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Influence of citric acid on the hydration of Portland cement / Göril Möschner in Cement and concrete research, Vol. 39 N° 4 (Avril 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 4 (Avril 2009) Titre : Influence of citric acid on the hydration of Portland cement Type de document : texte imprimé Auteurs : Göril Möschner, Auteur ; Barbara Lothenbach, Auteur ; Renato Figi, Auteur Année de publication : 2009 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Retardation;  Hydration  products;  Thermodynamic  calculations;  Admixture;  Modeling Résumé : Citric acid can be used to retard the hydration of cement. Experiments were carried out to investigate the influence of citric acid on the composition of solid and liquid phases during cement hydration. Analyses of the solid phases showed that dissolution of alite and aluminate slowed down while analyses of the pore solution showed that citric acid was removed almost completely from the pore solution within the first hours of hydration. The complexation of the ions by citrate was weak, which could also be confirmed by thermodynamic calculations. Only 2% of the dissolved Ca and 0.001% of the dissolved K formed complexes with citrate during the first hours. Thus, citric acid retards cement hydration not by complex formation, but by slowing down the dissolution of the clinker grains. Thermodynamic calculations did not indicate precipitation of a crystalline citrate species. Thus, it is suggested that citrate sorbed onto the clinker surface and formed a protective layer around the clinker grains retarding their dissolution. ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609000179 [article] Influence of citric acid on the hydration of Portland cement [texte imprimé] / Göril Möschner, Auteur ; Barbara Lothenbach, Auteur ; Renato Figi, Auteur . - 2009. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 4 (Avril 2009) Mots-clés : Retardation;  Hydration  products;  Thermodynamic  calculations;  Admixture;  Modeling Résumé : Citric acid can be used to retard the hydration of cement. Experiments were carried out to investigate the influence of citric acid on the composition of solid and liquid phases during cement hydration. Analyses of the solid phases showed that dissolution of alite and aluminate slowed down while analyses of the pore solution showed that citric acid was removed almost completely from the pore solution within the first hours of hydration. The complexation of the ions by citrate was weak, which could also be confirmed by thermodynamic calculations. Only 2% of the dissolved Ca and 0.001% of the dissolved K formed complexes with citrate during the first hours. Thus, citric acid retards cement hydration not by complex formation, but by slowing down the dissolution of the clinker grains. Thermodynamic calculations did not indicate precipitation of a crystalline citrate species. Thus, it is suggested that citrate sorbed onto the clinker surface and formed a protective layer around the clinker grains retarding their dissolution. ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609000179

Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements / Thomas Schmidt in Cement and concrete research, Vol. 39 N° 12 (Décembre 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1113-1121 Titre : Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements Type de document : texte imprimé Auteurs : Thomas Schmidt, Auteur ; Barbara Lothenbach, Auteur ; Michael Romera, Auteur Année de publication : 2010 Article en page(s) : pp. 1113-1121 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Sulfate  attack;  Ettringite;  Gypsum;  Thaumasite;  Temperature;Thermodynamic  modelling Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : The consequences of external sulfate attack were investigated by traditional test methods, i.e. length and mass change, as well as by a newly developed, surface sensitive ultrasonic method, using Leaky Rayleigh waves (1 MHz). The macroscopic changes are discussed and compared with thermodynamic calculations and microstructural findings (SEM/EDS). The results show that the main impact of limestone additions on resistance to sulfate degradation are physical — i.e. addition of a few percent in Portland cement reduces the porosity and increases the resistance of Portland cement systems to sulfate; but higher addition of 25% increase porosity and lower resistance to sulfate. The kinetics of degradation were dramatically affected by the solution concentration (4 or 44 g Na2SO4/l) and the higher concentration also resulted in the formation of gypsum, which did not occur at the low concentration. However the pattern of cracking was similar in both cases and it appears that gypsum precipitates opportunistically in pre-formed cracks so it is not considered as making a significant contribution to the degradation. At 8 °C limited formation of thaumasite occurred in the surface region of the samples made from cement with limestone additions. This thaumasite formation led to loss of cohesion of the paste and loss of material from the surface of the samples. However thaumasite formation was always preceded by expansion and cracking of the samples due to ettringite formation and given the very slow kinetics of thaumasite formation it was probably facilitated by the opening up of the structure due to ettringite induced cracking. The expansion of the samples showed a steady stage, followed by a rapidly accelerating stage, with destruction of the samples. The onset of the rapidly accelerating stage occurred when the thickness of the cracked surface layer reached about 1–1.5 mm–10–15% of the total specimen thickness (10 mm). DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002063 [article] Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements [texte imprimé] / Thomas Schmidt, Auteur ; Barbara Lothenbach, Auteur ; Michael Romera, Auteur . - 2010 . - pp. 1113-1121. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 12 (Décembre 2009) . - pp. 1113-1121 Mots-clés : Sulfate  attack;  Ettringite;  Gypsum;  Thaumasite;  Temperature;Thermodynamic  modelling Index. décimale : 691 Matériaux de construction. Pièces et parties composantes Résumé : The consequences of external sulfate attack were investigated by traditional test methods, i.e. length and mass change, as well as by a newly developed, surface sensitive ultrasonic method, using Leaky Rayleigh waves (1 MHz). The macroscopic changes are discussed and compared with thermodynamic calculations and microstructural findings (SEM/EDS). The results show that the main impact of limestone additions on resistance to sulfate degradation are physical — i.e. addition of a few percent in Portland cement reduces the porosity and increases the resistance of Portland cement systems to sulfate; but higher addition of 25% increase porosity and lower resistance to sulfate. The kinetics of degradation were dramatically affected by the solution concentration (4 or 44 g Na2SO4/l) and the higher concentration also resulted in the formation of gypsum, which did not occur at the low concentration. However the pattern of cracking was similar in both cases and it appears that gypsum precipitates opportunistically in pre-formed cracks so it is not considered as making a significant contribution to the degradation. At 8 °C limited formation of thaumasite occurred in the surface region of the samples made from cement with limestone additions. This thaumasite formation led to loss of cohesion of the paste and loss of material from the surface of the samples. However thaumasite formation was always preceded by expansion and cracking of the samples due to ettringite formation and given the very slow kinetics of thaumasite formation it was probably facilitated by the opening up of the structure due to ettringite induced cracking. The expansion of the samples showed a steady stage, followed by a rapidly accelerating stage, with destruction of the samples. The onset of the rapidly accelerating stage occurred when the thickness of the cracked surface layer reached about 1–1.5 mm–10–15% of the total specimen thickness (10 mm). DEWEY : 620.13 ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609002063

