How much water is there within calcium silicate hydrates? Probing water dynamics by Inelastic Neutron Scattering and Molecular Dynamics Simulations

Calcium silicate hydrate (C-S-H) is a disordered, nanocrystalline material acting as the primary binding phase in Portland cement. Thin films of water are present on the surfaces and inside the nanopores of C-S-H and related phases such as calcium-aluminum-silicate-hydrate (C-A-S-H), an Al-bearing substitute present in low-CO2 C-S-H cement. These water films control many of the chemical and mechanical properties of C-S-H, including drying shrinkage, ion transport, creep, and thermal behavior. Therefore, obtaining a fundamental understanding of their properties is essential. In this work, we have applied a combination of inelastic incoherent neutron scattering and molecular dynamics simulation methods to unravel the dynamics of water in synthetic C-(A)-S-H samples conditioned at five hydration states (from dry to fully hydrated) and with three different Ca/Si ratios (0.9, 1, and 1.3). Our results converge towards a picture where the evolution from thin layers of adsorbed water to bulk capillary water is dampened by the structure of C-(A)-S-H, in particular by the availability of Ca2+ sites which tend to keep the water in the form of structured surface layers.