Magnesium phosphate cements (MPCs) are a class of chemically bonded ceramics studied as bone grafting biomaterials. To gain control over the reaction is deemed necessary in order to modulate the release of heat and extend the working time. To this aim, the use of reaction modifiers has become customary in MPCs for applications. Citric acid (CA) has been considered particularly attractive in the design of Mg-based bio-cements because its biocompatibility [1]; in fact, it can be metabolized by the organism and citrate is present in human bone, where it plays a crucial role for its structural stability. Its mechanism of action in the MPC is poorly understood. For this reason, the cement formulation largely rests on an empirical basis.
In order to gain insights into the process, the interaction of CA with the MPC setting reaction has been investigated adopting an articulated approach. The heat released at different CA contents has been recorded with Isothermal conduction calorimetry. The amount of crystalline and amorphous phase has been determined X-ray powder diffraction, the induced microstructural modifications have been observed with electron microscopy. The complexing ability of the CA molecule in the mutating chemical environment has been studied in-situ with infrared spectroscopy, trying to address molecular issues. Moreover, X-ray photoelectron spectroscopy was employed to detect the interaction of CA with the solid surfaces.
The results indicated that the role of CA in MPC is manifold [2], and includes enhanced dissolution of MgO by promoting surface ligand-exchange reaction, which leads to a net acceleration of the first reaction step.
The Mg²⁺ ions released in solution are complexed by citrates. The degree of supersaturation is therefore reduced, delaying the nucleation of phosphates. The growth of stable nuclei, the crystal growth, and the amorphous-to-crystalline transformation are hindered due to citrate adsorption. The formed surface complexes are prevalently inner-sphere complexes exhibiting the combined coordination of hydroxyl and carboxylate groups. The mutating chemical environment dictates the coordination modes of citrate, the competition with phosphates, and the stable forms of phosphate products. This investigation is thought to convey crucial information to help in the effective material design.

[1] Klammert, U., Ignatius, I., Wolfram, U., Reuther, T., Gbureck, U. (2011) Acta Biomaterialia. DOI: 10.1016/j.actbio.2011.05.022.
[2] Viani, A., Mácová, P., Šev?ík, R., Zárybnická, L.  (2022) Ceramics International.   DOI: 10.1016/j.ceramint.2022.11.308