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Jorgen Skibsted - One of the best experts on this subject based on the ideXlab platform.
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the charge balancing role of calcium and alkali ions in per alkaline aluminosilicate glasses
Journal of Physical Chemistry B, 2018Co-Authors: Jorgen Skibsted, Rene Thomsen, Yuanzheng YueAbstract:The structural arrangement of alkali-modified calcium aluminosilicate glasses has implications for important properties of these glasses in a wide range of industrial applications. The roles of sodium and potassium and their competition with calcium as network modifiers in peralkaline aluminosilicate glasses have been investigated by 27Al and 29Si MAS NMR spectroscopy. The 29Si MAS NMR spectra are simulated using two models for distributing Al in the silicate glass network. One model assumes a hierarchical, quasi-heterogeneous aluminosilicate network, whereas the other is based on differences in relative lattice energies between Si–O–Si, Al–O–Al, and Si–O–Al linkages. A systematic divergence between these simulations and the experimental 29Si NMR spectra is observed as a function of the sodium content exceeding that required for stoichiometric charge-balancing of the negatively charged AlO4 tetrahedra. Similar correlations between simulations and experimental 29Si NMR spectra cannot be made for the excess...
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thermodynamic modeling of hydrated white portland cement metakaolin limestone blends utilizing hydration kinetics from 29Si MAS NMR spectroscopy
Cement and Concrete Research, 2016Co-Authors: Wolfgang Kunther, Zhuo Dai, Jorgen SkibstedAbstract:Abstract Hydration kinetics for the principal phases of Portland cement blends have been incorporated in thermodynamic modeling (GEMS package), utilizing degrees of hydration from 29 Si MAS NMR. An empirical relationship for the reaction of these phases is established which includes three variable parameters that all can be estimated from the degrees of hydration. This approach is compared with thermodynamic equilibrium modeling (full hydration) for white Portland cement–metakaolin (0–30 wt.%) blends and for ternary blends of white Portland cement (65 wt.%)–metakaolin–limestone. The predicted phase assemblages have been compared with the phases identified by XRD, 27 Al and 29 Si MAS NMR which reveals that the incorporation of hydration kinetics improves the agreement between modeling and experiments. The results show also that the formation of stratlingite depends critically on the quantity of charge-balancing anions in the AFm phases, especially carbonate and sulfate anions, and on the degree of hydration for metakaolin.
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thermodynamic modeling of portland cement metakaolin limestone blends
2015Co-Authors: Wolfgang Kunther, Zhuo Dai, Jorgen SkibstedAbstract:The partial replacement of Portland cement by different supplementary cementitious materials (SCM´s) has been investigated extensively in recent years with the aim of reducing the embodied CO2 of blended Portland cements. In this work, we have utilized the maximum cement substitution of 35 wt%, according to the standard EN 197-1, and investigated the effect of changing the metakaolin/limestone ratio on the hydrating phase assemblages. Paste samples of the hydrated cement blends have been characterized by XRD, 27Al and 29Si MAS NMR spectroscopy and the results are compared with thermodynamic modeling. 29Si MAS NMR is a very valuable technique for studies of hydrated cement blends, since it allows detection of amorphous and crystalline phases in an equal manner. The determined degrees of hydration have been implemented into thermodynamic modeling to improve the modeling approach and thereby the agreement between predicted and observed phase assemblages. A simple equation has been established for implementation of the hydration kinetics which employs only one MASs and one dissolution-rate parameter to describe the hydration successfully. The agreement between the experimental and modeled phase assemblages improves significantly when the hydration kinetics for the anhydrous alite, belite, and amorphous MK phases are implemented. The phase assemblages of the hydrated blends change only for very high MK contents from a C(-A)-S-H, calcite, portlandite, monocarbonate and ettringite system to a phase assemblage that in addition contains stratlingite and other AFm phases.
