Reactive powder concrete(RPC) is vulnerable to explosive spalling when exposed to high temperature.The characteristics of micro pore structure and vapor pressure of RPC are closely related to the thermal spalling.Applying mercury intrusion porosimetry(MIP) and scanning electron microscopy(SEM) techniques,the authors probed the characteristics of micro pore structures of plain RPC200 when heated from 20-350℃.The pore characteristics such as specific pore volume,threshold pore size and most probable pore size varying with temperatures were investigated.A vapor pressure kit was developed to measure the vapor pressure and its variation inside RPC200 at various temperatures.A thin-wall spherical pore model was proposed to analyze the thermo-mechanical mechanism of spalling,by which the stresses varying with the vapor pressure q(T) and the characteristic size of wall(K) at any point of interest were determined.It is shown that the pore characteristics including specific pore volume,average pore size,threshold pore size and most probable pore size rise significantly with the increasing temperature.200℃ appears to be the threshold temperature above which the threshold pore size and the most probable pore size climb up dramatically.The increase in the specific pore volume results from the growth both in quantity and in volume of the transition pores and the capillary pores.The appearance of the explosive spalling in RPC200 is mainly attributed to being unable to form pathways in favor of releasing water steam in RPC and to the rapid accumulation of high vapor pressures as well.The thin-wall sphere domain where the vapor pressure governs the spalling is bounded through the pore model. Reactive powder concrete (RPC) is vulnerable to explosive spalling when exposed to high temperature. These characteristics of micro pore structure and vapor pressure of RPC are closely related to the thermal spalling. Applying mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) techniques, the authors probed the characteristics of micro pore structures of plain RPC200 when heated from 20-350 ° C. The pore characteristics such as specific pore volume, threshold pore size and most probable pore size varying with temperatures were investigated. A vapor pressure kit was developed to measure the vapor pressure and its variation inside RPC200 at various temperatures. A thin-wall spherical pore model was proposed to analyze the thermo-mechanical mechanism of spalling, by which stresses times with the vapor pressure q (T) and the characteristic size of wall (K) at any point of interest were determined. It is shown that the pore characteristics include specific pore volume, average pore size, threshold pore size and most probable pore size rise significantly with the increasing temperature.200 ° C appears to be the threshold temperature above which the threshold pore size and the most probable pore size climb up dramatically. increase in the specific pore volume results from the growth both in quantity and in volume of the transition pores and the capillary pores. The appearance of the explosive spalling in RPC200 is mainly attributed to being unable to form pathways in favor of releasing water steam in RPC and to the rapid accumulation of high vapor pressures as well. The thin-wall sphere domain where the vapor pressure governs the spalling is bounded through the pore model.