Natural abundance ultrahigh-field 43Ca solid-state NMR in cement-based materials
Bibliographic record
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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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".