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aluminum incorporation in the c s h phase of white portland cement metakaolin blends studied by 27al and 29Si MAS NMR spectroscopy
Journal of the American Ceramic Society, 2014Co-Authors: Zhuo Dai, Thuan T Tran, Jorgen SkibstedAbstract:The composition and structure of the calcium-silicate-hydrate (C–S–H) phases formed by hydration of white portland cement–metakaolin (MK) blends have been investigated using 27Al and 29Si MAS NMR. This includes blends with 0, 5, 10, 15, 20, 25, 30 wt% MK, following their hydration from 1 d to 1 yr. 29Si MAS NMR reveals that the average Al/Si ratio for the C–S–H phases, formed by hydration of the portland cement–MK blends, increases almost linearly with the MK content but is invariant with the hydration time for a given MK content. Correspondingly, the average aluminosilicate chain lengths of the C–S–H increase with increasing MK content, reflecting the formation of a C–S–H with a lower Ca/Si ratio. The increase in Al/Si ratio with increasing MK content is supported by 27Al MAS NMR which also allows detection of stratlingite and fivefold coordinated aluminum, assigned to AlO5 sites in the interlayer of the C–S–H structure. Stratlingite is observed after prolonged hydration for MK substitution levels above 10 wt% MK. This is at a somewhat lower replacement level than expected from thermodynamic considerations which predict the formation of stratlingite for MK contents above 15 wt% after prolonged hydration for the actual portland cement–MK blends. The increase in fivefold coordinated Al with increasing MK content suggests that these sites may contribute to the charge balance of the charge deficit associated with the incorporation of Al3+ ions in the silicate chains of the C–S–H structure.
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13c chemical shift anisotropies for carbonate ions in cement minerals and the use of 13c 27al and 29Si MAS NMR in studies of portland cement including limestone additions
Cement and Concrete Research, 2013Co-Authors: Tine F Sevelsted, Duncan Herfort, Jorgen SkibstedAbstract:Abstract 13C isotropic chemical shifts and chemical shift anisotropy parameters have been determined for a number of inorganic carbonates relevant in cement chemistry from slow-speed 13C MAS or 13C{1H} CP/MAS NMR spectra (9.4 T or 14.1 T) for 13C in natural abundance. The variation in the 13C chemical shift parameters is relatively small, raising some doubts that different carbonate species in Portland cement-based materials may not be sufficiently resolved in 13C MAS NMR spectra. However, it is shown that by combining 13C MAS and 13C{1H} CP/MAS NMR carbonate anions in anhydrous and hydrated phases can be distinguished, thereby providing valuable information about the reactivity of limestone in cement blends. This is illustrated for three cement pastes prepared from an ordinary Portland cement, including 0, 16, and 25 wt.% limestone, and following the hydration for up to one year. For these blends 29Si MAS NMR reveals that the limestone filler accelerates the hydration for alite and also results in a smaller fraction of tetrahedrally coordinated Al incorporated in the C-S-H phase. The latter result is more clearly observed in 27Al MAS NMR spectra of the cement–limestone blends and suggests that dissolved aluminate species in the cement–limestone blends readily react with carbonate ions from the limestone filler, forming calcium monocarboaluminate hydrate.
Angel Palomo - One of the best experts on this subject based on the ideXlab platform.
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c s h gels interpretation of 29Si MAS NMR spectra
Journal of the American Ceramic Society, 2012Co-Authors: Ines Garcialodeiro, Ana Fernandezjimenez, Isabel Sobrados, Jesus Sanz, Angel PalomoAbstract:The detection of bridging tetrahedra, or Q2(L), in the silica chains constituting C–S–H gels is controversial. While some authors maintain that 29SiMAS-NMR can be used to distinguish between Q1 and Q2 units but not between the silicon environments associated with Q2 (intermediate position) and Q2(L) (intermediate and bridging position) units, others claim the contrary. The present article addresses this issue with a report on 29SiMAS-NMR studies of a number of calcium silicate hydrate gels. The findings showed that nuclear magnetic resonance can differentiate between Q2 and Q2(L) units in highly polymerized C–S–H gels with mean chain lengths (MCL) of over 12 and Ca/Si ratios of <1. At higher Ca/Si ratios, however, the MCL declined, with the concomitant rise in Q1 units. Under these conditions, in which it was practically impossible to distinguish between Q2 and Q2(L), the presence of a single signal for all Q2 units was assumed.
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opc fly ash cementitious systems study of gel binders produced during alkaline hydration
Journal of Materials Science, 2007Co-Authors: Angel Palomo, G Kovalchuk, L M Ordonez, Ana Fernandezjimenez, M C NaranjoAbstract:In the present paper, experiments were performed on blended cements containing 30% Portland cement clinker and 70% fly ash. The powdery material was mixed with deionised water for “normal” hydration, and with two different alkaline solutions for “normal” alkaline activation. The mechanical strength developed by this highly blended cement differed significantly depending on the hydrating solution used. XRD, FTIR and 29Si MAS-NMR characterisation studies were conducted to obtain information on the complex structural nature of the hardened matrices, which in all cases consisted of a mixture of amorphous gels (C-S-H + N-A-S-H gel). These highly blended cements are able to comply with the specifications defined in the European Standard EN-197-1:2000.
