A. Kunhi Mohamed,^2,6 P. Berruyer,¹ B. J. Walder,¹ J. Siramanont,^2,3 M. Harris,² M. Negroni,¹ S. C. Galmarini,⁴ S. C. Parker,⁵ K. L. Scrivener,² L. Emsley,¹ and P. Bowen²

² Laboratory of Construction Materials, Institut des Matériaux, EPFL Lausanne, CH-1015 Lausanne, Switzerland, ³ SCG CEMENT Co., Ltd., Saraburi 18260, Thailand, ⁴ Building Energy Materials and Components, EMPA, Switzerland, ⁵ Computational Solid State Chemistry Group, Department of Chemistry, University of Bath, UK, ⁶ Institute for Building Materials, Department of Civil, Environmental and Geomatic Engineering, ETH Zürich, CH-8073, Switzerland

Calcium silicate hydrate (C-S-H) is the main hydration product of ordinary Portland cement and has a layered calcium-silicate sheet structure with a disordered interlayer space that contains water molecules, hydroxyl groups and calcium ions. When present, aluminum can be incorporated into the C-S-H as aluminate species, forming calcium aluminate silicate hydrate (C-A-S-H). In C-A-S-H aluminum species are found having coordination numbers of four, five, and six. Here, using first principles calculations and advanced solid-state NMR techniques, we predict that at high Ca:Si and H₂O ratios, the stable coordination number of aluminum is six and that this aluminum species is incorporated into the bridging sites of the linear silicate chains. This is confirmed experimentally by one- and two-dimensional Dynamic Nuclear Polarization (DNP) enhanced ²⁷Al and ²⁹Si solid-state NMR experiments. In some of the highest field DNP MAS NMR experiments to date, we show that a narrow peak appearing at 5 ppm in the ²⁷Al NMR spectrum at 21.14 T can be assigned to the silicate-bridging [AlO₂(OH)₄]^5- sites. A DNP enhanced 2D ²⁷Al/²⁹Si refocused dipolar INEPT MAS NMR experiment at 9.40 T was also used to correlate NMR signals from ²⁷Al nuclei that are less than ~4.3 Å from ²⁹Si (Figure 1).  This experiment shows correlation of the 5 ppm ²⁷Al NMR signal with ²⁹Si NMR signals at -77 ppm.  Using DFT shielding calculations we show that this ²⁹Si chemical shift is consistent with six-coordinate aluminate inserted as a bridge between silicate chains. We therefore answer the debated and long-standing question of the structural nature of aluminum in C-A-S-H.

Figure 1:  DNP enhanced two-dimensional {²⁹Si}²⁷Al refocused dipolar INEPT MAS spectrum acquired for a synthetic C-A-S-H sample (Ca:Si ratio of 2.0 and Al:Si ratio of 0.07) at 9.40 T, 100 K, and 10 kHz MAS.