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The French national radioactive waste management agency (Andra) is studying the building of a deep geological disposal for High and Intermediate Level radioactive waste. If accepted, the disposal will lead all along the operating period to the excavation of a large amount of host rock, mainly constituted by argillite. With the aim of using this by-product, alkali-activated materials have been prepared and studied, involving two different alkaline silicate sources (solution and powder). In order to increase the thermal resistance, the addition of sand, as filler, has been studied. A standard test and a thermal shock, both up to 800 °C, are used to study the physical stability of the samples under these thermal constraints. Analyses by XRD and SEM allow correlating the thermal resistance to the structural evolution occurring during the thermal tests. It was shown that the alkaline-silicate solution composition is the main parameter to control the reactions during the thermal treatment. The use of the powder-based solution, containing sodium, leads to the formation of more viscous flow and less crystalline phases, which reduces the thermal resistance and the mechanical properties. The addition of filler in the samples increases the resistance to thermal sudden increase, but is not sufficient to obtain a correct resistance to shock. The utilization of flash-calcined argillite, containing carbonate species, gives a better crystallization during the thermal treatment and leads to more cracks but an increase of the mechanical properties. Using, these different compositions, a value of 20 MPa can be reached after thermal treatment. [ABSTRACT FROM AUTHOR]

Solution precursor plasma spraying (SPPS) is an innovative process for obtaining finely structured coatings from metallic salt solutions. Lanthanum and zirconium precursors were studied in order to understand their influence on the synthesis of lanthanum zirconate (La2Zr2O7) by SPPS. Thermal analysis revealed that the nature of the precursor affected the hydration shell as well as the formation temperature of zirconium oxide (ZrO2). The surface tensions of precursor solutions in various media were investigated and revealed the influence of the nature of the anion. Different solutions of precursor mixtures were used to obtain La2Zr2O7 splats on metallic substrates. A decrease in the surface tension of the solution led to an increase in the size of the splats. Coating mechanisms by SPPS are governed by the nature of the precursors and solvents. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a method for preparing nanomaterials shaped into powder and balls (diameter: 1–2mm) is developed. The synthesis is performed by an oil-drop process using a sol–gel method. During the preparation, the sol was improved by the inclusion of some additives such as methylethylketone (MEK), Butvar (But) and ethylmethacrylate (EMC). BaTiO3 and Ba0.67Sr0.33TiO3 perovskite-type compounds synthesized by both procedures present the same physical–chemical properties. The development of this sol–gel process enables the formation of these shapes in amounts as large as the 50g scale in a laboratory. The use of synthesized spherical pellets in a non-thermal plasma reactor enables the increased conversion of ethanol when acting as a pollutant. [Copyright &y& Elsevier]

Abstract: The purpose of this work is to identify the effect of the cation nature on mechanisms of the sand consolidation with alkaline silicate solution at low temperature (70°C). Three diluted lithium, sodium and potassium silicate solutions with [Si]=2.8mol/l were used to agglomerate sand composed of grains which mean diameter is 340μm. According to the cation, different behaviors were observed in terms of the drying time and the material cohesion. Essentially, the drying time increases with decreasing cation size. In contrast, the compressive strength raises when the cation size increases inducing intra-granular rupture highlighted by SEM observations. This could find an explanation in the cation hydration sphere of cations. The strength of the cation–water electrostatic interaction becomes less important as the size of the cations increase leading to more ionic bonds. Despite their strong consolidation, potassium-based materials have a high solubility in water. This result is consistent with the ionic nature of bonds. [Copyright &y& Elsevier]

This study was undertaken to valorize naturally occurring clays in the synthesis of new geopolymers materials from the mixture of commercial metakaolins and calcined clays (550 °C). For the purpose of this work, three raw clay samples were collected from various locations in Tunisia where clayey deposits extended over large areas. A simple synthesis method was adopted by mixing calcined clays and metakaolins with an alkali silicate solution of the desired Si/K ratio. The obtained geopolymers were stored for subsequent analyses. Both metakaolins and prepared geopolymers were characterized by FTIR, XRD, TG-DTA, SEM and mechanical properties. It was demonstrated that samples based on clay and different metakaolins present amorphous consolidated geopolymer materials. Also, the obtained geopolymer materials have a smooth and amorphous appearance characterizing the polycondensation reactions. Addition of natural clay decreased the compressive strength of the geopolymer materials, because of impurities (i.e. illite, calcite, iron). Despite their low kaolinite contents, raw clays from Tunisia provided good geopolymers in the presence of metakaolins and alkaline solution. [ABSTRACT FROM AUTHOR]

