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Jorgen Skibsted - One of the best experts on this subject based on the ideXlab platform.
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effect of temperature on the hydration of white portland cement metakaolin blends studied by 29si and 27al mas nmr
2020Co-Authors: Zhuo Dai, Jorgen SkibstedAbstract:Metakaolin can be considered as a model compound for an aluminosilicate-rich supplementary cementitious material (SCM) because of its composition and highly pozzolanic properties. This work presents a systematic investigation of the hydration of white Portland cement (wPc)–metakaolin (MK) blends cured at different temperatures by 29Si and 27Al MAS NMR with the main focus on the structure and composition of the C-(A)-S-H phase. White Portland cement–MK paste samples with wPc replacement levels of 0, 10, 20, and 30 wt% have been prepared and hydrated for 1–8 weeks at five different temperatures (5, 20, 40, 60, and 80 °C). Information on the degree of reaction for alite, belite, and MK is derived from the 29Si NMR spectra and shows that the hydration of the wPc–MK blends is accelerated with increasing temperature except for the blend with the highest MK level (30 wt%). The AlIV/Si ratio of the C-(A)-S-H phase is found to be independent on the hydration time but increases slightly with curing temperature. On the other hand, a significant increase in the average aluminosilicate chain lengths is observed for blends with increasing temperature, reflecting a decrease in the Ca/Si ratio for the C-(A)-S-H phase. Stratlingite is present in the wPc–MK blends at 20 °C and the fraction of this phase decreases with increasing curing temperature.
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role of calcium on chloride binding in hydrated portland cement metakaolin limestone blends
Cement and Concrete Research, 2017Co-Authors: Zhenguo Shi, Tone Anita Ostnor, Frank Winnefeld, Mette Rica Geiker, Klaartje De Weerdt, Barbara Lothenbach, Jorgen SkibstedAbstract:Abstract Chloride binding is investigated for Portland cement–metakaolin–limestone pastes exposed to CaCl 2 and NaCl solutions. The phase assemblages and the amount of Friedel's salt are evaluated using TGA, XRD and thermodynamic modeling. A larger amount of Friedel's salt is observed in the metakaolin blends compared to the pure Portland cement. A higher total chloride binding is observed for the pastes exposed to the CaCl 2 solution relative to those in the NaCl solution. This is reflected by the fact that calcium increases the quantity of Friedel's salt in the metakaolin blends by promoting the transformation of Stratlingite and/or monocarbonate to Friedel's salt. Calcium increases also the amount of chloride in the diffuse layer of the C-S-H for the pure cement. A linear correlation between the total bound chloride and the uptake of calcium from the CaCl 2 solution is obtained and found to be independent on the type of cement blend.
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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.
Moises Frias - One of the best experts on this subject based on the ideXlab platform.
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influence of zno on the activation of kaolinite based coal waste pozzolanic activity and mineralogy in the pozzolan lime system
Applied Clay Science, 2018Co-Authors: Rosario Garciagimenez, Lucia Fernandezcarrasco, S Martinezramirez, Vigil R De La Villa, Moises FriasAbstract:Abstract One inconvenience presented by the thermal activation of kaolinite-based wastes is their low content of metakaolinite, a highly pozzolanic product listed in current standards for the manufacture of commercial cements. The addition of a chemical activator during the thermal activation process is a priority line of research to increase the reactivity of the recycled metakaolinite. In this paper, an additional chemical activator, ZnO, is studied and its effect on both pozzolanic properties and the evolution of mineralogical phases in the thermal activation of coal waste with a reaction time of up to 90 days in the pozzolan/lime system. To do so, activation temperatures of between 550 °C/650 °C were selected and additions of chemical activator (ZnO) in percentages of between 0.0% and 3.0% by weight of coal waste, because it is an activator with a positive effect on a 100% natural kaolinite. The results showed that the incorporation of ZnO inhibited the reactivity of the recycled metakaolinite and in consequence, the capacity of the metakaolinite to react with the surrounding lime; even more so when the content of added chemical activator was raised, albeit with some exceptions, in the samples activated at 550 °C and 650 °C with 0.5% of chemical activator. In none of the cases under analysis was the chemical activator able to improve the properties of the metakaolinite in comparison with the properties of the reference sample activated only with temperature. The hydrated phases that appeared in the pozzolanic reaction were tetracalcium aluminate hydrate, Stratlingite, monosulfoaluminate hydrate and LDH (phyllosilicate/carbonate).
