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. © 2011 Elsevier Ltd. All rights reserved.