the concentration of sodium polyphosphate in the mixing solution affects the types of amorphous gel in the CAP system. It was determined that the conditions where HAp crystals are most abundant suggest that the appropriate ratios of sodium polyphosphate to water and polyphosphate to cement are 0.80 and 0.40, respectively.