Tracking the Early Hydration Reaction of Cementitious Calcium Silicate Hydrate via DNP-Enhanced Solid-State NMR

The early hydration of calcium silicate hydrate (C-S-H) plays a crucial role in the development of key mechanical properties in cement, yet an atomic-level description of this reaction remains elusive. Here, to understand the early stages of the hydration reaction, we introduce a method to quantify dilute silicate species as a function of reaction time using ex-situ solid-state ²⁹Si magic-angle spinning dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy. Samples are flash-frozen and separated by centrifugation, allowing for kinetic analysis of the supernatant and structural studies of the precipitate over time. With DNP-enhanced ²⁹Si NMR, we can determine the concentrations of various silicate species throughout the reaction and track the growth of the silicate chains, which form the dreierketten backbone of C-S-H. In our low Ca/Si ratio system, we observe a vast majority of the supernatant silicate species to be monomers, with small amounts of dimers. The initially precipitated C-S-H, which has a mean chain length of 2.6 and is composed primarily of dimers, is shown to significantly differ from the C-S-H present after 3 h of hydration, which has an average length of 4.2.