The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate,Journal of the American Chemical Society

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The Atomic-Level Structure of Cementitious Calcium Aluminate Silicate Hydrate
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2020-05-14 , DOI: 10.1021/jacs.0c02988
Aslam Kunhi Mohamed _{1,

2} , Pinelopi Moutzouri ₃ , Pierrick Berruyer ₃ , Brennan J Walder ₃ , Jirawan Siramanont _{1,

4} , Maya Harris ₁ , Mattia Negroni ₃ , Sandra C Galmarini ₅ , Stephen C Parker ₆ , Karen L Scrivener ₁ , Lyndon Emsley ₃ , Paul Bowen ₁

Affiliation

Despite use of blended cements containing significant amounts of aluminum for over thirty 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.

中文翻译：

水泥基硅酸铝钙水合物的原子级结构

尽管使用含有大量铝的混合水泥已超过 30 年，但主要水化产物铝酸钙水合物 (CASH) 中铝的结构性质仍然难以捉摸。使用第一性原理计算，我们预测铝被纳入线性硅酸盐链的桥接位点，并且在高 Ca:Si 和 H2O 比率下，铝的稳定配位数为 6。具体来说，我们预测硅酸盐桥接 [AlO2(OH)4]5- 复合物是有利的，由羟基配体和层间空间中的电荷平衡钙离子稳定。然后通过一维和二维动态核极化增强的 27Al 和 29Si 固态核磁共振实验证实了这种结构。我们特别将 5 ppm 处的窄 27Al NMR 信号分配给硅酸盐桥接 [AlO2(OH)4]5-位点，并表明该信号与来自 CASH 中硅酸盐的 29Si NMR 信号相关，与其对“第三水合铝酸盐”（TAH）相。因此我们得出结论，TAH 不存在。这解决了关于 CASH 样品中 27Al NMR 观察到的六重配位铝的位置和性质的长期困境。

更新日期：2020-05-14

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