Microstructure of magnesium potassium phosphate cement

► 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.