Influences of rehydration conditions on the mechanical and atomic structural recovery characteristics of Portland cement paste exposed to elevated temperatures

This study investigates the effects of rehydration conditions (different hydration durations at 20 degrees C/60% relative humidity (RH) and in water) on the mechanical and atomic structural recovery of cement paste heated to temperatures of 200, 500, 800, and 1000 degrees C. Variations in the mass, bulk density, compressive strength, and surface morphology of the paste due to heating and rehydration were characterized. In addition, variations in the hydration products and the mean chain length (MCL) of calcium silicate hydrates (C-S-H) in the pastes due to heating and rehydration were analyzed using X-ray diffraction and Si-29 solid-state nuclear magnetic resonance spectroscopy (Si-29 NMR), respectively. The compressive strength of the cement paste heated up to 500 degrees C recovered much more when the paste was placed in water while hot and water-rehydrated for 24 h than when it was rehydrated at 20 degrees C/60% RH. This mechanical recovery could be induced by accelerated hydration, which results in the formation and enhanced silicate polymerization of C-S-H. Si-29 NMR analysis further indicated that the mechanical recovery of cement paste was correlated linearly with the increase in the MCL of C-S-H. In contrast, no mechanical recovery was observed in the cement pastes heated at 800 degrees C and 1000 degrees C after rehydration due to the decomposition of C-S-H. (C) 2019 Elsevier Ltd. All rights reserved.