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The Transformation of Coal-Mining Waste Minerals in the Pozzolanic Reactions of Cements
Minerals, 2016Co-Authors: Rosario Giménez-garcía, Raquel Vigil De La Villa Mencía, Virginia Rubio, Moises FriasAbstract:The cement industry has the potential to become a major consumer of recycled waste materials that are transformed and recycled in various forms as aggregates and pozzolanic materials. These recycled waste materials would otherwise have been dumped in landfill sites, leaving hazardous elements to break down and contaminate the environment. There are several approaches for the reuse of these waste products, especially in relation to clay minerals that can induce pozzolanic reactions of special interest in the cement industry. In the present paper, scientific aspects are discussed in relation to several inert coal-mining wastes and their recycling as alternative sources of future eco-efficient pozzolans, based on activated phyllosilicates. The presence of kaolinite in this waste indicates that thermal treatment at 600 °C for 2 h transformed these minerals into a highly reactive metakaolinite over the first seven days of the pozzolanic reaction. Moreover, high contents of metakaolinite, together with silica and alumina sheet structures, assisted the appearance of layered double hydroxides through metastable phases, forming Stratlingite throughout the main phase of the pozzolanic reaction after 28 days (as recommended by the European Standard) as the reaction proceeded.
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mineralogical study of calcined coal waste in a pozzolan ca oh 2 system
Applied Clay Science, 2015Co-Authors: Rosario Garcia, Olga Rodriguez, Moises Frias, Raquel Vigil De La Villa, S Martinezramirez, Lucia Fernandezcarrasco, I S De Soto, E VillarcocinaAbstract:Abstract Activated carbon mining waste influences the formation of hydrated phases, their saturation indexes and mineral stability fields during pozzolanic reactions. The behavior of these reactions is predicted in this study by examining the influence of carbon waste at 600 °C over 2 h, by means of a thermodynamic model running on a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations (PHREEQC program: pH - RE dox- Eq uilibrium in Program C ). Experimental analysis of the solid phases shows early formation of monosulfoaluminate hydrate, C–S–H phases and subsequent precipitation of laminar minerals, tetracalcium aluminate hydrate, layered double hydroxide compounds (phyllosilicate/carbonate) (LDH) and Stratlingite. Monosulfoaluminate hydrate was formed on day one of the reaction for samples with activated coal mining waste. The thermodynamic calculations confirmed the experimental observations.
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Chemical and mineral transformations that occur in mine waste and washery rejects during pre-utilization calcination
International Journal of Coal Geology, 2014Co-Authors: R. Vigil De La Villa, Rosario García Giménez, Moises Frias, Sagrario Martínez-ramírez, Lucía Fernández-carrascoAbstract:Abstract The mineralogical content of coal mine waste consists primarily of inorganic compounds which can be converted into a metakaolin-based product under controlled activation conditions that is also a highly pozzolanic material. Activation temperatures ranging from 500 to 900 °C over 2 h retention time affect the mineralogy of coal mine waste, as well as the formation and evolution of the hydrated phases that form during the pozzolanic reaction. The hydrated phases formed during the pozzolanic reaction in the activated coal mine waste (ACMW)/Ca(OH) 2 system were C–S–H gels, Stratlingite, tetracalcium aluminate hydrate, LDH compounds (phyllosilicate/carbonate) and monosulfoaluminate. Low temperatures (600 °C) favored the formation of LDH compounds and Stratlingite; whereas monosulfoaluminate formed during the hydrated phase at higher temperatures (900 °C) during the first day of the pozzolanic reaction, and tetracalcium aluminate hydrate appeared as the dominant crystalline phase at 7 and 28 days.
