Abstract
TTCP cement recipes must distinguish molar ratios from weighed mass. This note converts equimolar TTCP/DCPA and TTCP/DCPD mixtures into weighing quantities and identifies checks for the resulting cement. The two dicalcium phosphate forms require different mass ratios and cannot be substituted without evaluation.
Balance the calcium and phosphate
Tetracalcium phosphate (TTCP) has formula Ca₄(PO₄)₂O. Dicalcium phosphate anhydrous (DCPA) is CaHPO₄; the dihydrate (DCPD) is CaHPO₄·2H₂O. One mole TTCP plus one mole of either dicalcium phosphate supplies five calcium and three phosphorus atoms: an overall Ca/P ratio of 1.667.
Using approximate formula masses TTCP 366.25, DCPA 136.06 and DCPD 172.09 g/mol gives the equimolar mass splits below. These are stoichiometric calculations for pure starting solids. A 50/50 mass mixture would instead contain a large molar excess of dicalcium phosphate.
For the anhydrous pair, 2 TTCP + 2 DCPA = HA. For the dihydrate pair, 2 TTCP + 2 DCPD = HA + 4 H₂O. These overall balances do not guarantee complete reaction. A published injectable system used equimolar TTCP/DCPD with a phosphate/HPMC liquid [1]; it is a distinct formulation, not a drop-in equivalent to an α-TCP cement.
| 10 g total solids | TTCP mass | Dicalcium phosphate mass |
|---|---|---|
| TTCP + DCPA | 7.29 g | 2.71 g DCPA |
| TTCP + DCPD | 6.80 g | 3.20 g DCPD |
| 50/50 TTCP/DCPA | 5.00 g | 5.00 g; not equimolar |
| With inert filler | Recalculate reactive fraction | Preserve ratio within that fraction |
Protect the reaction balance
1. Allow for the actual material
Use phase identity, hydration state and assay information to assess the weighed quantities. Loss on drying is not automatically a correction for crystal water: aggressive drying may change DCPD itself. If the lot contains secondary phases, a single moisture correction cannot restore the intended chemistry.
2. Define the additive basis
For an illustrative 10 g formulation with 20 wt% nonreactive filler, only 8 g remains for the reactive pair. An equimolar TTCP/DCPA fraction would contain about 5.83 g TTCP and 2.17 g DCPA, plus 2.00 g filler. Adding 2 g filler to 10 g reactive powder gives 16.7 wt% filler instead.
3. Verify the hardened product
Keep the liquid chemistry and test age fixed. Compare the expected apatite phase with residual TTCP or dicalcium phosphate using XRD. If the residual pattern changes across batches, check mixing, size distribution and reactive ratio before attributing the difference to purity alone.
Development decision
Use the molar calculation to define the reactive powder charge; use setting, delivery and phase data to select the working formulation. Preserve both the total-powder and reactive-powder ratios in every formulation record.
References
[1] Burguera EF, Xu HHK, Sun L. Injectable calcium phosphate cement: Effects of powder-to-liquid ratio and needle size. J Biomed Mater Res B Appl Biomater. 2008;84(2):493-502. doi:10.1002/jbm.b.30896.
https://pmc.ncbi.nlm.nih.gov/articles/PMC2652762/