Geopolymers are new binders resulting from the activation of an aluminosilicate source by an alkaline solution. It is well-established that the precursors that are used affect geopolymer formation. ...This study focuses on how to control the geopolymer setting time with an alkaline silicate solution. To conduct this investigation, various geopolymer mixtures based on different alkaline silicate solutions were characterized in terms of viscosity and setting time. To understand the link between geopolymer formation and the properties of the solution, the ionic conductivity and the viscosity of the solutions were measured. The solutions were also characterized by Raman spectroscopy. The characterization of the geopolymer mixture shows that the properties of the alkaline silicate solution highly depend on the viscosity and setting time of the geopolymer. Therefore, the alkali molar concentration (3 < M < 10 mol·L−1) influenced the properties of the solution (silicate species, ionic conductivity, and viscosity). A relationship among the chemical composition, the silicate species of the solution and the geopolymer setting time is highlighted depending on the nsinAl+nM ratio. For example, a ratio of nsinAl+nM>1.1 permits a slower geopolymerization reaction, and the setting time of the geopolymer mixture is higher.
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•The alkali molar concentration modifies the silicate species.•For a given metakaolin, the silicate solution can permit to adjust the geopolymer setting time.•The geopolymer setting time is dependant of the silicate species of the solution.
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•Metakaolins reactivity impacts the behavior in the presence of alkaline solution.•Highly reactive metakaolin accelerates the consolidation of the material.•The metakaolin reactivity ...can generate the formation of one or several networks.•Nanostructure variations after consolidation influence the mechanical properties.•A descriptive model of the mechanism of geopolymer formation was proposed.
Geopolymer materials are obtained by the alkaline activation of aluminosilicate sources, the best of which is metakaolin. However, every raw material is different, and very few comparative studies have been done on different metakaolin sources. The aim of this work is to develop methods for the prediction of the working properties of geopolymer materials based on the reactivity of the metakaolin employed. Infrared spectroscopy showed direct relationships between the wettability, the Si/Al ratio and the kinetics of conversion of Si–O–Si bonds to Si–O–Al bonds. Moreover, it was demonstrated that the presence of impurities and the reactivity of the metakaolin can generate the formation of one or several networks. Finally, a descriptive model of the mechanism of geopolymer formation was proposed that takes into account the quality of metakaolin used.
This work aims to study the effect of the addition of reactive or unreactive aluminosilicate sources on the geopolymerisation reaction, and therefore on the geopolymer's working properties. To this ...end, six formulations with different metakaolin content, as well as the potential addition of mica fillers, were synthetized and investigated over time. The structural evolution of the reactive mixtures was monitored by FTIR spectroscopy and thermal analysis, whereas the resulting geopolymers were characterized using X-ray diffraction and 27Al NMR. Finally, SEM observations of the microstructures of the consolidated materials were carried out. The results show that a rise in the metakaolin content modifies the dissolution step of the geopolymer formation by decreasing the amount of consumed water from 50 to 34%. Moreover, it increases the initial viscosity by 66% and decreases the setting time by 168 min. Besides, with the addition of mica fillers, the analysis of the FTIR spectra outlines a lower reactivity of the mixture and the formation of several geopolymer networks, ultimately leading to a heterogeneous microstructure. Finally, the mechanical properties are directly related to the level of amorphous phase in the geopolymer sample, the highest values being reached for 85% of amorphous phase.
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•The metakaolin content has a strong influence on the geopolymer formation.•The addition of mica affects the reactive mixture properties.•The use of mica fillers modifies the polycondensation reactions and the microstructure of the geopolymer synthetized.
The synthesis of geopolymer-silica composites was achieved at room temperature to determine the role of the silica source (quartz or amorphous silica) on the polycondensation rate and the mechanical ...properties of synthesized materials. Then, samples with a composition range from 100% quartz to 100% amorphous silica were formed, compared and characterized by XRD, infrared spectroscopy, thermal analysis, SEM, and compression tests. The results give evidence that the increase of amorphous silica in the mixture favors the polycondensation reaction (i.e., “geopolymerization”) to form consolidated materials whereas quartz led to heterogeneous materials without cohesion. These facts are explained by the modification of the Si/Al ratio in the geopolymer matrix due to the increase of quartz in the mixture.
