| Abstract | Portland cement is a primary construction material with an annual production and consumption in billions of tons. Considering the great demand for this material, it is necessary to have a deep understanding of its nanostructure and detailed knowledge of the chemical transformations that occur during hydrolysis. Solid State (SS) NMR has had a significant role in studies of cement-based materials and processes. Until now, most of the SS NMR studies concentrated on such nuclei as 29Si, 27Al, 17O and 1H. All these nuclei are important part of the hydrated cement framework and the related studies have provided a wealth of information regarding the nanostructure and hydrolysis in cement systems.1,2 The properties of cement systems, however, depend greatly on the coordination and local environment of calcium. The 43Ca NMR could therefore play a significant role in structural and chemical studies of these materials. Recent 43Ca SS NMR studies have demonstrated impressively that the technique is suitable for solving complicated structural problems not accessible by other methods.3,4 Great difficulties in obtaining natural abundance 43Ca SS NMR, however, have limited its applications mostly to pure compounds.The current work presents results of a systematic 43Ca ultra high field solid state NMR study on a series of cement based materials, aimed at determining the possibilities and the limitations of the method in cement and concrete research. The low natural abundance (0.135%) and small gyromagnetic ratio of 43Ca present a serious challenge even at a magnetic field of 21.1T. In the initial phase of this research we examined the spectra of a number of anhydrous cement related compounds of known structure and composition. The spectra of several materials of significance in cement research, such as beta dicalcium (b-C2S) and tri-calcium (C3S) silicate, and tri-calcium aluminate (C3A), were obtained for the first time. The relation of spectroscopic and structural parameters was at the center of this study and the assignment of the signals was assisted by the first principles calculations. Further, the method was extended to the study of hydrated cement phases. The calcium environment in the synthetic calcium silicate hydrate (C-S-H) of variable composition was probed in comparison to the C-S-H formed in the hydration of C3S. The 43Ca NMR spectra of hydrated C3S suggested similar nanostructural features to those of more-ordered analogs including the synthetic C-S-H samples and 11 Å tobermorite mineral. This observation supports the validity of using layered crystalline C-S-H systems as structural models for the nearly amorphous C-S-H that forms in the hydration of Portland cement. This study demonstrated that in-spite of the great complexity of the calcium silicate chemistry, 43Ca SS NMR is a useful and informative tool that provides valuable information in cement research. |
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