Shortly after the first 29Si MAS NMR study of silicates by Lippmaa, Mägi, Samoson, Engelhardt and Grimmer in 1980, the same group of researchers reported similar pioneering ivestigations for cement minerals and zeolites. Portland cement is mainly composed of calcium silicates, which during hydration form an amorphous calcium-silicate-hydrate (C-S-H) phase as the principal binding component responsible for the strength of hardened cement paste. Over the past decades, 29Si MAS NMR has been instrumental in exploring the structure and composition of this phase. In addition, small amounts of aluminum and other ions (Na, K, Fe) are incorporated in the C-S-H phase, reflecting the cement blend composition and pore-solution chemistry during hydration. Here, the role and structural environments of Al in the C-S-H structure have been unraveled by 27Al NMR. This presentation will illustrate some of these achievements on the structure of the C-(A)-S-H phase and recent applications of 27Al and 29Si MAS NMR in the development of sustainable Portland cement binders, where a significant amount of the CO2-intensive clinkers are replaced by alternative materials such as calcined clays with significantly lower CO2 footprint. In 1986, Lippmaa and co-workers presented a correlation between 27Al isotropic chemical shifts and the mean T-O-T bond angle between framework tetrahedra for condensed aluminosilicates and zeolites (JACS 108, 1730 (1986)). This correlation, known as the ‘Lippmaa correlation’, has been widely used in assignments of 27Al resonances for a variety of zeolite structures. However, over the past decade, atomistic simulations of Al framework sites in zeolites have questioned its validity, in particular for highly silicious zeolites. The final part of the presentation will describe recent ultra-high field 27Al MAS NMR studies of ZSM-5 and ZSM-23 zeolites and discuss resonance assignment in terms of the Lippmaa correlation and DFT calculations of 27Al chemical shifts (JPC C 123, 7831(2019)).

29Si and 27Al MAS NMR studies of Portland cements and zeolites

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.