Microstructure of magnesium potassium phosphate cement paste is characterized by a reaction product-to-space ratio. Degree of reaction of magnesium potassium phosphate cement is limited by amounts of magnesia, phosphate and water. An optimal magnesia-to-phosphate molar ratio gives the highest strength in magnesium potassium phosphate cement paste. Value of the optimal magnesia-to-phosphate molar ratio depends on water-to-cement ratio of paste.