Ceramics in the Ca2P2O7–Ca(PO3)2 System Obtained by Annealing of the Samples Made from Hardening Mixtures Based on Calcium Citrate Tetrahydrate and Monocalcium Phosphate Monohydrate

Abstract

Calcium phosphate ceramics in the Ca2P2O7–Ca(PO3)2 system was obtained by annealing of hardened samples formed from highly loaded suspensions (HLS). HLSs included water and a homogenized powder mixture of calcium citrate tetrahydrate Ca3(C6H5O7)2 ⋅ 4H2O and monocalcium phosphate monohydrate Ca(H2PO4)2 ⋅ H2O. The latter was taken in the amount providing the molar ratio of Ca/P = 1 in the powder mixture, as well as at a molar ratio of Ca/P < 1 so that the excess of monocalcium phosphate monohydrate Ca(H2PO4)2 ⋅ H2O provided the formation of the calcium polyphosphate Ca(PO3)2 phase upon annealing. The samples were molded by extrusion of the HLS through a die with a diameter of 2 mm by sandwiching powder cords. After hardening and drying, the phase composition of all obtained samples included brushite CaHPO4 ⋅ 2H2O, monetite CaHPO4, monocalcium phosphate monohydrate Ca(H2PO4)2 · H2O, and octacalcium phosphate Ca8(HPO4)2(PO4)4H2O. According to the XRD data, the phase composition of all ceramic samples after annealing in a range of 800–1000°C included calcium β-pyrophosphate β-Ca2P2O7, and a ceramic sample obtained from a powder mixture with the highest excess of monocalcium phosphate monohydrate Ca(H2PO4)2 · H2O additionally included tromelite Ca4P6O19. Hardening HLSs containing water and a homogenized powder mixture of calcium citrate Ca3(C6H5O7)2 · 4H2O and monocalcium phosphate monohydrate Ca(H2PO4)2 · H2O can be used for layer-by-layer extrusion molding. The resulting calcium phosphate ceramics with lowered sintering temperature can be used for the design of a resorbable matrices of tissue engineering constructs.