Abstract: The characterization of the behavior of inorganic foams in aqueous media is defined as a function of various parameters (initial pH value of the solution, alkaline element and aging of the foam). The final products and their microstructure depend on the alkaline cation in relation with the hydration shell. Durability tests revealed that the geopolymer foam network is based on three compounds: a dense geopolymer depending on the alkaline element, amorphous silica and M2Si2O5. For potassium foam, a fourth component was also evidenced: a zeolite that is partly responsible for the degradation of this foam. [Copyright &y& Elsevier]

Abstract: This work aims to study the effect of the pH of a sodium silicate solution on the consolidation of silica-based aggregates at low temperature (70°C). A diluted solution obtained by adding 50wt% of distilled water into a concentrated commercial solution was used as starting solution. Solutions with an increasing pH values were prepared by alkalinizing the diluted solution with solid NaOH. This leads to the depolymerization of silicate species present in solution. Consolidated materials were obtained by impregnating silica sand with the solution. In order to increase the surface reactivity, the sand was also functionalized by a prior basic attack. Results reveal that the increase in the pH value or the functionalization of the sand surface leads to improve mechanical properties of materials suggesting chemical reactions between the components confirmed by FTIR experiments. Actually, the νasSi-O-Si broad band with a sharp peak located around 1015cm-1, shifts firstly to lower and then to higher wavenumber characteristic of dissolution / precipitation reactions. Moreover, the enhancement of the pH value induces intra-granular break characterized by SEM observations confirming the best consolidation. However, these samples are more soluble in water related to the excess in sodium that induces more ionic bonds. [Copyright &y& Elsevier]

Abstract: A study based on the use of sodium silicate gels as binder for cold consolidation of silica-based aggregates has been investigated. The gels used as precursor of binder were synthesized by adding hydrochloric acid to a concentrated sodium silicate solution. Consolidated materials were obtained by mixing the previous solution before gelation with granular materials (fine silica powder and sand). The study of the gel–silica–sand ternary system shows that the existence domain of materials depends on the sand size distribution. The microstructure of gel–silica–sand ternary samples reveals the presence of the three components with a partial attack of grain surface. This was confirmed by FTIR experiments during the monitoring of the synthesis. Actually, the νasSih name="sbnd" />Si broad band resulting from the average of the contribution of the set of Q4, Q3 and Q2 units with a sharp peak located around 1078cm−1, firstly shifts to lower wavenumber until 21days and then to higher wavenumber characteristic of dissolution/precipitation reactions. On the other hand, the consolidation of the material is strong when the amount of fine silica in the material is high leading to efficient mechanical properties. Therefore, consolidation could be explained by the dissolution of small particles of silica and their precipitation into the grain boundary of sand. [Copyright &y& Elsevier]

Abstract: Both concentrated and diluted sodium silicate solutions have been investigated by combining 29Si NMR spectroscopy and SAXS experiments. The chemical nature of the entities responsible for the high siliceous species solubility observed in such alkaline concentrated sodium silicate solutions and their evolution according to dilution have been identified. For the most concentrated solution ([Si]=7mol/l; pH=11.56; Si/Na atomic ratio=1.71), the results evidence the preponderant presence of neutral Si7O18H4Na4 complexes, which behave like colloids of about 0.6–0.8nm able to form very small aggregates with an average size lower than 3nm. Addition of distilled water to this initial concentrated solution leads, on one hand, to a doubling of the colloid size, i.e. 1.2–1.5nm, and, on the other hand, to a progressive decrease of the aggregate size until their total disappearance. Such a behavior could be explained by considering, first, the dissociation of the neutral Si7O18H4Na4 complexes present in the concentrated solution into Na+ ions and charged (Si7O18H4Na4− n ) n − complexes (with 1⩽ n ⩽4) and, second, the condensation of these siliceous charged species in order to form larger (Si7 y O18 y − z H4 y −2 z Na(4− n )y) n y− colloids. The mean size of these colloids suggests that the condensation occurs between 2 and 8 (Si7O18H4Na4− n ) n − groups. [ABSTRACT FROM AUTHOR]