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

Abstract

Magnesium phosphate cements (MPCs) are formed when an aqueous solution of ammonium dihydrogen phosphate (ADP, NH 4 H 2 PO 4) 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.

Effect of Water Content and MgO / ADP Ratio on the Properties of Magnesium Phosphate Cement

“…According to the Brazilian standard NBR NM 65:2003 ("Portland Cement -Determination of Setting Time), measurement of the setting time should be performed using the Vicat apparatus. However, recent studies 3,11,[19][20][21][22][23]26,[28][29][30] have shown that the setting time for magnesium phosphate cements can be indirectly determined by calorimetry assays. These authors observed a direct relationship between the results obtained by calorimetric measurements and using the Vicat apparatus.…”

“…Recent studies have evaluated the effect of boric acid (H 3 BO 3 ) content 19 , MgO/ADP ratio 20 , and microwave calcination of MgO 21,22 on the properties and durability 23 of MPCs. Therefore, since a consolidated dosage method for MPCs has not been reported yet, this study aims to evaluate the influence of water content and MgO/ADP ratio on the properties of MPC using MgO calcined at different temperatures, formation reactions (setting time and phases formed), microstructure, porosity, and mechanical strength of these materials.…”