Abstract
The production of cement has a major carbon footprint corresponding to roughly 8% of the global greenhouse gas CO2 emissions. In this process, calcination of limestone contributes to more than half of the CO2 emissions. Therefore, a direct way of lowering its environmental impact is to reduce the amount of limestone needed as raw material. Supplementary cementitious materials (SCMs) represent a valuable replacement since SCM-blended cements simultaneously meet the structural qualities for a durable binder and lower the carbon footprint. The SCM materials are generally rich in silica and alumina, which will lead to a significant change in chemical composition and structure of the main hydration phase, the calcium-silicate-hydrate (C-S-H). Although the effects of SCMs on cement hydration have been widely studied, important questions regarding the atomic-level structure of C-S-H, its binding of ions and structural stability are not fully understood yet.
This PhD thesis focusses on aluminium-substituted C-S-H phases, C-A-S-H, addressing three main topics that are relevant for the structure and composition of this main hydration product in blended cements. (i) Qualitative and quantitative analysis of Al and Na in synthesized C-A-S-H samples by structural studies of C-A-S-H phases formed in alkaline solutions with variable Ca/Si and Al/Si ratios, utilizing multi-nuclear 23Na, 27Al and 29Si solid-state NMR spectroscopy. A systematic method for simulations of the 29Si NMR spectra for C-A-S-H phase is developed. 27Al NMR is coupled with mass balance calculations to determine the Al/Si ratio of the structure for the different Al coordination states. These values are correlated with Al/Si ratios from 29Si NMR to elucidate the type of Al in the silicate chain. Furthermore, the roles of sodium in the C-A-S-H phase with different Ca/Si ratios are investigated. (ii) Structural changes upon storage of Al-free C-S-H and Al-containing C-S-H phases. The structural stability of C-S-H and C-A-S-H phase under storage times up to 7.5 years have been studied using thermogravimetric analyses (TGA) and powder X-ray diffraction (XRD) in combination with 29Si and 27Al NMR. These techniques have provided information about changes in the hydration state, the interlayer spacing of the C-S-H, the local environments of silicate units and the coordination environment for Al. The main effect is a release of interlayer water with time as seen by a shrinkage of the interlayer space. Moreover, a structural rearrangement of the silicate chains is associated with this process as observed by a broadening of the 29Si resonances and a re-distribution over the silicate sites, which is accompanied by changes in the coordination environment of Al. (iii) The effects of alkali ions on C-S-H phases have been studied for C-S-H prepared in equilibrium with NaOH and KOH solutions to investigate the structural impact of alkalis on the structure of the C-S-H phase. A new silicate species is resolved in the 29Si NMR spectra which intensity grows with increasing alkali concentration. Finally, it is shown that important structural information may also be gained from 23Na NMR experiments, in particular 23Na MQMAS NMR experiments at very high magnetic field, which resolve several distinct Na environments.
This PhD thesis focusses on aluminium-substituted C-S-H phases, C-A-S-H, addressing three main topics that are relevant for the structure and composition of this main hydration product in blended cements. (i) Qualitative and quantitative analysis of Al and Na in synthesized C-A-S-H samples by structural studies of C-A-S-H phases formed in alkaline solutions with variable Ca/Si and Al/Si ratios, utilizing multi-nuclear 23Na, 27Al and 29Si solid-state NMR spectroscopy. A systematic method for simulations of the 29Si NMR spectra for C-A-S-H phase is developed. 27Al NMR is coupled with mass balance calculations to determine the Al/Si ratio of the structure for the different Al coordination states. These values are correlated with Al/Si ratios from 29Si NMR to elucidate the type of Al in the silicate chain. Furthermore, the roles of sodium in the C-A-S-H phase with different Ca/Si ratios are investigated. (ii) Structural changes upon storage of Al-free C-S-H and Al-containing C-S-H phases. The structural stability of C-S-H and C-A-S-H phase under storage times up to 7.5 years have been studied using thermogravimetric analyses (TGA) and powder X-ray diffraction (XRD) in combination with 29Si and 27Al NMR. These techniques have provided information about changes in the hydration state, the interlayer spacing of the C-S-H, the local environments of silicate units and the coordination environment for Al. The main effect is a release of interlayer water with time as seen by a shrinkage of the interlayer space. Moreover, a structural rearrangement of the silicate chains is associated with this process as observed by a broadening of the 29Si resonances and a re-distribution over the silicate sites, which is accompanied by changes in the coordination environment of Al. (iii) The effects of alkali ions on C-S-H phases have been studied for C-S-H prepared in equilibrium with NaOH and KOH solutions to investigate the structural impact of alkalis on the structure of the C-S-H phase. A new silicate species is resolved in the 29Si NMR spectra which intensity grows with increasing alkali concentration. Finally, it is shown that important structural information may also be gained from 23Na NMR experiments, in particular 23Na MQMAS NMR experiments at very high magnetic field, which resolve several distinct Na environments.
| Original language | English |
|---|
| Place of publication | Aarhus |
|---|---|
| Publisher | Århus Universitet |
| Number of pages | 309 |
| Publication status | Published - Apr 2021 |