Research highlights ► A systematic study of the microstructure of magnesium potassium phosphate cement (MKPC) is presented, starting from rational paste formulation and phase identification to microstructure observation and compositional analysis. With a wide range of calculated magnesium to phosphate (M/P) molar ratios, the MgO_KH2PO4_H2O system exhibits more or less similar brief setting period, but significant differences in physical and chemical properties. ► The experimental results show that the mechanical strength of MKPC paste may be dictated by the rational formulation of the starting materials, in turn the crystallinity and growth of the main reaction product of the ternary heterogeneous system. The major crystalline phases observed in MKPC are magnesium potassium phosphate hexahydrate, as well as the residuals of raw materials, including magnesia and potassium phosphate. ► With a very big size and a flat surface, the raw materials residuals can be easily identified by their appearances. The hexahydrate polycrystals, however, generally elongate along the crystallographic a-axis as its isostructure struvite. The final form of the hexahydrate developed in the cement pastes largely depends on the growth conditions, which is closely related to the M/P ratio adopted. With an M/P ratio as low as mere 2, the hexahydrate would grow into slender acicular polycrystals with large aspect ratio. ► As the M/P ratio increases, the hexahydrate could grow much larger in size and turn into bladed and prismatic habits with wrinkled surface. To achieve the most rapid strength development and higher efficiency in materials consumption, an M/P ratio as high as 10 should be used. As a result, a more compacted microstructure will be formed, in which the polycrystals of the reaction products covered by a subhedral surface might be difficult to be recognized. ► The microstructural observations of the different crystalline phases in the MKPC system are verified by compositional analysis which agree well with the theoretical calculations. It is believed that the current study provides sound information for further development of the ternary cementing system for rapid repair.

Abstract The ever growing concerns over global warming and other detrimental ecological changes have spurred the research and development of new types of cementitious materials. Along this line, magnesium potassium phosphate cement (MKPC) appears as one of the most promising candidates, especially in the field of rapid repair. A systematic study of the microstructure of MKPC is conducted, starting from paste design and phase identification, to morphology examination and compositional analysis. The experimental results show that the mechanical properties of the cement pastes are influenced by the crystallinity and growth of magnesium potassium phosphate hexahydrate, the principal reaction product of the heterogeneous system. The final form of the hexahydrate developed in the matrix largely depends on the growth conditions, which is closely related to the calculated molar ratio of magnesia to phosphate (M/P). It is demonstrated that a poor crystal growth and a significant amount of unreacted raw materials can be observed in the cement pastes with low M/P ratios, while a better crystal growth associated with high early strength can be achieved for the pastes with an M/P ratio as high as ten folds of the theoretical one. Finally, the microstructural observations of the different crystalline phases in MKPC are further verified by compositional analysis which agree well with the theoretical values.