Chemical kinetics of C-S-H decomposition in hardened cement paste subjected to elevated temperatures up to 800oC

To achieve both qualitative and quantitative understanding of the decomposition of hardened cement paste (HCP) at elevated temperatures, X-ray diffraction (XRD) tests were conducted on HCP samples after exposure to various peak temperatures from 400 to 800°C. The kinetic equations of C-S-H decomposition at 600, 700 and 800°C are proposed. The quantitative measurement using α-Al2O3 as a reference phase, revealed that the decomposition of C-S-H started at 560°C but became significant only above 600°C. The C-S-H decomposition rate increased dramatically with temperatures above 600°C. The decomposition of C-S-H influenced the strength loss of high-performance concrete (HPC) exposed to temperatures above 600°C but had little or no effect on the occurrence of explosive spalling. This test result confirms the vapour pressure mechanism for explosive spalling of HPC. It is also postulated that the strength loss of concrete exposed to temperatures below 600°C must be mainly caused by the HCP pore-structure coarsening.