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

Abstract

This study investigates the effects of rehydration conditions (different hydration durations at 20 °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 °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 29Si solid-state nuclear magnetic resonance spectroscopy (29Si NMR), respectively. The compressive strength of the cement paste heated up to 500 °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 °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. 29Si 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 °C and 1000 °C after rehydration due to the decomposition of C-S-H.