► The increase of amorphous silica in the mixture favors the polycondensation reaction. ► Quartz source leads to a lack of silica inducing a non consolidated material. ► The mechanical properties increase with the amount of amorphous silica. ► Si/Al and mechanical properties decrease with quartz sand addition. ► For Si/Al constant, the quartz sand addition increases the mechanical properties.
Increasing the amount of amorphous silica in a mixture containing silica and quartz favors a polycondensation reaction (i.e., geopolymerization) and improves the mechanical properties of the ...synthesized materials. The study aimed to investigate the polycondensation reaction during the consolidation step of geopolymer formation and examine the various equilibriums at different temperatures. In total, eleven compositions with various amounts of amorphous silica S (high reactivity) and quartz Q (low reactivity) (from 100%Q to 100%S) were synthesized in basic media with metakaolin. The synthesized samples were characterized by thermal analyses and mercury porosimetry. Correlations between the loss of water and the molar ratio of each composition were investigated. The existence of four reactions during the consolidation process was demonstrated: (i) the reorganization of the species; (ii) the dissolution of the metakaolin; (iii) the formation of oligomers; and (iv) the reaction of polycondensation. Moreover, two types of networks were shown, a silicate solution network for quartz-rich samples and a geopolymeric network for amorphous silica-rich samples. The nature of the primary network and the reactivity of the synthesized sample depend on the reactivity of the silica source used.
► The existence of four reactions during the consolidation process was demonstrated ► The amount of trapped water within the network decreases with the temperature of reaction. ► The nature of the main network depends on the reactivity of the silica source used. ► The reaction temperature has a direct impact on the reaction kinetics.
During the synthesis of geopolymer materials, all of the phenomena and reactions that occur should be included in the analysis of the synthesis. This work aims to study the role that siliceous ...species play in an activation solution in the presence of pure metakaolin. The formation of a gel phase during the synthesis of a K-geopolymer was shown. Different mixtures were analyzed by infrared spectroscopy. During material consolidation, there is always competition between the geopolymer network and the gel that is governed by a change in the siliceous species. The heat treatment of various gels and solids provided evidence of various networks in geopolymer materials. Finally, the change in the crystallinity of the silica in the activation solution led to a change in the kinetics of the polycondensation reactions, in agreement with previous work.
The activation of metakaolin using an alkaline solution is a method for producing high quality consolidated materials. Si, Al and K are the main factors that influence the nature of the consolidated ...materials. This study focuses on the effects of the composites on the final network formation. Two types of alkaline solutions, one (S
c
) based on a commercial potassium silicate solution and the other (S
l
) synthesized by mixing KOH and amorphous silica, were studied, and their effects on the network formation of consolidated materials were compared and discussed. Four types of materials were observed and identified based on different Si, Al and K contents, including a geopolymer, gel, sedimented materials and hardening materials. These behaviors are manifested in the various polycondensation mechanisms and microstructures, which were characterized using FTIR spectroscopy, thermal analysis, X-ray diffraction measurements and Scanning electron microscope observations. The influence of Si, Al and K is discussed according to network formation.
The synthesis of geopolymer-silica composites was achieved at room temperature to determine the role of the silica source (quartz or amorphous silica) on the polycondensation rate and the mechanical ...properties of synthesized materials. Then, samples with a composition range from 100% quartz to 100% amorphous silica were formed, compared and characterized by XRD, infrared spectroscopy, thermal analysis, SEM, and compression tests. The results give evidence that the increase of amorphous silica in the mixture favors the polycondensation reaction (i.e., "geopolymerization") to form consolidated materials whereas quartz led to heterogeneous materials without cohesion. These facts are explained by the modification of the Si/Al ratio in the geopolymer matrix due to the increase of quartz in the mixture
The activation of metakaolin using an alkaline solution is a method for producing high quality consolidated materials. Si, Al and K are the main factors that influence the nature of the consolidated ...materials. This study focuses on the effects of the composites on the final network formation. Two types of alkaline solutions, one (Sc) based on a commercial potassium silicate solution and the other (S l ) synthesized by mixing KOH and amorphous silica, were studied, and their effects on the network formation of consolidated materials were compared and discussed. Four types of materials were observed and identified based on different Si, Al and K contents, including a geopolymer, gel, sedimented materials and hardening materials. These behaviors are manifested in the various polycondensation mechanisms and microstructures, which were characterized using FTIR spectroscopy, thermal analysis, X-ray diffraction measurements and Scanning electron microscope observations. The influence of Si, Al and K is discussed according to network formation.
