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.

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“Pure shift” methods can greatly improve the resolution of proton NMR spectra.

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.