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Changes to the Triaxial Composition of the Hydrated Phases (CaO/Al2O3/SiO2) in the Metakaolin/Lime System
Journal of the American Ceramic Society, 2012Co-Authors: Rosario García Giménez, Raquel Vigil De La Villa, Olga Rodriguez, Moises FriasAbstract:This study examines the composition of certain hydrates (calcium silicate, aluminum silicate, and related phases) produced by the pozzolanic reaction of waste-paper sludge that had previously been activated at different temperatures. It summarizes and compares the evolution of the oxide compounds, and records their stability intervals. Changes to their mineralogical composition were analyzed using X-Ray Diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The hydration products from 1 to 360 days of curing time were CSH gels, hydrotalcite-type compounds (LDH), and Stratlingite (C2ASH8). CSH gels were employed as substrates for growing other materials and their morphologies were modified from fibrous to hexagonal layers. The composition of the LDH-type compounds observed in the carbonate group varied with changes in curing time. Two LDH-type compound types were identified: (a) with and (b) without magnesium. Stratlingite was the only stable material after long-curing times.Peer reviewe
R P Nogueira - One of the best experts on this subject based on the ideXlab platform.
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on the hydration of belite ye elimite ferrite byf cement pastes effect of the water to cement ratio and presence of fly ash
Cement and Concrete Research, 2020Co-Authors: Guilherme Yuuki Koga, Blandine Albert, R P NogueiraAbstract:Abstract This work reports the evolution of the phase assemblage, pore solution, porosity, and mechanical resistance of Belite-Ye'elimite-Ferrite (BYF) cement pastes. Different water-to-cement (w/c) ratios were considered and the addition of fly ash (FA) evaluated. The initial 1-day hydration of BYF cement paste was characterized by the ye'elimite hydration accompanied by a pronounced alkalinity increase controlled mainly by the molar ratio of calcium sulfate to ye'elimite. Throughout the 1-year test, the phase assemblages of 0.50 and 0.67 w/c samples were similar, composed of ettringite, AFm, Stratlingite, silicious hydrogarnet, and C-S-H. The hydration of the 0.40 w/c cement paste was almost complete within a month. Additions of 25 wt% FA increased the fraction of Stratlingite and ettringite without long-term strength losses compared to plain BYF cement paste with the same water-to-binder ratio. Results from different characterization techniques for both liquid and solid phases at early-age (first 24 h) were complemented by thermodynamic calculations.
Barbara Lothenbach - One of the best experts on this subject based on the ideXlab platform.
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role of calcium on chloride binding in hydrated portland cement metakaolin limestone blends
Cement and Concrete Research, 2017Co-Authors: Zhenguo Shi, Tone Anita Ostnor, Frank Winnefeld, Mette Rica Geiker, Klaartje De Weerdt, Barbara Lothenbach, Jorgen SkibstedAbstract:Abstract Chloride binding is investigated for Portland cement–metakaolin–limestone pastes exposed to CaCl 2 and NaCl solutions. The phase assemblages and the amount of Friedel's salt are evaluated using TGA, XRD and thermodynamic modeling. A larger amount of Friedel's salt is observed in the metakaolin blends compared to the pure Portland cement. A higher total chloride binding is observed for the pastes exposed to the CaCl 2 solution relative to those in the NaCl solution. This is reflected by the fact that calcium increases the quantity of Friedel's salt in the metakaolin blends by promoting the transformation of Stratlingite and/or monocarbonate to Friedel's salt. Calcium increases also the amount of chloride in the diffuse layer of the C-S-H for the pure cement. A linear correlation between the total bound chloride and the uptake of calcium from the CaCl 2 solution is obtained and found to be independent on the type of cement blend.
