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Abstract Materials such as ceramic matrix composites are developed for mechanical applications at high temperature, but their cost remains a limitation. Consequently, the use of acid-based geopolymer matrices may be an alternative to reduce costs. In this study, the sample was prepared from metakaolin and phosphoric acid. Fourier Transform InfraRed and Nuclear Magnetic Resonance spectroscopies, X-Ray Diffraction and thermal measurements were used to understand the structural evolution of acid-based geopolymers (binders) during consolidation and after thermal treatments. According to the results, the consolidation of the binder has been divided into four steps: the dissolution of the metakaolin, the polycondensation reactions forming AlPO 4 entities and hydrated phases, the breakdown of Si O Al bonds with formation of various hydrated silica networks and finally the completion of the networks. After a thermal treatment at 1000 °C, the binder consists of AlPO 4 phases, quartz and vitreous silica. Highlights • The polycondensation mechanism of acid-based geopolymers is highlighted. • The different networks formed are evidenced from spectroscopy and XRD data. • Several hydrated phases and AlPO 4 polymorphs crystallize during heat treatment. [ABSTRACT FROM AUTHOR]

• Geopolymer foams were produced from phosphoric acid and various aluminosilicates. • The use of a mechanically produced foam is compatible with the process. • Their thermal conductivity is low and they can be manipulated. • They resist to water, low and high temperature with no decrease of the properties. • Upscaling of the production process is possible. This paper aims to determine the feasibility of phosphoric acid-based geopolymer highly porous foams elaborated by a surfactant induced mechanical foaming. The processing and consolidation parameters of the basalt fibers reinforced foams were determined. The influence of the aluminosilicate source on the synthesis of usable samples is reported, showing that the Si/Al ratio as well as the reactivity of the source is crucial to ensure a good consolidation. The effect of the Al/P composition of the geopolymer foam on its properties is also discussed. Two optimal formulations with very good properties (water and temperature resistance, mechanical strength, low thermal conductivity) could be established. Their working properties as well as their main physico-chemical characteristics, allowing to understand their formation mechanism were studied. Both withstood immersion in water and exposition to high temperatures. Their size may also be increased without any difference. Their insulating (61 to 75 mW.m−1K−1) and mechanical performances (80 to 190 kPa) are reported, as well as the study of their networks and microstructures. [ABSTRACT FROM AUTHOR]

Two metakaolins exhibiting different reactivities were used to study geopolymers obtained via phosphoric acid activation. The study of consolidation of samples with various Si/Al and Al/P showed that the Al- and Si-rich and P-poor compositions were more likely to consolidate properly. The geopolymers formed at 20 or 70 °C kept the same crystalline impurities as the starting metakaolins, the samples remaining mostly amorphous. The microstructure of geopolymers formed with less reactive metakaolin was smooth and dense, while the geopolymers prepared from the most reactive one were more porous and rough. Finally, tests on water and fire resistances of all the samples were reported. [ABSTRACT FROM AUTHOR]

Lanthanum silicated apatites with nominal composition La 9.33+ x (SiO 4 ) 6 O 2+3 x /2 (−0.2 < x < 0.27) have been successfully synthesized by solid state reaction using a new reagent La 2 O 2 CO 3 and amorphous SiO 2 precursors. The formation mechanism of La 2 O 2 CO 3 reagent, which cannot be purchased, has been followed by in-situ temperature depend XRD of La 2 O 3 under CO 2 atmosphere. The stability of this reagent during the synthesis step allowed to limit the formation of secondary phase La 2 Si 2 O 7 and made the weighting of the reagent easier. High purity powders could be synthesized at the temperature of 1400 °C. Dense pellets (more than 98.5%) were obtained by isostatic pressing of powders calcined at 1200 °C and then sintered at 1550 °C. Traces of La 2 SiO 5 secondary phase present in synthesized powder disappeared after densification and pure oxyapatite materials were obtained for all the compositions. Electrical measurements confirmed that conductivity behaviors of the sintered pellets were dependent to the oxygen over-stoichiometry. Indeed, a relatively high conductivity of 1 × 10 −2 S cm −1 was exhibited at 800 °C for the nominal composition La 9.60 (SiO 4 ) 6 O 2.405 with low activation energy around 0.79 eV. The ionic conductivity properties were comparable with that of the earlier obtained materials. [ABSTRACT FROM AUTHOR]

Geopolymers are obtained from an alkali silicate solution and aluminosilicate sources. The source commonly used geopolymer is metakaolin. The chemical composition, extraction site or calcination process of metakaolin influence its reactivity and thus the properties of the consolidated samples. This work focused on clarifying how the properties of aluminosilicate-based raw materials evolve when different metakaolin sources are mixed. The study involved mixing different metakaolins to evaluate their physico-chemical properties. The different samples were characterized by measuring their granulometry, wettability and zeta potential. Structural data were obtained from X-ray diffraction and 27Al nuclear magnetic resonance spectroscopy. It appears that the properties of the mixtures can be expressed as a function of different parameters. Granulometric properties directly depend on the quantity of each source, wettability is related to the amount of available amorphous aluminum in the sources, and zeta potential is strongly influenced by the source with the highest amount of siliceous-based impurities. This methodological approach can be applied to geopolymer synthesis. [ABSTRACT FROM AUTHOR]

Acid-based geopolymers are interesting materials, as they usually present higher mechanical and thermal properties than their alkali-based counterparts do, but they are difficult to use for complex shapes due to the high viscosity of their reactive mixture. This can be overcome by the addition of water in the formulation. In this study, geopolymers have been synthesized from metakaolin and phosphoric acid with variable [P] concentration (by a modification of either the water content or Al/P ratio). Measurements of the viscosity, density and consolidation times have been performed on reactive mixtures differentiated by their water content. Furthermore, a study of the geopolymers' microstructure and mechanical properties confirmed their evolution with [P]. Finally, a characterization by thermal and structural analyzes of their structure (Fourier transform infrared spectroscopy and X-ray diffraction), have been carried out in order to assess the influence of the water content on the different amorphous networks. It appears that the addition of water influences the structure of the material, as it leads to the formation of new amorphous networks. More specifically, for Al/P = 1 and variable water amounts, the networks are mainly based on the coexistence of Al–O–P with an hydrated silica network, while for variable Al/P, the Al–O–P network coexists with the unreacted metakaolin. In addition, it seems that all the properties are governed by the parameter n P /n Al + n H2O. Image 1 • Addition of water and Al/P change lead to the formation of new amorphous. • The properties of acid geopolymer are governed by the parameter n P /n Al + n H2O. • The availability of reactive aluminous or phosphate species control the setting time and viscosity. [ABSTRACT FROM AUTHOR]

La (1− x ) Sr x Fe (1− y ) Ga y O 3− δ perovskites are some of the most promising materials for oxygen transport membranes due to their high oxygen semi-permeation and good chemical stability at high temperature under a wide range of oxygen partial pressures (from 0.21 to 10 −19 atm.). This paper is focused on a better understanding of the influence of the Ga and Sr substitution ratios in the La (1− x ) Sr x Fe (1− y ) Ga y O 3− δ perovskite series on oxygen semi-permeation performances, oxygen diffusion and surface exchange coefficients. This study identifies the optimal compositions of the La (1− x ) Sr x Fe (1− y ) Ga y O 3− δ perovskite series. In addition, this study shows that the rate-determining step of oxygen transport through the membrane depends strongly on the Ga and Sr substitution ratios. [ABSTRACT FROM AUTHOR]