In this paper, a computer model is established to simulate the microstructure of magnesia-phosphate cement paste at the micro-scale. The chemical reaction in the paste is analyzed first, based on which the microstructure is modeled through interacted growing and eroding spherical particles representing the solid phases. The pore structure of the simulated paste is characterized by a closing operation based algorithm. It is found that the simulated pore size distribution curves are consistent with corresponding experimental results. © 2014 Elsevier B.V.