Natural abundance ultrahigh-field 43Ca solid-state NMR in cement-based materials

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 \uc5 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.Le ciment Portland est un mat\ue9riau de construction primaire dont la production et la consommation annuelles totales s?\ue9l\ue8vent \ue0 des milliards de tonnes. \uc9tant donn\ue9 la forte demande pour ce mat\ue9riau, il devient n\ue9cessaire de comprendre \ue0 fond sa nanostructure et de conna\ueetre en d\ue9tail les transformations chimiques qui se produisent au cours de l?hydrolyse. La r\ue9sonance magn\ue9tique nucl\ue9aire (RMN) en phase solide a jou\ue9 un r\uf4le significatif dans les \ue9tudes des mat\ue9riaux et des proc\ue9d\ue9s \ue0 base de ciment. Jusqu?\ue0 maintenant, la plupart des \ue9tudes articul\ue9es sur la RMN en phase solide se sont int\ue9ress\ue9es \ue0 des nucl\ue9i tels que 29Si, 27Al, 17O et 1H. Tous ces nucl\ue9i constituent une partie importante de la structure du ciment hydrat\ue9, et les \ue9tudes connexes ont g\ue9n\ue9r\ue9 toute une masse de renseignements concernant la nanostructure et l?hydrolyse dans les formules de ciment. Les propri\ue9t\ue9s des formules, cependant, d\ue9pendent dans une large mesure de la coordination et de l?environnement calcique local. La RMN du 43Ca pourrait donc jouer un r\uf4le important dans les \ue9tudes structurales et chimiques de ces mat\ue9riaux. De r\ue9centes \ue9tudes de RMN du 43Ca en phase solide ont d\ue9montr\ue9 de fa\ue7on \ue9clatante que la technique convient \ue0 la r\ue9solution de probl\ue8mes structuraux complexes, que n?autorisent pas les autres m\ue9thodes. Des difficult\ue9s majeures li\ue9es \ue0 l?obtention de la RMN d?une teneur isotopique naturelle de 43Ca en phase solide ont toutefois limit\ue9 ses applications essentiellement aux compos\ue9s purs.Les travaux actuels pr\ue9sentent les r\ue9sultats d?une \ue9tude syst\ue9matique de RMN \ue0 champ ultra-\ue9lev\ue9 du 43Ca en phase solide, pour une s\ue9rie de mat\ue9riaux cimentaires, visant la d\ue9termination des possibilit\ue9s et des limitations de la m\ue9thode dans le cadre de la recherche sur les ciments et les b\ue9tons. La faible teneur isotopique naturelle (0,135 %) et le rapport gyromagn\ue9tique peu \ue9lev\ue9 du 43Ca pr\ue9sentent un d\ue9fi de taille, m\ueame \ue0 un champ magn\ue9tique de 21,1 T. Dans la phase initiale de cette recherche, nous avons examin\ue9 les spectres d?un certain nombre de substances voisines du ciment anhydre, d?une structure et d?une composition connues. Les spectres de plusieurs mat\ue9riaux importants dans la recherche sur le ciment, comme le silicate de b\ueata-dicalcium ( f-C2S), le silicate de tri-calcium (C3S) et l?aluminate de tri-calcium (C3A), ont \ue9t\ue9 obtenus pour la premi\ue8re fois. La relation entre les param\ue8tres spectroscopiques et structuraux \ue9tait au centre de notre \ue9tude, et l?assignation des signaux a \ue9t\ue9 r\ue9alis\ue9e \ue0 l?aide des calculs des premiers principes. En outre, on a \ue9largi cette m\ue9thode \ue0 l?\ue9tude des phases du ciment hydrat\ue9. L?environnement calcique dans l?hydrate de silicate de calcium (C-S-H) synth\ue9tique de diverses compositions a \ue9t\ue9 sond\ue9 en comparaison avec le C-S-H form\ue9 lors de l?hydratation du C3S. Les spectres de RMN du 43Ca du C3S hydrat\ue9 ont sembl\ue9 indiquer des caract\ue9ristiques nanostructurales semblables \ue0 celles d?analogues plus ordonn\ue9s, y compris les \ue9chantillons de C-S-H synth\ue9tiques et le min\ue9ral tobermorite \ue0 11 \uc5. Cette observation admet la validit\ue9 de l?utilisation de syst\ue8mes de C-S-H cristallins en couches comme mod\ue8les structuraux pour le C-S-H quasi-amorphe qui se forme au cours de l?hydratation du ciment Portland. Cette \ue9tude a d\ue9montr\ue9 qu?en d\ue9pit de la grande complexit\ue9 de la chimie des silicates de calcium, la RMN du 43Ca en phase solide demeure un outil utile et profitable qui g\ue9n\ue8re une information fort pr\ue9cieuse au domaine de la recherche sur les ciments.A version of this document is published in: 2008-2009 Annual Report, National Ultrahigh-Field NMR Facility for Solids, pp. 36-37Une version de ce document se trouve dans: 2008-2009 Annual Report, National Ultrahigh-Field NMR Facility for Solids, pp. 36-37Peer reviewed: NoNRC publication: Ye