Effects of rehydration conditions on the mechanical recovery and the properties of portland cement paste subjected to high temperature

Abstract

When concrete is exposed to fire, decomposition of the cement paste results in critical damage to the sustainability of the building. In this study, the mechanical properties of ordinary cement paste heated to different temperatures (200 ℃, 500 ℃, 800 ℃, and 1000 ℃) and the effects of various recurring conditions (in 20 ℃ 60%RH and in water for different recurring times) on the mechanical recovery were investigated. The results showed remarkable compressive strength enhancements immediately after heating to 200 ℃ and for samples water recurred after heating to 500 ℃. For the samples heated to 200 ℃, this was due to the hydration of the anhydrate clinker phase. And for the samples heated to 500 ℃ and water recurred, this was due to the reaction between water and amorphous CaO (produced by the decomposition of Ca(OH)2), which forms hydrates. On the other hand, the samples heated to 800 ℃ and 1000 ℃ did not show mechanical recovery due to water recurring. Also, 29Si nuclear magnetic resonance analysis showed that the mean chain length of calcium silicate hydrates was directly proportional to the compressive strength.