Aluminosilicate network formation during geopolymerization followed by in-situ 27Al nutation NMR

In classical cement systems, hydration reactions can typically be stopped by a solvent exchange (such as isopropanol) or by drying1. Subsequently, the chemical reactions are studied by separating and characterizing independently the solid and the liquid phases at different times, to follow their respective compositions and to establish a reaction process by finding chemical intermediates and products. As for geopolymers, they are formed by a dissolution-condensation mechanism resulting from the mixing a solid aluminosilicate source (for example metakaolin) with a highly concentrated alkali-silicate solution. The properties of the suspension do not allow to employ phase separation. This is the reason why the reaction mechanism leading to geopolymers is still said to be unclear, because it has only been studied by indirect methods so far, such as calorimetry, time-resolved rheology or small-angle scattering2 for instance. In-situ static 27Al NMR has already been used as a direct method to probe and quantify the aluminate species in the liquid phase during geopolymerization, using the quadrupolar nature of 27Al nuclei. Aluminum is not present in the liquid state at the very beginning of the process but goes to the initial aluminosilicate powder to the final solid product, naturally making it the nucleus of interest for an NMR study. While dissolved species are mobile enough for the quadrupolar interaction to be averaged, the quadrupolar coupling persists in less mobile species or in solids, leading to different nutation behaviors. In the present study, it will be demonstrated that a nutation experiment, which simply consists in varying the pulse length and measuring the resulting signal, allows filtering out the reactant aluminosilicate source from the 27Al NMR signal to detect reaction intermediates, and apparently also products. The evolution of the 27Al NMR signal was followed over longer periods of time up to several days during the geopolymerization process of metakaolin-based systems. It was shown that more than two steps can be identified in the geopolymerization process, depending on the frequency of the radiofrequency field applied during the experiment. Simulation of nutation curves at different times of the reactions allowed to follow the evolution of the quadrupolar coupling constant, and gave insight on the aluminate intermediates. Finally, the NMR results were confronted to time-resolved rheology and isothermal calorimetry in order to understand processes occurring on different time scales. 1- Collier et al. The influence of water removal techniques on the composition and microstructure of hardened cement pastes, Cement and Concrete Research, 38(6) (2008) pp. 737-744. 2- Steins et al. Structural Evolution during Geopolymerization from an Early Age to Consolidated Material, Langmuir, 28 (2012) pp. 8502-8510. 3- Favier et al., Mechanical properties and compositional heterogeneities of fresh geopolymer pastes, Cement and Concrete Research, 48 (2013) pp. 9-16

Preconditioning of Specimens - Drying Influence on Alkali-Activated and Geopolymer Mortar

Alkali-activated materials (AAM) are now seriously considered by the cement industry as an economical alternative to Portland cement, especially for its low CO2 footprint. However, their durability still remain to be assessed in more details. The aim of this study is to focus on the sample preconditioning conditions required for testing, especially the drying stage involved in most of the current tests. Four alkali-activated binders were studied: a geopolymer (Na-silicate activated metakaolin), a Na-carbonate activated slag (GGBS), a Na-silicate activated slag and a Na-silicate activated mixture of 50% metakaolin with 50% GGBS. After an endogenous cure of 28 days at 20°C, mortar specimens were dried at different temperatures (from 20°C to 125°C) until mass stabilization. Drying kinetics and released water contents were evaluated, as well as physical, mechanical and mineralogical analyses at the end of drying. Optimal drying temperature for each alkali-activated binder was determined by coupling mechanical strength measurements and mercury intrusion porosimetry. This study revealed that an inappropriate drying temperature could modify the porosity of some classes of AAM, and reduced the compressive strength by up to 30 to 40%. Antagonistic behaviors were observed in the four alkali-activated materials studied, therefore one should be careful about selecting preconditioning protocols for assessing the properties and the durability of these binders

Characterization of geopolymer porosity : temporal evolution and study of the confined water

