Coupled effect of elevated thermal exposure and nanochannel confined water on the phase transformations and mechanical changes in calcium silicate hydrates paste

The increasing frequency and intensity of fires and floods pose significant threats to building infrastructure worldwide. To prevent such disasters, it is wise to review the behavior of cementitious materials under the extreme conditions of both heat and humidity. In the present work, a nanoscale study is performed to analyze the morphology, bonding arrangement, phase transformations and mechanical properties of low density calcium silicate hydrates, exposed to temperatures from 300 to 1300 K and various water content. Reactive molecular dynamics method is used to give an insight into the atomistic mechanisms behind the thermal changes of CSH gel. Our findings show that heating CSH grains results in dehydration, volumetric shrinkage, partial decomposition and recrystallization in the CSH grains. This behavior is governed by a competition between water evaporation, porosity increase and volume shrinkage, leading to various changes in the mechanical strength of CSH grains.