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.
In complex mixtures, proton nuclear magnetic resonance (NMR) spectra are often very crowded, making spectral analysis complicated or even impossible, particularly when detailed structural information about the mixture components is needed. A new 1D NMR method (fluorine-edited selective TOCSY acquisition, FESTA) is introduced that facilitates the structural analysis of mixtures of species that contain fluorine. It allows simplified H spectra to be obtained that show only those protons that are in a spin system coupled to fluorine of interest. The new method is illustrated by factorizing a complexH spectrum into subspectra for individual spin systems involving different F sites.
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