Change in microstructure of hardened cement paste subjected to elevated temperatures

To achieve both qualitative and quantitative understanding of the change in microstructure of hardened cement paste (hcp) due to elevated temperatures, X-ray diffraction, mercury intrusion porosimetry tests, and scanning electron microscope (SEM) observations were conducted. The kinetics equations of C–S–H decomposition at 600, 700 and 800 °C are proposed. The C–S–H decomposition started at 560 °C but became significant only above 600 °C. The decomposition rate increased dramatically with temperature above 600 °C, which must be a main reason for the strength loss of concrete exposed to temperatures above 600 °C. The measured hcp pore size distributions confirm the coarsening effect of elevated temperatures on the pore-structure, which was verified by the SEM observations. The pore-structure coarsening can be regarded as the formation of equivalent cracks which should be responsible for the reduction in mechanical properties of HPC, especially when no chemical decomposition of hcp occurs below 400 °C.