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influence of calcium to silica ratio on aluminium uptake in calcium silicate hydrate
Cement and Concrete Research, 2016Co-Authors: E Lhopital, Barbara Lothenbach, Dmitrii A KulikAbstract:Aluminium uptake in calcium silicate hydrate (C-S-H) was studied at different Ca/Si ratios from 0.6 to 1.6 at 20 degrees C. Aluminium incorporation in C-S-H depends on the total amount of aluminium present in the sample. At low Al/Si ratio (<= 0.05), the major part of the aluminium is taken up in C-S-H while at higher Al/Si ratio, the precipitation of Stratlingite and/or katoite limits the Al/Si ratio in C-S-H to approximate to 0.15, regardless of the Ca/Si ratio. A strong correlation between the aqueous aluminium concentration and the aluminium uptake in the solid phase is observed. At high Ca/Si ratios, aluminium in C-S-H is observed mainly as octahedrally coordinated Al(VI) in TAH and the aluminium uptake increases with the aqueous aluminium concentration. At low Ca/Si ratio (<= 0.8), aluminium is observed mainly as tetrahedrally coordinated Al(IV) and low aqueous aluminium concentrations (below detection limit) indicate a high affinity of aluminium towards C-S-H. (C) 2016 Elsevier Ltd. All rights reserved.
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influence of calcium to silica ratio on aluminium uptake in calcium silicate hydrate
Cement and Concrete Research, 2016Co-Authors: E Lhopital, Barbara Lothenbach, Dmitrii A KulikAbstract:Abstract Aluminium uptake in calcium silicate hydrate (C–S–H) was studied at different Ca/Si ratios from 0.6 to 1.6 at 20 °C. Aluminium incorporation in C–S–H depends on the total amount of aluminium present in the sample. At low Al/Si ratio (≤ 0.05), the major part of the aluminium is taken up in C–S–H while at higher Al/Si ratio, the precipitation of Stratlingite and/or katoite limits the Al/Si ratio in C–S–H to ≈ 0.15, regardless of the Ca/Si ratio. A strong correlation between the aqueous aluminium concentration and the aluminium uptake in the solid phase is observed. At high Ca/Si ratios, aluminium in C–S–H is observed mainly as octahedrally coordinated Al(VI) in TAH and the aluminium uptake increases with the aqueous aluminium concentration. At low Ca/Si ratio (≤ 0.8), aluminium is observed mainly as tetrahedrally coordinated Al(IV) and low aqueous aluminium concentrations (below detection limit) indicate a high affinity of aluminium towards C–S–H.
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incorporation of aluminium in calcium silicate hydrates
Cement and Concrete Research, 2015Co-Authors: E Lhopital, Dmitrii A Kulik, Le G Saout, Barbara Lothenbach, Karen ScrivenerAbstract:Calcium silicate hydrate (C-S-H) was synthetized at 20 degrees C to investigate the effect of aluminium uptake (Al/Si = 0-0.33) in the presence and absence of alkalis on the composition and the solubility of a C-S-H with a Ca/Si equal to 1.0. C-S-H incorporates aluminium readily resulting in the formation of C-A-S-H at Al/Si At higher Al/Si ratios, in addition to C-A-S-H, katoite and/or Stratlingite are present. Aluminium is mainly taken up in the bridging position of the silica dreierketten structure, which increases the chain length. The aluminium uptake in C-S-H increases with the aqueous aluminium concentrations. The presence of potassium hydroxide leads to higher pH values, to the destabilisation of Stratlingite and to higher dissolved aluminium concentrations, which favours the aluminium uptake in C-S-H. Potassium replaces partially the calcium ions on the surface and interlayer, thus leading to more negative surface charge and to shortening of chain length. (C) 2015 Elsevier Ltd. All rights reserved.
