Influence of Cu2+-doping in β-TCP on the hydration kinetics of brushite cement investigated by Rietveld analysis and heat flow calorimetryPart of:
Brushite (CaHPO4∙2H2O) formation in calcium phosphate generally occurs under acidic conditions. One example is the acid-base-reaction of monocalcium phosphate monohydrate (MCPM, Ca(H2PO4)2∙H2O) and β-tricalcium phosphate (β-TCP, β-Ca3(PO4)2), which react with H2O according to (1).
Ca(H2PO4)2∙H2O + β-Ca3-xCux(PO4)2 + 7 H2O → 4 CaHPO4∙2H2O (1)
Because of the very rapid reaction of this cement system, a setting retarder is necessary. Phytic acid (IP6, C6H6(OPO3H2)6) is an additive to decelerate the reaction, while further improving injectability of the cement pastes .The use of Cu2+ as dopant for β-Ca3(PO4)2 leads to increased microbacterial activity, hence preventing inflammation  and has positive effects on angiogenesis and wound healing .
The limit of substitution of Cu2+ ions in the structure of β-Ca3(PO4)2 was investigated by Rietveld refinement of powder XRD samples with different amounts of Cu2+ doping up to 20 mol-%. Its effect on the hydration was investigated by isothermal heat flow calorimetry and quantitative in-situ XRD. β-TCP was mixed with MCPM with a molar ratio of β-TCP/MCPM = 1.62 and a water to solid ratio of 0.3 ml/g. The amount of IP6 was set at 2 wt.-% referred to the solid content, all measurements were conducted at 23 °C.
Rietveld refinement showed a significant decrease of the lattice parameters a and c with an increasing amount of Cu2+ up to 15 mol-% for Ca2+. Furthermore the refinement of the site occupancies showed an incorporation of Cu2+ ions at the Ca(5) and Ca(3) atom site. For Cu2+ amounts above 15 mol-% the formation of the secondary phase Ca3Cu3(PO4)4 was observed, whereby the limit of solid solution could be set between 10 and 15 mol-%. Heat flow calorimetry and in-situ XRD measurements showed a clear increasing retardation of the reaction with increasing amount of Cu2+ in the dry powder. Besides a significant decrease of the reaction degree could be determined.
Consequently, the studies provided a deeper understanding of Cu2+ doping in the crystal structure of β-TCP and demonstrated a large impact of Cu2+ on the hydration of β-TCP and MCPM.
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