Mechanism of magnesium phosphate cement retardation by citric acid

Citric acid modulates the release of heat and the rates of reaction in magnesium phosphate cements, chemically-bonded ceramics employed in biomaterials, for the encapsulation of nuclear wastes and in civil engineering. To gain knowledge on the mechanism of action of citric acid and, therefore, help in the effective material design, the reaction was studied in-situ to address molecular issues. The results indicated that citric acid enhances 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.