The ideal mechanical strength and critical porosity of calcium phosphate cement (CPC) were estimated to help determine ways to improve its properties. CPC at various porosities was made by packing CPC paste, at various powder‐to‐liquid (P/L) ratios (2.0–6.0), into a mold under various pressures (0–173 MPa). The mechanical strength of CPC, in terms of diametral tensile strength (DTS), increased with decreases in porosity. Intercrystalline fracture was observed in specimens made without the application of pressure, while fracture within the crystals increased with the packing pressure. These observations support the application of the relationship between DTS and porosity in fractographic equations. The ideal wet DTS and critical porosity of CPC were estimated to be 102 MPa and 63%, respectively. The minimum porosity of the currently used CPC was approximately 26–28%, even when it was packed under 173 MPa, and the maximum DTS value was thus approximately 13–14 MPa. Because reducing the porosity of currently used CPC would be difficult, we conclude that in CPC‐related research, we should focus on ways in which to accelerate bone‐replacing behavior, in addition to improving the mechanical strength of CPC. © 1995 John Wiley & Sons, Inc.
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