Confined water redistribution governs nanomechanical property changes of cement pastes curing at different air pressures

The water status within concrete under low air pressure (AP) condition has been reported to affect the long-term properties of concrete in plateau regions, however, the underlying mechanism for the microstructural changes is not fully understood yet. In this paper, the confined water statuses within cement pastes curing at 98% RH and different APs (i.e. 101 kPa, 60 kPa, and 20 kPa) were first demonstrated and then correlated to the nanomechanical property changes of calcium-silicate-hydrates (C-S-H) phase by means of the 1H NMR relaxometry and nanoindentation techniques. The results show that low AP conditions increase the evaporable water content and drive the water movement from interlayer spaces to gel pores during wetting at 98% RH. According to the nanoindentation investigations, low AP conditions increase the volume fraction of high-density (HD) C-S-H and improve the nanomechanical and creep characteristics and bending strength of hardened cement pastes, which is attributed to the water redistribution within C-S-H phase.