Effect of β-TCP granularity on setting time and strength of calcium phosphate hydraulic cements

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The effects of β-tricalcium phosphate (β-TCP) granularity on the properties of calcium phosphate hydraulic cements (CPHC) have been investigated. A model system based on mixtures of (β-TCP) and aqueous solution of orthophosphoric acid has been used. Powders with different shapes (irregular agglomerates or spheres) and sizes (d 50=7 to 130 μm) were prepared from two different calcium phosphate sources: Ca-deficient hydroxyapatite (DAP) or hydroxyapatite dicalcium phosphate mixtures (HAP-DCP). The cements exhibited setting times (ST) ranging from 65 to 510 s; they are mainly affected by the specific surface area (S BET) of the β-TCP powders, longer ST corresponding to lower S BET. In general, lower S BET and shorter ST values were obtained with HAP-DCP powders. Diametral tensile strengths (DTS) ranging from 3.5 to 10.4 MPa were obtained. The results show that DTS is affected in a complex way by the experimental variables. In general, DTS is higher with irregular agglomerates compared to spheres (+1.2 MPa), while better results are obtained with HAP-DCP powders (+0.9 MPa). The dependence of DTS on particle size is variable according to powder source and shape. The highest DTS (10.4 MPa) was obtained with irregular agglomerates prepared from HAP-DCP mixtures (d 50=16 μm), and the lowest (3.5 MPa), with irregular agglomerates prepared with DAP (d 50=123 μm). It can be concluded from this work that specific CPHC formulations can give quite different cement properties, such as setting time and ultimate mechanical strength, depending on the characteristics of the raw materials used. In the case of β-TCP based cements, the granularity of the starting cement powder, including particle size, shape and specific surface area, is of crucial importance and should be specified when the performances of different formulations are to be compared.