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

摘要

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 deg C. The kinetic equations of C-S-H decomposition at 600, 700 and 800 deg C are proposed. The quantitative measurement using #alpha#-Al_2O_3 as a reference phase, revealed that the decomposition of C-S-H started at 560 deg C but became significant only above 600 deg C. The C-S-H decomposition rate increased dramatically with temperatures above 600 deg C. The decomposition of C-S-H influenced the strength loss of high-performance concrete (HPC) exposed to temperatures above 600 deg 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 deg C must be mainly caused by the HCP pore-structure coarsening.