The discovery of calcium phosphate cements (CPCs) constitutes a significant advance in the field of biomaterials for bone reconstruction, especially for injectable compositions which allow implantation using minimally invasive surgical techniques [1-5]. These formulations most often consist of combinations of calcium orthophosphates. They have been extensively developed to obtain biocompatible materials showing improved mechanical properties in comparison to ceramics, while preserving their ability to be re-sorbed in vivo and replaced over time by newly formed bone. To be of practical use in a surgery room, the cements must reach a suitable mechanical strength after an acceptable time. Indeed, from the moment it is prepared, the cement paste changes from a fluid to a rigid state until its plasticity is completely lost and a specific mechanical resistance is reached (generally between 5 and 50 MPa in compression). Throughout this setting period, there is a time window during which the cement can be handled (e.g. for surgical implantation). The hardening process then continues while the mechanical strength of the cement paste gradually increases. Therefore, determining the initial and final setting times of injectable bone cements is of great importance. For this purpose, two standardized methods are classically used: the Gillmore needles [6,7] or Vicat needle [8] tests. These methods are based on the concept of "visible indentation", which is operator-dependent and can result in poor reproducibility from one research group to another. Furthermore, the methods are unable to provide information about the progress of the chemical reaction controlling the setting process. Monitoring chemical characteristics (i.e. nature of the products and their relative amount) and physical properties (i.e. porosity and mechanical strength) of the cement as a function of time most often entails the collection of samples at regular intervals and requires rather sophisticated characterization techniques, including X-ray diffraction, solid-state NMR, FTIR, mercury intrusion porosimetry, calorimetric analyses, scanning electron microscopy, compressive strength measurements or gas adsorption/desorption for surface area determination.
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