When a concrete structure is exposed to high temperature, the mechanical damage and chemical transformation take place simultaneously, which will change the microstructure of material. On the other hand, the mechanical properties and transport properties depend on the development of microstructure of cement paste. In order to study the microstructure changes at high temperature, in this contribution the cement paste samples were firstly heated to varied temperatures from 100 °C to 1000 °C with heating rate of 5 °C/min, and then naturally cooled to indoor temperature. In the microstructural analysis program, the environmental scanning electron microscope (ESEM), mercury intrusion porosimetry (MIP) and thermogravimetry analysis (TGA) were used. The images captured by ESEM were analyzed by segmentation and binary image processing in order to calculate the volume fractions of hydration products and porosity. The change on density distribution of hydration products was characterized by histogram of ESEM image at different temperatures. The porosity and pore size distribution of same samples were studied by MIP and the chemical decomposition was analyzed by TGA as well. Through the analysis of the information obtained from TGA and microstructure measurements, it is found that hydrated calcium silicate (CSH gel) and portlandite dehydrated into sparse crystalline particles when temperature reached 1000 °C. The porosity and the connectivity of pore of cement paste increased along with temperature increase. The results of this research provide a fundamental understanding on how the fire sapling occurs when concrete structures were exposed to high temperature.
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