Local Structures and Electronic States of C‑S-H–Sodium–H2O Interface: NMR and DFT Studies

Since the accident at the Fukushima Nuclear Power Station, the stabilization of radioactive nuclides such as alkali metals like Cs has become very important. Cementation is a widespread technique for nuclear waste immobilization due to the low leachability of cementitious materials. Calcium silicate hydrate (C-S-H) is the main component of cement and is known to efficiently retain radioactive ions. In the present study, first-principle quantum chemical calculations were conducted for the C-S-H–Na+(H2O) n (n = 0–8) hydration system to elucidate the specific adsorption capacity of C-S-H. In addition, nuclear magnetic resonance (NMR) measurement was performed to detect the chemical shift of 23Na on C-S-H. The calculations showed that Na+ takes two tapped states close to the C-S-H surface. One is the contact form where Na+ is directly bound to the hydroxyl group (OH) of silanol of C-S-H with one or two hydrogen bonds, and subsequently, three or four water molecules are bound to the Na+ ion. The other one is the solvent separated (ss) form where hydrated Na+ interacts with silanol of the C-S-H surface via H2O. The contact form is more stable in energy than the ss form, although the energy difference is small. Both forms are in equilibrium state on C-S-H. The calculated chemical shifts of 23Na on C-S-H (−5.14 ppm) were in excellent agreement with that measured by NMR spectroscopy (−4.62 ppm). The electronic states and chemical shifts of Na+ on C-S-H were discussed based on theoretical results.