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Autef A.,CNRS Heterogeneous Materials Study Group | Joussein E.,CNRS Research Group on Water, Soil and Environment | Gasgnier G.,Imerys | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of Non-Crystalline Solids

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. © 2013 Elsevier B.V. Source

Autef A.,CNRS Heterogeneous Materials Study Group | Joussein E.,CNRS Research Group on Water, Soil and Environment | Gasgnier G.,Imerys | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of Non-Crystalline Solids

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. © 2012 Elsevier B.V. All rights reserved. Source

Joulia A.,CNRS Heterogeneous Materials Study Group | Joulia A.,University of Limoges | Joulia A.,French National Center for Space Studies | Vardelle M.,University of Limoges | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of the European Ceramic Society

Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La2Zr2O7, Gd2Zr2O7, La2Ce2O7-γ and La2(Zr0.7Ce0.3)O7-γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400°C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar(g)/3%H2(g) atmosphere. © 2013 Elsevier Ltd. Source

Hounsi A.D.,University of Lome | Lecomte-Nana G.L.,CNRS Heterogeneous Materials Study Group | Djeteli G.,University of Lome | Blanchart P.,CNRS Heterogeneous Materials Study Group
Construction and Building Materials

The present work aimed to investigate the influence of mechanical activation of raw kaolin on the final compressive strength of as-obtained geopolymers regarding the curing profile. A commercial raw kaolin containing 81.5 mass% of kaolin (labeled KBip) was used. Mechanical activation was performed by dry ball-milling of raw kaolin at 250 rpm for 1 h. The curing temperatures were 40 °C, 70 °C or 100 °C for 24 h, 48 h or 72 h. The compressive tests were conducted on geopolymers after the 2nd, 7th, 14th and 28th days of ageing. Mechanical activation was performed to improve mechanical properties. Results showed that without mechanical activation, the optimal curing condition was 24 h at 70 °C and the compressive strength was 15 MPa after 28 days of ageing. Under mechanical activation, improvement of the compressive strength was obtained with a curing time of 72 h at 70 °C (to reach 35% increase) or with a curing temperature of 100 °C (for 76% improvement). The formation of alkaline aluminosilicate gels and new crystalline hydrated phases controlled the strength development of geopolymers while the occurrence of carbonated species was responsible for the degradation of mechanical properties. © 2013 Elsevier Ltd. All rights reserved. Source

Prud'Homme E.,CNRS Heterogeneous Materials Study Group | Michaud P.,CNRS Heterogeneous Materials Study Group | Joussein E.,GRESE | Rossignol S.,CNRS Heterogeneous Materials Study Group
Journal of Non-Crystalline Solids

The development of porous materials based on geopolymers allows the benefits of eco-consumption, good insulating properties and good mechanical properties to be combined. For geopolymers to be useful, the development of an understanding of their properties under various conditions is important. Attention was given to the structural evolution of porous materials with heating. The structural evolutions were investigated using thermal analysis and infrared spectroscopy. The formation of a crystalline phase was observed and identified as a zeolite. To elucidate the roles played by the raw materials, silicon concentration and potassium concentration on the formation of the crystalline phase, three parameters in the foam synthesis process were modified. The influence of silica fume and metakaolin on the appearance of the crystalline phase was examined through the reactivity of the raw materials in contact with a potassium solution. Different behaviors directly linked to the network composition were observed. Silica-fume-based samples showed phase crystallization and dehydroxylation due to the large concentration of silicon, which likely led to the formation of hydrated silica species. The formation of the zeolite-phase network upon the application of heat depended only on the silica dissolution and was linked to the composition of silica in terms of the silicon species and grain size. Studying the crystalline phase formation by variation of the amount of potassium allowed the determination of a maximum silicon concentration and a minimum potassium concentration. The formation of the zeolite phase occurred at a potassium concentration greater than 3.5 mol·L - 1 and a silicon concentration lower than 16.5 mol·L - 1, which confirmed the presence of F-zeolites. © 2012 Elsevier B.V. All rights reserved. Source

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