Multinuclear solid-state NMR investigation of local structure in aluminosilicate cementitious materials

二次胶凝材料是添加到水泥中以产生所需的性能(如高抗压强度)的材料。人们已经发现，在试验的各种材料中，炉渣产生的性能最好。众所周知，胶凝材料含有结构无序，因此，基于衍射的技术，其中研究材料的时间和空间平均结构，对于表征它们的用途是有限的。相比之下，固态MAS核磁共振通过影响核磁共振参数的相互作用提供了一个强大的材料局部结构探测器，使其成为表征无序材料的理想位置。以前对本项目所研究的胶凝材料的低场MAS核磁共振研究发现，理想的性能(高抗压强度)与低配位Si和Al的存在之间存在关联。低配位物种提供高抗压强度的形成机制和结构-性能关系都不完全清楚。因此，在整个PHD过程中，将使用多核固体核磁共振波谱对一系列材料的结构进行表征，并研究Al、Si和其他阳离子物种的局部几何结构。我们已经制备了一系列样品，并使用X射线衍射、X射线荧光以及低场29Si和27Al MAS核磁共振光谱对其进行了初步研究。首先，将通过进行高场常规的27Al和29Si(可能还有25 mg和43Ca)MAS核磁共振实验，以及二维27Al MQMAS和27Al-29Si异核关联实验，对这些样品进行进一步的研究。这些材料中的结构无序导致核磁共振谱中峰的展宽和重叠，因此谱将需要分析拟合以识别单个峰，对于四极核，将需要使用四极参数的切赛克分布进行拟合，以便提取关于目前存在的核磁共振参数分布的信息。MQMAS光谱提供的较高分辨率将使存在的低配位Al物种的数量和类型得以确定。将对具有不同化学成分、结构、来源等的铝硅酸盐材料进行分析，特别是调查硅和铝物种的数量和类型以及它们的配位数，以试图了解它们之间的关系。还将通过改变温度、加热时间和其他可能的参数来研究高温加热对低配位Al和Si形成的影响。核磁共振将被用来研究这些材料中的局部结构，预计这将提供关于形成低配位物种的机制以及如何设计和合成具有最佳化学成分和结构的材料的有价值的信息。在可能的情况下，将通过使用DFT计算来辅助解释实验核磁共振数据，在可能的情况下，通常是在模型系统上，使用平面波基组和GIPAW方法来计算核磁共振参数。

Secondary cementitious materials are materials added to cements in order to produce desirable properties (such as high compressive strength). It has been found that amongst the wide variety of materials trialled, slags produce the best properties. Cementitious materials are known to contain structural disorder, and as a result diffraction-based techniques, where the time and space averaged structure of a material is studied, are of limited utility for their characterisation. By contrast, solid-state MAS NMR provides a powerful probe of the local structure of materials, through the interactions that affect the NMR parameters, making it ideally placed for characterisation of disordered materials.Previous low-field MAS NMR studies of the cementitious materials under study in this project have found a correlation between desirable properties (high compressive strength) and the presence of low-coordinate Si and Al. Neither the mechanism of formation or the structure-property relationship whereby low coordination species provide high compressive strength is fully understood. Therefore, throughout the course of this PhD structural characterisation of a range of materials and an investigation the local geometry of Al, Si and other cationic species will be carried out using multinuclear solid-state NMR spectroscopy.A series of samples have been previously prepared and have been initially investigated using X-ray diffraction, X-ray fluorescence and low field 29Si and 27Al MAS NMR spectroscopy. Initially, further investigation of these samples will be carried out by performing higher field conventional 27Al and 29Si (and possibly 25Mg and 43Ca) MAS NMR experiments, and two-dimensional 27Al MQMAS and 27Al-29Si heteronuclear correlation experiments. The structural disorder in these materials causes the broadening and overlapping of peaks in the NMR spectra, and so spectra will require analytical fitting in order to identify individual peaks and, for quadrupolar nuclei, fitting using a Czjzek distribution of quadrupolar parameters will be needed in order to extract information on the distribution of NMR parameters present. The higher resolution offered by MQMAS spectra will enable the number and type of low coordinate Al species present be determined. An analysis of aluminosilicate materials with different chemical composition, structure, origin etc. will be performed, investigating, in particular, the number and type of Si and Al species and their coordination number in order to try and understand how these are related. The effect of high temperature heating on the formation of low coordinated Al and Si will also be investigated by varying the temperature, the time spent heating and other possible parameters. NMR will be used to investigate the local structure in these materials and it is anticipated that this will provide valuable information on both the mechanism the formation of low coordinate species and on how to design and synthesise materials with optimal chemical composition and structure.Interpretation of experimental NMR data will be aided by using DFT calculations, where possible, typically on model systems, using planewave basis sets and the GIPAW approach for calculating NMR parameters.