Although magnesia-phosphate cements have been studied and applied in several fields for many years, the theoretical background on this kind of chemically bonded ceramics has not been sufficiently well established for the quantitative prediction of material properties. In this study, the stoichiometric factors of the chemical reaction in magnesium potassium phosphate cement (MKPC) paste are analyzed, and the degree of reaction of this cement is defined. Based on the stoichiometric factors and the degree of reaction, the porosity of MKPC paste, which is essential for predictions of both the mechanical and transport properties, is calculated. In addition, the pore structure is simulated by a newly developed computer model. The calculated porosities and the simulated pore structures are both found to be consistent with the results measured by mercury intrusion porosimetry (MIP). © 2014 Elsevier Ltd.