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 Al-27 and Si-29 solid-state NMR experiments. We notably assign a narrow Al-27 NMR signal at 5 ppm to the silicate-bridging [AlO2(OH)(4)](5-) sites and show that this signal correlates to Si-29 NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a "third aluminate hydrate" (T ) 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 Al-27 NMR in C-A-S-H samples.

Kunhi Mohamed, A., Moutzouri, P., Berruyer, P., Walder, B., Siramanont, J., Harris, M., et al. (2020). The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 142(25), 11060-11071 [10.1021/jacs.0c02988].

The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate

Negroni, Mattia;
2020

Abstract

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 Al-27 and Si-29 solid-state NMR experiments. We notably assign a narrow Al-27 NMR signal at 5 ppm to the silicate-bridging [AlO2(OH)(4)](5-) sites and show that this signal correlates to Si-29 NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a "third aluminate hydrate" (T ) 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 Al-27 NMR in C-A-S-H samples.
Articolo in rivista - Articolo scientifico
Calcium silicate; Calculations; Crystal atomic structure; Hydrates; Hydration; Nuclear magnetic resonance spectroscopy; Silicates; Silicon; Sodium Aluminate
English
2020
142
25
11060
11071
open
Kunhi Mohamed, A., Moutzouri, P., Berruyer, P., Walder, B., Siramanont, J., Harris, M., et al. (2020). The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 142(25), 11060-11071 [10.1021/jacs.0c02988].
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/491062
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