ABSTRACT
It has recently been demonstrated that the addition of zinc can enhance the mechanical strength of tricalcium silicates (C3S) upon hydration, but the structure of the main hydration product of cement, calcium silicate hydrate (C-S-H), in zinc-modified formulations remains unresolved. Here, we combine 29Si DNP-enhanced solid-state nuclear magnetic resonance (NMR), density functional theory (DFT)-based chemical shift computations, and molecular dynamics (MD) modeling to determine the atomic-level structure of zinc-modified C-S-H. The structure contains two main new silicon species (Q(1,Zn) and Q(2p,Zn)) where zinc substitutes Q(1) silicon species in dimers and bridging Q(2b) silicon sites, respectively. Structures determined as a function of zinc content show that zinc promotes an increase in the dreierketten mean chain lengths.
Subject(s)
Silicon , Zinc , Zinc/chemistry , Calcium Compounds/chemistry , Silicates/chemistryABSTRACT
In a recent article, we described the edxia framework, a user-friendly framework to analyse the microstructure of cementitious materials using SEM-EDS hypermaps. The manual approach presented was shown to be efficient to answer the relevant scientific questions. However, it is limited for batch analysis and (semi-)automated treatments. In this article, we show how the framework can be used to customise the analysis to the problem at hand. We first present some possible extensions, and then we provide a simple example of automatic clustering, using the flexible Python scientific libraries which will allow to define more custom workflows in the future.
ABSTRACT
Despite use of blended cements containing significant amounts of aluminum for over 30 years, the structural nature of aluminum in the main hydration product, calcium aluminate silicate hydrate (C-A-S-H), remains elusive. Using first-principles calculations, we predict that aluminum is incorporated into the bridging sites of the linear silicate chains and that at high Ca:Si and H2O ratios, the stable coordination number of aluminum is six. Specifically, we predict that silicate-bridging [AlO2(OH)4]5- complexes are favored, stabilized by hydroxyl ligands and charge balancing calcium ions in the interlayer space. This structure is then confirmed experimentally by one- and two-dimensional dynamic nuclear polarization enhanced 27Al and 29Si solid-state NMR experiments. We notably assign a narrow 27Al NMR signal at 5 ppm to the silicate-bridging [AlO2(OH)4]5- sites and show that this signal correlates to 29Si NMR signals from silicates in C-A-S-H, conflicting with its conventional assignment to a "third aluminate hydrate" (TAH) phase. We therefore conclude that TAH does not exist. This resolves a long-standing dilemma about the location and nature of the six-fold-coordinated aluminum observed by 27Al NMR in C-A-S-H samples.
ABSTRACT
The applicability, reliability, and repeatability of 29Si MAS NMR for determination of the quantities of alite (Ca3SiO5) and belite (Ca2SiO4) in anhydrous Portland cement was investigated in detail for 11 commercial Portland cements and the results compared with phase quantifications based on powder X-ray diffraction combined with Rietveld analysis and with Taylor-Bogue calculations. The effects from paramagnetic ions (Fe3+) on the spinning sideband intensities, originating from dipolar couplings between 29Si and the spins of the paramagnetic electrons, were considered and analyzed in spectra recorded at four magnetic fields (4.7-14.1T) and this has led to an improved quantification of alite and belite from (29)Si MAS NMR spectra recorded at "high" spinning speeds of nu(R)=12.0-13.0kHz using 4 or 5mm rotors. Furthermore, the impact of Fe3+ ions on the spin-lattice relaxation was studied by inversion-recovery experiments and it was found that the relaxation is overwhelmingly dominated by the Fe3+ ions incorporated as guest-ions in alite and belite rather than the Fe3+ sites present in the intimately mixed ferrite phase (Ca2Al(x)Fe(2-)(x)O5).
