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

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.