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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.
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.
The study of the feasibility of geopolymer formulations was realized from several aluminosilicate sources. Six of them led to the formation of geopolymer-type materials and were confronted with three ...other reference metakaolins. Several fundamental parameters were identified. A correlation was revealed between these parameters and the reactivity of the mixture, the microstructure and the behavior according to the time of consolidated materials from the various data issues from structural and mechanical characterizations. It has proved possible to predict the behavior of the reactive mixture until its evolution after consolidation for any raw material containing a metakaolinitic phase. All the results allowed to establish a model based on the existence of various geopolymer-type networks: (i) the first one in whom the aluminum atoms participate with the silicon as formers of the network (aluminum-rich) and (ii) the second where the aluminum atoms play the role of compensators in a network rich in silicon.
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•Determination of the aluminosilicate parameters controlling the feasibility of geopolymer formulations•Prediction of the behavior of the reactive mixture geopolymer in correlation with working properties•Establishment of a predictive model based on the existence of various geopolymer networks
Selective laser sintering of porcelain Danezan, A.; Delaizir, G.; Tessier-Doyen, N. ...
Journal of the European Ceramic Society,
02/2018, Letnik:
38, Številka:
2
Journal Article
Recenzirano
Selective Laser Sintering (SLS) technique is used to fabricate 3D porcelain products with complex shapes. Commercial powder has been studied and optimized in terms of morphology and particle size ...distribution in order to get a perfect powder layerwise which remains the critical step of such a technique. The influence of laser energy density (through the laser power and scan speed) and hatching space have been investigated to determine the optimized parameters that allow the greater densification of this complex multi-materials composed of kaolinite, quartz and potassium feldspar. The laser-sintered porcelain products which exhibit about 60% of porosity have been post-treated at 1350°C under vacuum or air to further improve densification.
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 properties and structures of three commercial metakaolins were evaluated for their use in geopolymer materials. The dehydroxylation processes used on these three metakaolins (rotary, flash or ...oven processes) induce varying reactivities and purities according to the results obtained by FTIR spectroscopy, XRD, 27Al MAS-NMR, TEM characterization, and DTA. The flash-calcined metakaolin is the most reactive one. After geopolymerization, the impurities (such as quartz or micas) and the reactivities of the raw metakaolins induce the formation of different networks within the geopolymer material. This study demonstrates the main role of the metakaolin dehydroxylation process, the effect of impurities and the use of the wettability as an indicator of metakaolin reactivity.
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•Impurities in the metakaolin influence the properties of the geopolymer.•The reactivity induced by calcination counterbalance the influence of impurities.•The calcination process influences the formation of Si- and Al-amorphous phases.•The reactivity differences induce the formation of different networks.•The wettability is an indicator of metakaolin reactivity.
Thanks to their design characteristics (i.e., colors, brightness, opacity, etc.) and/or physical properties (i.e., durability, low thermal conductivity, tightness, etc.), glazes find numerous ...applications, from art ornamenting to protection against corrosion. Glazing consists in coating a substrate by fusing various mineral substances over it. This is a low cost process and hence can be applied on large surfaces. Conventional glazing process needs a relatively high temperature treatment (i.e., up to 1400 °C) that heat-sensitive substrates do not sustain.
Thermal spraying may be a good solution to prevent the substrate from thermal degradation. Flame spraying was considered as the spray technique due to its low operating cost and the possibility to adapt the glaze transition temperature to the operating parameters.
When spraying glazes, the coating formation mechanism is different from the one encountered with crystallized ceramic materials. Indeed, the high surface tension of those feedstock prevents the particles from being totally spread (i.e., “dewetting” phenomena). Here, the coating results from the coalescence of impinging particles to form a monolayer.
The effects of glaze morphology on coatings were studied in this paper. Chemical analysis also permitted to determine the influence of spray parameters on glaze compositions, that can affect glazes thermal properties and hence modify coating structures. At last, the effects of operating parameters on coating architecture were analyzed by experimental design.
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
Many substrates do not sustain the conventional glazing process (i.e., vitreous glazing) due to the relatively high temperature required by this treatment (i.e., up to 1400 °C in some cases) to fuse ...glazes after their application on the surface to be covered. Flame spraying could appear as a solution to circumvent this limitation and to avoid thermal decomposition of substrates. This contribution describes some structural attributes of glaze coatings manufactured by flame spraying. It also discusses the influence of the feedstock powder morphology and some of its physical properties on coating characteristics.PUBLICATION ABSTRACT