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characterisation of fly ashes potential reactivity as alkaline cements
Fuel, 2003Co-Authors: Ana Fernandezjimenez, Angel PalomoAbstract:A representative group of Spanish fly ashes has been characterised in order to determine its capacity for being alkali activated and give place to a material with cementitious properties. The characterisation studies have been carried out through chemical analysis, laser granulometry, Blaine, BET, particle size distribution, XRD and 29Si MAS NMR. Compressive mechanical strength test was used to determine the reactivity of the fly ashes as alkaline binders. The results obtained have demonstrated that all investigated fly ashes are suitable to be alkali activated. Additionally it has also been demonstrated that the key factors of their potential reactivity are: the reactive silica content, the vitreous phase content and the particle size distribution.
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characterisation of fly ashes potential reactivity as alkaline cements
Fuel, 2003Co-Authors: Ana Fernandezjimenez, Angel PalomoAbstract:A representative group of Spanish fly ashes has been characterised in order to determine its capacity for being alkali activated and give place to a material with cementitious properties. The characterisation studies have been carried out through chemical analysis, laser granulometry, Blaine, BET, particle size distribution, XRD and 29Si MAS NMR. Compressive mechanical strength test was used to determine the reactivity of the fly ashes as alkaline binders. The results obtained have demonstrated that all investigated fly ashes are suitable to be alkali activated. Additionally it has also been demonstrated that the key factors of their potential reactivity are: the reactive silica content, the vitreous phase content and the particle size distribution.
Hans J. Jakobsen - One of the best experts on this subject based on the ideXlab platform.
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improved quantification of alite and belite in anhydrous portland cements by 29Si MAS NMR effects of paramagnetic ions
Solid State Nuclear Magnetic Resonance, 2009Co-Authors: Soren L Poulsen, Hans J. Jakobsen, Vanessa Kocaba, Gwenn Le Saout, Karen Scrivener, Jorgen SkibstedAbstract:The applicability, reliability, and repeatability of Si-29 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 Si-29 and the spins of the paramagnetic electrons, were considered and analyzed in spectra recorded at four magnetic fields (4.7-14.1 T) and this has led to an improved quantification of alite and belite from Si-29 MAS NMR spectra recorded at "high" spinning speeds of v(R) = 12.0-13.0 kHz using 4 or 5 mm 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 (Ca2AlxFe2-xO5). (C) 2009 Elsevier Inc. All rights reserved.
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a new aluminium hydrate species in hydrated portland cements characterized by 27al and 29Si MAS NMR spectroscopy
Cement and Concrete Research, 2006Co-Authors: Morten Daugaard Andersen, Hans J. Jakobsen, Jorgen SkibstedAbstract:Abstract Recent 27 Al MAS NMR studies of hydrated Portland cements and calcium-silicate-hydrate (C-S-H) phases have shown a resonance from Al in octahedral coordination, which cannot be assigned to the well-known aluminate species in hydrated Portland cements. This resonance, which exhibits the isotropic chemical shift δ iso = 5.0 ppm and the quadrupole product parameter P Q = 1.2 MHz, has been characterized in detail by 27 Al MAS and 27 Al{ 1 H} CP/MAS NMR for different hydrated white Portland cements and C-S-H phases. These experiments demonstrate that the resonance originates from an amorphous or disordered aluminate hydrate which contains Al(OH) 6 3− or O x Al(OH) 6- x (3+ x )− units. The formation of the new aluminate hydrate is related to the formation of C-S-H at ambient temperatures, however, it decomposes by thermal treatment at temperatures of 70–90 °C. From the experiments in this work it is proposed that the new aluminate hydrate is either an amorphous/disordered aluminate hydroxide or a calcium aluminate hydrate, produced as a separate phase or as a nanostructured surface precipitate on the C-S-H phase. Finally, the possibilities of Al 3+ for Ca 2+ substitution in the principal layers and interlayers of the C-S-H structure are discussed.