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influence of slag chemistry on the hydration of alkali activated blast furnace slag part i effect of mgo
Cement and Concrete Research, 2011Co-Authors: Ben M Haha, Barbara Lothenbach, Le G Saout, Frank WinnefeldAbstract:The hydration and microstructural evolution of three alkali activated slags (AAS) with Al{sub 2}O{sub 3} contents between 7 and 17% wt.% have been investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na{sub 2}SiO{sub 3}{center_dot}5H{sub 2}O. The formation of C(-A)-S-H and hydrotalcite was observed in all samples by X-ray diffraction, thermal analysis and scanning electron microscopy. Higher Al{sub 2}O{sub 3} content of the slag decreased the Mg/Al ratio of hydrotalcite, increased the Al incorporation in the C(-A)-S-H and led to the formation of straetlingite. Increasing Al{sub 2}O{sub 3} content of the slag slowed down the early hydration and a lower compressive strength during the first days was observed. At 28 days and longer, no significant effects of slag Al{sub 2}O{sub 3} content on the degree of hydration, the volume of the hydrates, the coarse porosity or on the compressive strengths were observed.
Frank Winnefeld - One of the best experts on this subject based on the ideXlab platform.
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role of calcium on chloride binding in hydrated portland cement metakaolin limestone blends
Cement and Concrete Research, 2017Co-Authors: Zhenguo Shi, Tone Anita Ostnor, Frank Winnefeld, Mette Rica Geiker, Klaartje De Weerdt, Barbara Lothenbach, Jorgen SkibstedAbstract:Abstract Chloride binding is investigated for Portland cement–metakaolin–limestone pastes exposed to CaCl 2 and NaCl solutions. The phase assemblages and the amount of Friedel's salt are evaluated using TGA, XRD and thermodynamic modeling. A larger amount of Friedel's salt is observed in the metakaolin blends compared to the pure Portland cement. A higher total chloride binding is observed for the pastes exposed to the CaCl 2 solution relative to those in the NaCl solution. This is reflected by the fact that calcium increases the quantity of Friedel's salt in the metakaolin blends by promoting the transformation of Stratlingite and/or monocarbonate to Friedel's salt. Calcium increases also the amount of chloride in the diffuse layer of the C-S-H for the pure cement. A linear correlation between the total bound chloride and the uptake of calcium from the CaCl 2 solution is obtained and found to be independent on the type of cement blend.
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influence of slag chemistry on the hydration of alkali activated blast furnace slag part i effect of mgo
Cement and Concrete Research, 2011Co-Authors: Ben M Haha, Barbara Lothenbach, Le G Saout, Frank WinnefeldAbstract:The hydration and microstructural evolution of three alkali activated slags (AAS) with Al{sub 2}O{sub 3} contents between 7 and 17% wt.% have been investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na{sub 2}SiO{sub 3}{center_dot}5H{sub 2}O. The formation of C(-A)-S-H and hydrotalcite was observed in all samples by X-ray diffraction, thermal analysis and scanning electron microscopy. Higher Al{sub 2}O{sub 3} content of the slag decreased the Mg/Al ratio of hydrotalcite, increased the Al incorporation in the C(-A)-S-H and led to the formation of straetlingite. Increasing Al{sub 2}O{sub 3} content of the slag slowed down the early hydration and a lower compressive strength during the first days was observed. At 28 days and longer, no significant effects of slag Al{sub 2}O{sub 3} content on the degree of hydration, the volume of the hydrates, the coarse porosity or on the compressive strengths were observed.