Solid solution between Al-ettringite and Fe-ettringite (Ca6[Al1 − xFex(OH)6]2(SO4)3·26H2O) / Göril Möschner in Cement and concrete research, Vol. 39 N° 6 (Juin 2009) 

Public ISBD [article]  in Cement and concrete research > Vol. 39 N° 6 (Juin 2009) . - pp. 482–489 Titre : Solid solution between Al-ettringite and Fe-ettringite (Ca6[Al1 − xFex(OH)6]2(SO4)3·26H2O) Type de document : texte imprimé Auteurs : Göril Möschner, Auteur ; Barbara Lothenbach, Auteur ; Frank Winnefeld, Auteur Année de publication : 2009 Article en page(s) : pp. 482–489 Note générale : Génie Civil Langues : Anglais (eng) Mots-clés : Ettringite;  Thermodynamic  calculations;  Hydration  products Résumé : The solid solution between Al- and Fe-ettringite Ca6[Al1 − xFex(OH)6]2(SO4)3·26H2O was investigated. Ettringite phases were synthesized at different Al/(Al + Fe)-ratios (= XAl,total), so that XAl increased from 0.0 to 1.0 in 0.1 unit steps. After 8 months of equilibration, the solid phases were analyzed by X-ray diffraction (XRD) and thermogravimetric analysis (TGA), while the aqueous solutions were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). XRD analyses of the solid phases indicated the existence of a miscibility gap between XAl,total = 0.3–0.6. Some of the XRD reflections showed two overlapping peaks at these molar ratios. The composition of the aqueous solutions, however, would have been in agreement with both, the existence of a miscibility gap or a continuous solid solution between Al- and Fe-ettringite, based on thermodynamic modeling, simulating the experimental conditions. ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609000490 [article] Solid solution between Al-ettringite and Fe-ettringite (Ca6[Al1 − xFex(OH)6]2(SO4)3·26H2O) [texte imprimé] / Göril Möschner, Auteur ; Barbara Lothenbach, Auteur ; Frank Winnefeld, Auteur . - 2009 . - pp. 482–489. Génie Civil Langues : Anglais (eng) in Cement and concrete research > Vol. 39 N° 6 (Juin 2009) . - pp. 482–489 Mots-clés : Ettringite;  Thermodynamic  calculations;  Hydration  products Résumé : The solid solution between Al- and Fe-ettringite Ca6[Al1 − xFex(OH)6]2(SO4)3·26H2O was investigated. Ettringite phases were synthesized at different Al/(Al + Fe)-ratios (= XAl,total), so that XAl increased from 0.0 to 1.0 in 0.1 unit steps. After 8 months of equilibration, the solid phases were analyzed by X-ray diffraction (XRD) and thermogravimetric analysis (TGA), while the aqueous solutions were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). XRD analyses of the solid phases indicated the existence of a miscibility gap between XAl,total = 0.3–0.6. Some of the XRD reflections showed two overlapping peaks at these molar ratios. The composition of the aqueous solutions, however, would have been in agreement with both, the existence of a miscibility gap or a continuous solid solution between Al- and Fe-ettringite, based on thermodynamic modeling, simulating the experimental conditions. ISSN : 0008-8846 En ligne : http://www.sciencedirect.com/science/article/pii/S0008884609000490