Mechanism of setting reaction in magnesia-phosphate cements

This paper tries to explain the setting mechanism of magnesia-phosphate cements obtained by mixing equal qualities of magnesia (MgO) and monoammonium dihydrogen phosphate (NH4H2PO4) or MAP. As soon as mixing water is added to the mixture, the MAP goes into solution till saturation while magnesia starts to be wetted. The drop in pH observed during this period leads to the dissociation of magnesia through an acid-base reaction. The kinetics of this reaction depends upon the MgO surface structure. Mg2+ ions present in solution are complexed into Mg (H2O)62+ and can substitute to molecules of water and be adsorbed on the MgO surface. These complexes remain attached to the surface and recover it progressively. Ions PO43− and NH4+ and Mg(H2O)62+ complexes can come close to the surface and develop a struvite network, owing to hydrogen bonds. This explanation results from analyses of solutions, X-ray diffractometry, and scanning electron microscopy.