Decoupling skeleton stability and water dynamics in C–S–H by in situ SAXS and H2O/D2O-atmosphere SANS

Humidity-induced microstructural changes in calcium silicate hydrate (C–S–H) have been widely studied by small-angle X-ray scattering (SAXS) and proton nuclear magnetic resonance (1H NMR) relaxometry; yet neither technique has been able to separate the contributions of the Ca–Si skeletal deformation from those of water dynamics. In this study, we combine in situ humidity-controlled SAXS with isotopologue-selective small-angle neutron scattering (SANS), under H2O and D2O vapor atmospheres, on hardened cement paste with and without a polyurethane ether modifier (PUE). H2O atmosphere SANS profiles present no significant changes during several hours of humidity cycling, indicating that the Ca–Si skeleton remains stable for short timescales. D2O-atmosphere SANS reveals progressive pore-size–resolved water loss from large to small pores, which is suppressed in PUE-modified specimens. These results demonstrate that short-term SAXS profile changes arise from water dynamics, rather than skeletal deformation, and that PUE acts through nanoscale water-transport restriction, instead of physical restraint of C–S–H sheets.