Phosphoric acid-modified magnesium oxychloride: Study of water stability, kinetics, and pair distribution function

Magnesium oxychloride (MOC) excels in performance applications due to inherent structural strength, fire retardant properties, and numerous other attributes. To avoid the slow degradation of MOC when exposed to water, phosphoric acid is usually added, effectively increasing retention of structural properties (water stability). While this is an effective method, it is poorly understood. Additions of 2.5 wt.% and above had positive impacts on the water stability, preserving ~50 wt.% crystalline MOC after water stability tests. Phosphoric acid addition also impacted the reaction kinetics, increasing the activation energy of curing from 72.2 to 87.6-95.2 kJ/mol. Using synchrotron X-ray scattering and pair distribution function analysis, we identified an unreported amorphous phase formed when phosphoric acid is added; this phase contains structural motifs related to MgHPO ₄ ·3H ₂ O (newberyte), Mg ₂ P ₂ O ₇ ·3.5H ₂ O (magnesium pyrophosphate), and amorphous MOC phase. The short-range order of the samples show a prominent peak at ~3.2 Å that grows with increasing acid addition, believed to be a combination of newberyte (~3.4 Å Mg–P), pyrophosphate (~3.25 Å Mg–P), and MOC (~3.15 Å Mg–Mg). We propose that the increased water stability observed is due to this combined amorphous phase, which retains the low water solubility properties of MgHPO ₄ ·3H ₂ O and Mg ₂ P ₂ O ₇ ·3.5H ₂ O, effectively protecting the MOC crystalline phase.