The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate

Despite
use of blended cements containing significant amounts of
aluminum for over 30 years, the structural nature of aluminum in the
main hydration product, calcium aluminate silicate hydrate (C-A-S-H),
remains elusive. Using first-principles calculations, we predict that
aluminum is incorporated into the bridging sites of the linear silicate
chains and that at high Ca:Si and H2O ratios, the stable
coordination number of aluminum is six. Specifically, we predict that
silicate-bridging [AlO2(OH)4]5– complexes are favored, stabilized by hydroxyl ligands and charge
balancing calcium ions in the interlayer space. This structure is
then confirmed experimentally by one- and two-dimensional dynamic
nuclear polarization enhanced 27Al and 29Si
solid-state NMR experiments. We notably assign a narrow 27Al NMR signal at 5 ppm to the silicate-bridging [AlO2(OH)4]5– sites and show that this signal correlates
to 29Si NMR signals from silicates in C-A-S-H, conflicting
with its conventional assignment to a “third aluminate hydrate”
(TAH) phase. We therefore conclude that TAH does not exist. This resolves
a long-standing dilemma about the location and nature of the six-fold-coordinated
aluminum observed by 27Al NMR in C-A-S-H samples.