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 H 2 O ratios, the stable coordination number of aluminum is six. Specifically, we predict that silicate-bridging [AlO 2 (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 27 Al and 29 Si solid-state NMR experiments. We notably assign a narrow 27 Al NMR signal at 5 ppm to the silicate-bridging [AlO 2 (OH) 4 ] 5- sites and show that this signal correlates to 29 Si 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 27 Al NMR in C-A-S-H samples.