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

To achieve both qualitative and quantitative understanding of the decomposi tion of hardened cement paste (HCP) at elevated temperatures, X-ray diffrac tion (XRD) tests were conducted on HCP samples after exposure to various pe ak temperatures from 400 to 800 degreesC. The kinetic equations of C-S-H de composition at 600, 700 and 800 degreesC are proposed. The quantitative mea surement using alpha -Al2O3 as a reference phase, revealed that the decompo sition of C-S-H started at 560 degreesC but became significant only above 6 00 degreesC. The C-S-H decomposition rate increased dramatically with tempe ratures above 600 degreesC. The decomposition of C-S-H influenced the stren gth loss of highperformance concrete (HPC) exposed to temperatures above 60 0 degreesC but had little or no effect on the occurrence of explosive spall ing. This test result confirms the vapour pressure mechanism for explosive spalling of HPC It is also postulated that the strength loss of concrete ex posed to temperatures below 600 degreesC must be mainly caused by the HCP p ore-structure coarsening.