Sodium phosphate-derived calcium phosphate cements

Abstract

Calcium phosphate cements (CPC) were synthesized by the acid-base reaction between sodium phosphate, NaH 2PO 4 or -(-NaPO 3-)- n, as the acid solution, and calcium aluminate cements (CAC) as the base reactant at 25 °C. The extent of reactivity of -(-NaPO 3-)- n with CAC was much higher than that of NaH 2PO 4, thereby resulting in a compressive strength of > 20 MPa. Sodium calcium orthophosphate (SCOP) salts as amorphous reaction products were responsible for the development of this strength. When this CPC specimen was exposed in an autoclave, in-situ amorphous → crystal conversions, such as SCOP → hydroxyapatite (HOAp), and Al 2O 3· xH 2O → γ-A100H, occurred at ≈ 100 °C, while the rate of reaction of the residual CAC with the phosphate reactant was increasingly accelerated by hydrothermal catalysis. Based upon this information, we prepared lightweight CPC specimens by hydrothermally treating a low-density cement slurry (1.28 g/cc) consisting of CAC powder, -(-NaPO 3-)- n solution, and mullite-hollow microspheres. The characteristics of the autoclaved lightweight specimens were a compressive strength of > 9.0 MPa, water permeability of ≈ 5.0 × 10 −3 milli darcy, and a low rate of alkali carbonation. The reasons for such a low carbonation rate reflected the presence of a minimum amount of residual CAC, in conjunction with the presence of HOAp and γ-A100H phases that are unsusceptible to wet carbonation.