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hydration of portland cement in the presence of clay minerals studied by 29Si and 27al MAS NMR spectroscopy
Advances in Cement Research, 2003Co-Authors: H Kroyer, Hans J. Jakobsen, Holger Lindgreen, Jorgen SkibstedAbstract:The effects of the addition of clay minerals on the hydration reactions and kinetics of alite and belite are investigated for a white Portland cement (WPC)–kaolinite (20 wt%) blend and a mixture of 90 wt% WPC and 10 wt% bentonite using 29Si MAS NMR spectroscopy. 29Si MAS NMR spectra, used to follow the hydration from 12 h to one year, demonstrates that the two clay minerals have an accelerating effect on the hydration of both alite and belite. The relative intensities for the resonances from the calcium–silicate–hydrate (C–S–H) phase in these spectra show that addition of kaolinite results in a C–S–H phase that on average has longer chains of SiO4 tetrahedra as compared to the results for pure WPC. On the other hand, addition of bentonite results in a C–S–H with slightly shorter average SiO4 chain lengths. Furthermore, the 29Si MAS NMR spectra demonstrate that the basic structure of the kaolinite admixture is not affected by the hydrating cement and that kaolinite is not consumed during hydration. This ob...
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29Si chemical shift anisotropies in calcium silicates from high field 29Si MAS NMR spectroscopy
Inorganic Chemistry, 2003Co-Authors: Michael Ryan Hansen, Hans J. Jakobsen, Jorgen SkibstedAbstract:29Si chemical shift anisotropy (CSA) data have been determined from (29)Si MAS NMR spectra recorded at 14.1 T for a number of synthetic calcium silicates and calcium silicate hydrates. These are beta- and gamma-Ca(2)SiO(4), Ca(3)SiO(4)Cl(2), alpha-dicalcium silicate hydrate (alpha-Ca(2)(SiO(3)OH)OH), rankinite (Ca(3)Si(2)O(7)), cuspidine (Ca(4)Si(2)O(7)F(2)), wollastonite (beta-Ca(3)Si(3)O(9)), pseudowollastonite (alpha-Ca(3)Si(3)O(9)), scawtite (Ca(7)(Si(6)O(18))CO(3).2H(2)O), hillebrandite (Ca(2)SiO(3)(OH)(2)), and xonotlite (Ca(6)Si(6)O(17)(OH)(2)). The (29)Si MAS NMR spectra of rankinite and wollastonite clearly resolve manifolds of spinning sidebands from two and three Si sites, respectively, allowing the CSA parameters to be obtained with high precision for each site. For the (29)Si Q(1) sites in rankinite and cuspidine, the CSA asymmetry parameters (eta(sigma) approximately 0.6) contrast the general expectation that sorosilicates should possess small eta(sigma) values as a result of the nearly axially symmetric environments of the SiO(4) tetrahedra. The (29)Si CSA parameters provide an improved insight into the electronic and geometric environments for the SiO(4) tetrahedra as compared to the values solely for the isotropic chemical shift. It is shown that the shift anisotropy (delta(sigma)) and the CSA asymmetry parameter (eta(sigma)) allow a clear distinction of the different types of condensation of SiO(4) tetrahedra in calcium silicates. This relationship may in general be valid for neso-, soro-, and inosilicates. The CSA data determined in this work may form a valuable basis for (29)Si MAS NMR studies of the structures for tobermorites and calcium silicate hydrate phases resulting from hydration of Portland cements.
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incorporation of aluminum in the calcium silicate hydrate c s h of hydrated portland cements a high field 27al and 29Si MAS NMR investigation
Inorganic Chemistry, 2003Co-Authors: Morten Daugaard Andersen, Hans J. Jakobsen, Jorgen SkibstedAbstract:The calcium silicate hydrate (C−S−H) phase resulting from hydration of a white Portland cement (wPc) in water and in a 0.3 M NaAlO2 solution has been investigated at 14 and 11 hydration times, respectively, ranging from 6 h to 1 year by 27Al and 29Si MAS NMR spectroscopy. 27Al MAS NMR spectra recorded at 7.05, 9.39, 14.09, and 21.15 T have allowed a determination of the 27Al isotropic chemical shift (δiso) and quadrupolar product parameter (PQ = CQ ) for tetrahedrally coordinated Al incorporated in the C−S−H phase and for a pentacoordinated Al site. The latter site may originate from Al3+ substituting for Ca2+ ions situated in the interlayers of the C−S−H structure. The spectral region for octahedrally coordinated Al displays resonances from ettringite, monosulfate, and a third aluminate hydrate phase (δiso = 5.0 ppm and PQ = 1.20 MHz). The latter phase is tentatively ascribed to a less-crystalline aluminate gel or calcium aluminate hydrate. The tetrahedral Al incorporated in the C−S−H phase has been quan...