Ce travail s’inscrit dans le cadre de l’étude de liants aluminosilicatés que sont les géopolymères. La première partie de ce travail a consisté à caractériser la texture poreuse des géopolymères, par des techniques intrusives (porosimétrie à eau, adsorption-désorption d’azote, intrusion mercure) et non-intrusives (diffusion des rayons X et des neutrons aux petits angles). Le terme « texture poreuse » regroupe la forme et la taille des pores, le volume poreux, la surface spécifique et la connectivité des pores. En parallèle, l’évolution de la texture poreuse et des propriétés mécaniques a été suivie sur une période de deux ans, en évitant les échanges avec le milieu extérieur afin d’étudier l’évolution intrinsèque des géopolymères. La seconde étape a consisté à étudier les propriétés thermodynamiques, la structure et la dynamique de l’eau confinée dans la porosité, par calorimétrie différentielle à balayage basse température, par diffusion des neutrons et par des essais de migration. La structure poreuse des géopolymères est complexe, puisqu’il s’agit d’une porosité multi échelle, méso- et macroporeuse, essentiellement ouverte et connectée. Elle consiste en un réseau vermiculaire de mésopores et un réseau de macropores connecté via les mésopores. La taille caractéristique (comprise entre 4 et 10 nm environ) et le volume des mésopores dépendent de la formulation de la pâte de géopolymère, à savoir de la teneur en eau, du rapport molaire Si/Al et de la nature du cation compensateur de charge. Il a été montré que les géopolymères étudiés sont très poreux, la porosité représentant entre 40 et 50 % du volume total du matériau. Le volume mésoporeux représente entre 7 et 15 % du volume total, le reste étant attribué à un volume macroporeux. Au cours du temps, la porosité des géopolymères se ferme légèrement, ceci étant attribué à un mécanisme de dissolution-reprécipitation au niveau des murs de pores. Les propriétés mécaniques atteignent un maximum entre 7 et 10 jours, puis sont stables dans le temps lorsque les échantillons sont conservés à 20°C et à l’abri du séchage ou de la carbonatation de la solution porale. Par ailleurs, trois types d’eau ont été mises en évidence au sein des pores : (i) l’eau liée chimiquement et/physiquement à la surface des parois, (ii) l’eau libre confinée dans les mésopores, et (iii) l’eau libre dans les macropores. A l’échelle locale, les molécules d’eau possède une mobilité proche de celle de l’eau libre, tandis qu’à l’échelle macroscopique, une diminution d’un ordre de grandeur du coefficient de diffusion a été observé, avec un effet probable de la taille des mésopores.In this study, we have investigated the porous network of geopolymers. The first step consisted in characterizing the structure of the porous network by the means of both intrusive experimental techniques (water porosimetry, gas sorption and mercury intrusion) and non-intrusive techniques (small-angle X-ray and neutron scattering). By the same time, the evolutions of the porous structure as well as the mechanical properties were followed over time. The second step was to determine the structure, the thermodynamics and the dynamics of water confined in the porosity by differential scanning calorimetry, quasi-elastic neutron scattering and migration tests.Geopolymer pore structure is a complex multi-scale porosity, a meso- and macroporous network, essentially open and connected. It consists in a vermicular mesoporous network which connects the macropores. The mesopore characteristic size depends on the formulation of the geopolymer paste and is ranged between about 4 and 10 nm. Geopolymer have a total pore volume comprised between 40 and 50 %, the mesoporous volume represents between 7 and 15 % of the material global volume. The majority of the pore volume is then attributed to macropores. A slight closure of porosity was observed with time and was attributed to a dissolution-precipitation mechanism occurring at pore wall interfaces. The mechanical properties reach a maximum within 10 days, and then are stable over time when the samples were kept from drying and carbonation and at the temperature of 20°C. Besides, three kinds of water were highlighted inside the porosity: (i) an interfacial water linked at the pore surfaces, (ii) free water inside the mésopores and (iii) free water inside macropores. At local time scale, the mobility of water was found close to the one of free water, and at the macroscopic scale, a decrease in diffusion coefficient of one order of magnitude was observed, together with an effect of mesopore size

Effect of composition and aging on the porous structure of metakaolin-based geopolymers

International audienceA combination of intrusive and small-angle scattering techniques is used to characterize the porous structure of metakaolin-based geopolymers. The influence of the geopolymer paste composition and the aging time in a 100percent relative humidity environment at 20DC are studied. The effect of the alkali activator, the water amount and the silica amount were investigated. The results show a strong "ink-bottle" effect indicating a two-level pore structure: a meso-and macroporous network. Both alkali activator and water amount have a significant impact on porosity and microstructure in the studied formulation range. After a period of one month, the pore structure is stable over a period of one year except for a small closure of porosity, revealed by nitrogen sorption and small-angle neutron scattering (SANS). These results highlight the geopolymer stability in the curing conditions. For the first time, SANS combined with the contrast matching technique was used to determine a fraction of closed porosity in geopolymer pastes. It was found that the geopolymer porosity is mainly open, the closed porosity representing less than 5percent of the pore volume after six months of aging

Robustness to water and temperature, and activation energies of metakaolin-based geopolymer and alkali-activated slag binders

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Effect of drying temperature on the properties of alkali-activated binders - Recommendations for sample preconditioning

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Impact of aluminum on the structure of geopolymers from the early stages to consolidated material

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Behavior of calcined clay based geopolymers under sulfuric acid attack: Meta-illite and metakaolin

International audienceThis paper studies the behavior of four different calcined clay-based geopolymers under sulfuric acid attack and gives insights into their degradation mechanisms. Pastes were cast using metakaolin and meta-illite and were activated by two different alkaline solutions. Mineralogical and microscopic characterizations were performed on pastes before and after attack. Results indicated that all pastes were affected by low leaching of alkali cations, and thus by a linkage disequilibrium of the geopolymer network. Meta-illite based geopolymers, which had not been addressed in the literature until now, hold promise for improving the durability of materials in aggressive environments

Shrinkage mitigation of metakaolin-based geopolymer activated by sodium silicate solution

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