The Atomic-Level Structure of Cementitious Calcium Silicate Hydrate

Efforts
to tune the bulk physical properties of concrete are hindered
by a lack of knowledge related to the atomic-level structure and growth
of calcium silicate hydrate phases, which form about 50–60%
by volume of cement paste. Here we describe the first synthesis of
compositionally uniform calcium silicate hydrate phases with Ca:Si
ratios tunable between 1.0 and 2.0. The calcium silicate hydrate synthesized
here does not contain a secondary Ca­(OH)2 phase, even in
samples with Ca:Si ratios above 1.6, which is unprecedented for synthetic
calcium silicate hydrate systems. We then solve the atomic-level three-dimensional
structure of these materials using dynamic nuclear polarization enhanced 1H and 29Si nuclear magnetic resonance experiments
in combination with atomistic simulations and density functional theory
chemical shift calculations. We discover that bridging interlayer
calcium ions are the defining structural characteristic of single-phase
cementitious calcium silicate hydrate, inducing the strong hydrogen
bonding that is responsible for stabilizing the structure at high
Ca:Si ratios.