Effect of MgO/NH4H2PO4 Ratio on the Properties of Magnesium Phosphate Cements

Magnesium phosphate cements (MPCs) are formed when an aqueous solution of ammonium dihydrogen phosphate (ADP, NH₄H₂PO₄) and magnesium oxide (MgO) react in the presence of other aggregates for filling or reinforcement (e.g., fibers, waste, and sand). Therefore, understanding how the MgO/ADP ratio influences phase formation to produce cementitious matrices with better performance is essential. MgO dissolved in an acid solution undergoes numerous transformations because of which different opinions exist regarding the mechanisms by which the setting reactions and formation of hydrated phases occurs in MPCs. In this study, the effect of different MgO/ADP ratios on the properties of MPCs was evaluated regarding phase formation using X-ray diffraction, setting time, and pore size distribution. Increased ADP concentration was found to increase the concentration of hydrated phases, however, this increases the pore content while decreasing the apparent density of MPCs. Compared with compositions with lower MgO/ADP ratios, the composition with the highest MgO/ADP ratio required higher setting time due to the lower concentration of hydrated phases.

Keywords:
Chemically bonded phosphate ceramics; cement; magnesium phosphate; NH₄H₂PO₄