High Performance Concrete (HPC) has been defined as concrete with improved overall performance compared to conventional concrete. The high density of the microstructure is responsible for high strength and improved durability. However, the dense microstructure may have an adverse effect on the fire resistance. The formation of vapor goes along with expansion and the built-up of large stresses in the pore system, since the dense microstructure does hardly permit any pressure relief through the pores. The capillary pore structure, particularly the connectivity of the pores, plays a dominant role in this respect. In this contribution, a 3-D pore structure of cement paste, simulated with the numerical simulation program HYMOSTRUC, will be presented. The specific geometrical parameters, including the pore size distribution and the connectivity of capillary pores, can be deduced from the program. The potential of the program for analyzing the susceptibility of high performance concrete to spalling if subjected to fire loads will be discussed.