Characterization of and Structural Insight into Struvite-K, MgKPO 4 ·6H 2 O, an Analogue of Struvite

Struvite-K (MgKPO ·6H O) is a magnesium potassium phosphate mineral with naturally cementitious properties, which is finding increasing usage as an inorganic cement for niche applications including nuclear waste management and rapid road repair. Struvite-K is also of interest in sustainable phosphate recovery from wastewater and, as such, a detailed knowledge of the crystal chemistry and high-temperature behavior is required to support further laboratory investigations and industrial applications. In this study, the local chemical environments of synthetic struvite-K were investigated using high-field solid-state Mg and K MAS NMR techniques, alongside P MAS NMR and thermal analysis. A single resonance was present in each of the Mg and K MAS NMR spectra, reported here for the first time alongside the experimental and calculated isotropic chemical shifts, which were comparable to the available data for isostructural struvite (MgNH PO ·6H O). An high-temperature XRD analysis of struvite-K revealed the presence of a crystalline-amorphous-crystalline transition that occurred between 30 and 350 °C, following the single dehydration step of struvite-K. Between 50 and 300 °C, struvite-K dehydration yielded a transient disordered (amorphous) phase identified here for the first time, denoted δ-MgKPO . At 350 °C, recrystallization was observed, yielding β-MgKPO , commensurate with an endothermic DTA event. A subsequent phase transition to γ-MgKPO was observed on further heating, which reversed on cooling, resulting in the α-MgKPO structure stabilized at room temperature. This behavior was dissimilar from that of struvite exposed to high temperature, where NH liberation occurs at temperatures >50 °C, indicating that struvite-K could potentially withstand high temperatures via a transition to MgKPO .