Experimental study of phosphate salts influencing properties of magnesium phosphate cement

Magnesium phosphate cement (MPC) was prepared with dead burned magnesia oxide and phosphate salts, such as ammonium dihydrogen phosphate (NH4H2PO4) and potassium dihydrogen phosphate (KH2PO4). The properties of MPC prepared with varied phosphate salts were of much difference. In this paper the exothermia kinetics, mechanical properties and phase compositions of MPC prepared by mixing MgO with either ammonium dihydrogen phosphate or potassium dihydrogen phosphate, or a combination of both were studied. Besides, the effects of sodium triphosphate (Na5P3O10) on the microstructure and mechanical properties of MPC were also investigated. The results revealed that MPC prepared with ammonium dihydrogen phosphate exhibited substantially higher early strength and hydration temperature than that of magnesium potassium phosphate, while the later demonstrated better strength development. The strength of MPC prepared with equivalent mixture of ammonium dihydrogen phosphate and potassium dihydrogen phosphate was higher than that of MPC containing magnesium ammonium phosphate or magnesium potassium phosphate only. Moreover, the testing presented that the addition of sodium triphosphate improved the mechanical properties of MPC significantly. It also prolongs the setting time. Finally, the reaction products and their micro-morphology of MPC paste were examined and analyzed by X-ray diffraction (XRD) and Scanning Electronic Microscopic (SEM).