Mechanism of setting reaction in magnesia-phosphate cements

This paper tries to explain the setting mechanism of magnesia-phosphate cem ents 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 sta rts to be wetted. The drop in pH observed during this period leads to the d issociation of magnesia through an acid-base reaction. The kinetics of this reaction depends upon the MgO surface structure. Mg2+ ions present in solu tion are complexed into Mg (H2O)(6)(2+) and can substitute to molecules of water and be adsorbed on the MgO surface. These complexes remain attached t o the surface and recover it progressively. Ions PO43- and NH4+ and Mg(H2O) (6)(2+) complexes can come close to the surface and develop a struvite netw ork, owing to hydrogen bonds. This explanation results from analyses of sol utions, X-ray diffractometry, and scanning electron microscopy. (C) 2000 El sevier Science Ltd. All rights reserved.