Morten Daugaard Andersen - One of the best experts on this subject based on the ideXlab platform.
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the effect of alkali ions on the incorporation of aluminum in the calcium silicate hydrate c s h phase resulting from portland cement hydration studied by 29Si MAS NMR
Journal of the American Ceramic Society, 2013Co-Authors: Jorgen Skibsted, Morten Daugaard AndersenAbstract:The incorporation of aluminum in the calcium–silicate–hydrate (C–S–H) phases formed by hydration of three different white Portland cements has been investigated by 29Si MAS NMR. The principal difference between the three cements is their bulk Al2O3 contents and quantities of alkali (Na+ and K+) ions. 29Si MAS NMR allows indirect detection of tetrahedral Al incorporated in the silicate chains of the C–S–H structure by the resonance from Q2(1Al) sites. Analysis of the relative 29Si NMR intensities for this site, following the hydration for the three cements from 0.5 d to 30 weeks, clearly reveals that the alkali ions promote the incorporation of Al in the bridging sites of the dreierketten structure of SiO4 tetrahedra in the C–S–H phase. The increased incorporation of Al in the C–S–H phase with increasing alkali content in the anhydrous cement is in accord with a proposed substitution mechanism where the charge deficit, obtained by the replacement of Si4+ by Al3+ ions in the bridging sites, is balanced by adsorption/binding of alkali ions in the interlayer region most likely in the near vicinity of the AlO4 tetrahedra. This result is further supported by similar 29Si MAS NMR experiments performed for the white Portland cements hydrated in 0.30M NaOH and NaAlO2 solutions.
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a new aluminium hydrate species in hydrated portland cements characterized by 27al and 29Si MAS NMR spectroscopy
Cement and Concrete Research, 2006Co-Authors: Morten Daugaard Andersen, Hans J. Jakobsen, Jorgen SkibstedAbstract:Abstract Recent 27 Al MAS NMR studies of hydrated Portland cements and calcium-silicate-hydrate (C-S-H) phases have shown a resonance from Al in octahedral coordination, which cannot be assigned to the well-known aluminate species in hydrated Portland cements. This resonance, which exhibits the isotropic chemical shift δ iso = 5.0 ppm and the quadrupole product parameter P Q = 1.2 MHz, has been characterized in detail by 27 Al MAS and 27 Al{ 1 H} CP/MAS NMR for different hydrated white Portland cements and C-S-H phases. These experiments demonstrate that the resonance originates from an amorphous or disordered aluminate hydrate which contains Al(OH) 6 3− or O x Al(OH) 6- x (3+ x )− units. The formation of the new aluminate hydrate is related to the formation of C-S-H at ambient temperatures, however, it decomposes by thermal treatment at temperatures of 70–90 °C. From the experiments in this work it is proposed that the new aluminate hydrate is either an amorphous/disordered aluminate hydroxide or a calcium aluminate hydrate, produced as a separate phase or as a nanostructured surface precipitate on the C-S-H phase. Finally, the possibilities of Al 3+ for Ca 2+ substitution in the principal layers and interlayers of the C-S-H structure are discussed.
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incorporation of aluminum in the calcium silicate hydrate c s h of hydrated portland cements a high field 27al and 29Si MAS NMR investigation
Inorganic Chemistry, 2003Co-Authors: Morten Daugaard Andersen, Hans J. Jakobsen, Jorgen SkibstedAbstract:The calcium silicate hydrate (C−S−H) phase resulting from hydration of a white Portland cement (wPc) in water and in a 0.3 M NaAlO2 solution has been investigated at 14 and 11 hydration times, respectively, ranging from 6 h to 1 year by 27Al and 29Si MAS NMR spectroscopy. 27Al MAS NMR spectra recorded at 7.05, 9.39, 14.09, and 21.15 T have allowed a determination of the 27Al isotropic chemical shift (δiso) and quadrupolar product parameter (PQ = CQ ) for tetrahedrally coordinated Al incorporated in the C−S−H phase and for a pentacoordinated Al site. The latter site may originate from Al3+ substituting for Ca2+ ions situated in the interlayers of the C−S−H structure. The spectral region for octahedrally coordinated Al displays resonances from ettringite, monosulfate, and a third aluminate hydrate phase (δiso = 5.0 ppm and PQ = 1.20 MHz). The latter phase is tentatively ascribed to a less-crystalline aluminate gel or calcium aluminate hydrate. The tetrahedral Al incorporated in the C−S−H phase has been quan...