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influence of the calcium sulphate source on the hydration mechanism of portland cement calcium sulphoaluminate clinker calcium sulphate binders
Cement & Concrete Composites, 2011Co-Authors: Laure Pelletierchaignat, Frank Winnefeld, Barbara Lothenbach, Gwenn Le Saout, Christian J Muller, Charlotte FamyAbstract:Abstract Two different binders composed of Portland cement, calcium sulphoaluminate clinker and calcium sulphate were studied from early hydration to 28 days, one containing gypsum and one containing anhydrite at equimolar CaSO4 amount. Sodium gluconate was used as retarder to obtain a sufficient fluidity to cast the samples. Solids were analyzed by X-ray diffraction, scanning electron microscopy and thermogravimetric analysis and quantified by Rietveld refinement, while pore solutions were measured by ion chromatography. Thermodynamic modelling was used to model the hydration process of the ternary binders. This combined study allowed a precise understanding of the hydration process over time and the determination of the composition of the crystalline and of the X-ray amorphous hydrates present in the binders, which cannot be determined by conventional methods. Results show that the hydration mechanisms are similar in presence of gypsum or anhydrite, the difference being in the kinetics of reactions due to the slower dissolution of anhydrite compared to gypsum in the presence of sodium gluconate. The hydration starts with the formation of ettringite and of some X-ray amorphous hydrates. In the anhydrite-bearing system, the ettringite-forming reaction is stronger delayed by the addition of the retarder compared to the gypsum-bearing system. This stronger delay results in the formation of a significant amount of X-ray amorphous hydrates. The hydrates amorphous fraction is composed of different phases and its chemical composition is changing over time. During early hydration, it is mainly composed of aluminium hydroxide and Stratlingite, while in the anhydrite-bearing system it can additionally contain some monosulphoaluminate. At later ages, the aluminium hydroxide content decreases and additional monosulphoaluminate and a C-S-H type phase are formed.
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hydration of calcium sulfoaluminate cements experimental findings and thermodynamic modelling
Cement and Concrete Research, 2010Co-Authors: Frank Winnefeld, Arbara LothenbachAbstract:Abstract Calcium sulfoaluminate cements (CSA) are a promising low-CO 2 alternative to ordinary Portland cements and are as well of interest concerning their use as binder for waste encapsulation. In this study, the hydration of two CSA cements has been investigated experimentally and by thermodynamic modelling between 1 h and 28 days at w / c ratios of 0.72 and 0.80, respectively. The main hydration product of CSA is ettringite, which precipitates together with amorphous Al(OH) 3 until the calcium sulfate is consumed after around 1–2 days of hydration. Afterwards, monosulfate is formed. In the presence of belite, Stratlingite occurs as an additional hydration product. The pore solution analysis reveals that Stratlingite can bind a part of the potassium ions, which are released by the clinker minerals. The microstructure of both cements is quite dense even after 16 h of hydration, with not much pore space available at a sample age of 28 days. The pore solution of both cements is dominated during the first hours of hydration by potassium, sodium, calcium, aluminium and sulfate; the pH is around 10–11. When the calcium sulfate is depleted, the sulfate concentration drops by a factor of 10. This increases pH to around 12.5–12.8. Based on the experimental data, a thermodynamic hydration model for CSA cements based on cement composition, hydration kinetics of clinker phases and calculations of thermodynamic equilibria by geochemical speciation has been established. The modelled phase development with ongoing hydration agrees well with the experimental findings.
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the ternary system portland cement calcium sulphoaluminate clinker anhydrite hydration mechanism and mortar properties
Cement & Concrete Composites, 2010Co-Authors: Laure Pelletier, Frank Winnefeld, Barbara LothenbachAbstract:Abstract Binders composed of ordinary Portland cement (OPC), calcium sulphoaluminate clinker (CSA) and anhydrite ( C S ¯ ) were examined in order to study the impact of variations of the OPC:CSA: C S ¯ ratio on the hydration process and related mortar properties. A first sample series had various anhydrite contents and fixed OPC to CSA ratio, and a second various OPC contents and fixed CSA to C S ¯ ratio. Experiments made on pastes and thermodynamic modelling showed that the phase assemblage formed during the hydration of the binders was not very sensitive to changes in modal composition, while the ettringite to monosulphoaluminate volume ratio was influenced. All mixes started to hydrate with the formation of ettringite during a reaction involving C 4 A 3 S ¯ and calcium sulphate. This generated high early strength. Until about 7 d, mainly the CSA clinker reacted, and 15–20% of the dry binder was converted to ettringite. From about 7 d on, the OPC clinker phase alite reacted significantly, Stratlingite, C–S–H and monosulphoaluminate formed, while the ettringite content decreased. According to the laboratory experiments, the CSA clinker was mainly responsible for the early mechanical properties, while OPC played an important role at later ages.
