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 hours of hydration, which has an average length of 4.2.