Zhuo Dai - One of the best experts on this subject based on the ideXlab platform.
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thermodynamic modeling of hydrated white portland cement metakaolin limestone blends utilizing hydration kinetics from 29Si MAS NMR spectroscopy
Cement and Concrete Research, 2016Co-Authors: Wolfgang Kunther, Zhuo Dai, Jorgen SkibstedAbstract:Abstract Hydration kinetics for the principal phases of Portland cement blends have been incorporated in thermodynamic modeling (GEMS package), utilizing degrees of hydration from 29 Si MAS NMR. An empirical relationship for the reaction of these phases is established which includes three variable parameters that all can be estimated from the degrees of hydration. This approach is compared with thermodynamic equilibrium modeling (full hydration) for white Portland cement–metakaolin (0–30 wt.%) blends and for ternary blends of white Portland cement (65 wt.%)–metakaolin–limestone. The predicted phase assemblages have been compared with the phases identified by XRD, 27 Al and 29 Si MAS NMR which reveals that the incorporation of hydration kinetics improves the agreement between modeling and experiments. The results show also that the formation of stratlingite depends critically on the quantity of charge-balancing anions in the AFm phases, especially carbonate and sulfate anions, and on the degree of hydration for metakaolin.
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thermodynamic modeling of portland cement metakaolin limestone blends
2015Co-Authors: Wolfgang Kunther, Zhuo Dai, Jorgen SkibstedAbstract:The partial replacement of Portland cement by different supplementary cementitious materials (SCM´s) has been investigated extensively in recent years with the aim of reducing the embodied CO2 of blended Portland cements. In this work, we have utilized the maximum cement substitution of 35 wt%, according to the standard EN 197-1, and investigated the effect of changing the metakaolin/limestone ratio on the hydrating phase assemblages. Paste samples of the hydrated cement blends have been characterized by XRD, 27Al and 29Si MAS NMR spectroscopy and the results are compared with thermodynamic modeling. 29Si MAS NMR is a very valuable technique for studies of hydrated cement blends, since it allows detection of amorphous and crystalline phases in an equal manner. The determined degrees of hydration have been implemented into thermodynamic modeling to improve the modeling approach and thereby the agreement between predicted and observed phase assemblages. A simple equation has been established for implementation of the hydration kinetics which employs only one MASs and one dissolution-rate parameter to describe the hydration successfully. The agreement between the experimental and modeled phase assemblages improves significantly when the hydration kinetics for the anhydrous alite, belite, and amorphous MK phases are implemented. The phase assemblages of the hydrated blends change only for very high MK contents from a C(-A)-S-H, calcite, portlandite, monocarbonate and ettringite system to a phase assemblage that in addition contains stratlingite and other AFm phases.
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aluminum incorporation in the c s h phase of white portland cement metakaolin blends studied by 27al and 29Si MAS NMR spectroscopy
Journal of the American Ceramic Society, 2014Co-Authors: Zhuo Dai, Thuan T Tran, Jorgen SkibstedAbstract:The composition and structure of the calcium-silicate-hydrate (C–S–H) phases formed by hydration of white portland cement–metakaolin (MK) blends have been investigated using 27Al and 29Si MAS NMR. This includes blends with 0, 5, 10, 15, 20, 25, 30 wt% MK, following their hydration from 1 d to 1 yr. 29Si MAS NMR reveals that the average Al/Si ratio for the C–S–H phases, formed by hydration of the portland cement–MK blends, increases almost linearly with the MK content but is invariant with the hydration time for a given MK content. Correspondingly, the average aluminosilicate chain lengths of the C–S–H increase with increasing MK content, reflecting the formation of a C–S–H with a lower Ca/Si ratio. The increase in Al/Si ratio with increasing MK content is supported by 27Al MAS NMR which also allows detection of stratlingite and fivefold coordinated aluminum, assigned to AlO5 sites in the interlayer of the C–S–H structure. Stratlingite is observed after prolonged hydration for MK substitution levels above 10 wt% MK. This is at a somewhat lower replacement level than expected from thermodynamic considerations which predict the formation of stratlingite for MK contents above 15 wt% after prolonged hydration for the actual portland cement–MK blends. The increase in fivefold coordinated Al with increasing MK content suggests that these sites may contribute to the charge balance of the charge deficit associated with the incorporation of Al3+ ions in the silicate chains of the C–S–H structure.
