Hydration process and microstructure of magnesium potassium phosphate cement with nitrate solution

Magnesium phosphate potassium cement (MKPC) can be potentially used for solidification/stabilization (S/S) treatment of hazardous wastes. The influence of inorganic salts on the hydration process and microstructure of cementitious materials must be considered, especially to assess the performance of the S/S treatment of wastes with a high nitrate content. Hence, in this study, the hydration process and microstructure of MKPC specimens were investigated, along with their variations in compressive strength setting time, pH, and conductivity. The pore structure, phases composition, and elemental composition of the specimens were investigated using the Brunauer-Emmett-Teller method, X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results showed that although the potassium nitrate (KNO₃) solution did not affect the formation of struvite-K, it affected the crystallization degree of struvite-K, where its microstructure changed from dense, plate-like and prismatic crystals into loose, cluster-like crystals at higher amounts of nitrate. Furthermore, the addition of nitrate delayed the setting time and slowed the pH growth during the hydration process of the MKPC. The nitrate solution hindered the hydration process of the MKPC specimens and increased their porosity. When the amount of nitrate was less than 5 wt%, the effect of nitrate on the hydration and hardening of the MKPC was small. However, at higher amounts of nitrate, the nitrate ions had a negative effect on the compressive strength development of the MKPC and the hydration process was delayed.