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Abstract The influence of spark plasma sintering (SPS) parameters (temperature, time, pressure) and the role of particle size on densification, microstructure and mechanical properties of commercial additive-free TiB 2 , SiC and composites thereof were studied by X-ray diffraction, scanning electron microscopy, the ultrasonic method and indentation. Three particle sizes of SiC and 2 of TiB 2 were processed. An optimal cycle was found for TiB 2 and SiC: 2000 °C, 3 min dwell time, and 100 MPa applied at 600 °C. The relative density of pure SiC increases linearly from 70% to 90% when the initial particle size decreases from 1.75 µm to 0.5 µm. Pure TiB 2 was densified up to 87%. Using 2.5 wt% SiC in TiB 2 , the relative density increases to 97%. Young's modulus and the hardness of all samples were measured, with results discussed. The higher properties were obtained for additive-free TiB 2 –5%SiC with a relative density of 97% and with the Young's modulus and Vickers hardness values being close to 378 GPa and 23 GPa, respectively. [ABSTRACT FROM AUTHOR]

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]

The effects of alkaline solution and calcined clays on vibrational spectra and mechanical properties were studied on geopolymer samples prepared from two alkaline solutions, calcined clays and three metakaolins. Structural evolution of the prepared geopolymers was characterized using FTIR spectroscopy and compression tests. Our results indicated high positive correlation between reactivity and compressive strength of the studied samples. Commercial potassium silicate and laboratory prepared k-silicate solutions were used to assess their effects on the prepared geopolymers. It was also observed that various secondary phases appeared, causing a slight change in the species' reactivity. Finally, it was concluded that changing alkaline solution and calcined clays decreased the mechanical properties, but increased the FTIR shift values. However, further work is needed to study the effects of alkaline solutions and natural clays from Tunisia on the properties of the prepared geopolymer. This is compulsory for prospective application of those products in various industries. [ABSTRACT FROM AUTHOR]

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]

The present work carried out the influence of curing cycles on the performance of laterite-based geopolymer composites. To do so, the end products were obtained by altering laterite with 15, 20, and 25 wt% of rice husk ash (RHA). Alkaline solution in a constant solid/liquid ratio of 0.35 was added together with fine and coarse aggregates (representing equal and double weight of laterite, respectively). The different obtained matrices were treated in the following three curing cycles before characterization: room temperature curing (RTC), oven curing at 80 °C (OTC) and controlled humidity steam curing at 80 °C (STC). The mechanical tests carried out at 28 days give the following maximum values for each curing mode: 16.40, 28.82 and 56.41 MPa for RTC, OTC, and STC modes respectively. This means that when the samples, submitted in a moisture-controlled environment, the end products are more stable, less porous and resistant. Regarding the physical properties, the results show that the maximum value of open porosity is 11.62% corresponding to a matrix that was cured at room temperature without rice husk ash added, while the minimum value of 7% corresponds to a matrix that was cured under controlled humidity and containing 20% rice husk ash. The optimum and minimum absorption values are 2.70 and 4.60% respectively for the OTC and RTC curing modes. As for bulk densities, the optimum value is 2.64 g cm−3 for the matrix having 15% rice husk ash and the minimum value is 2.33 g cm−3 for a matrix having 20% rice husk ash, for OTC and STC curing modes respectively. The appropriate curing type for laterite-based geopolymer is when the humidity is controlled. • Laterite, aggregates and rice husk ash were used to produce geopolymer composites. • Geopolymer composites were cured at Room curing temperature (RTC), Oven curing and Steam curing (STC). • Applying the steam curing (STC) under controlled humidity (>65%) optimized the geopolymerization reaction. • High mechanical (56.41 MPa) was achieved on sample containing 25 wt% of RHA. • This sample also exhibits compact and dense structure with few accessible voids. [ABSTRACT FROM AUTHOR]

• Binary mixtures with pyroclastic deposits (Mt. Etna volcano) and metakaolin. • Alkaline activating solutions based on Na or K cation. • Polycondensation reaction monitoring on fresh pastes through In situ FTIR analysis. • Performance in terms of structure, compressive strengths and porosity. The huge availability of volcanic deposits in Mt. Etna volcano and the wide use of volcanic materials as aggregates in local architecture encouraged us to apply them in alkaline environment to produce geopolymeric binders to restore historical buildings of Mt. Etna area. Two clusters of samples for each volcanic precursor (volcanic ash and ghiara paleo-soil) were produced, using 10 and 20% wt. of metakaolin and two different alkaline solutions based on sodium and potassium respectively. A comparative study was made in relation to activators used, focusing on evaluation of polycondensation step (in ATR); chemical and structural characterization (FT-IR); TGA analysis, qualitative XRD analysis and mechanical compressive test after 7 and 21 curing days. ATR results confirmed the occurrence of geopolymerization for all samples; FT-IR analysis showed a higher content of carbonates in potassium-samples, decreasing at increasing of metakaolin content. A low weight lost was recorded in consolidated samples, as well as a partial dissolution of volcanic phases. Higher mechanical compressive strengths for K-samples, reaching 89 MPa as highest value after 21 days, differently to sodium-samples (51 MPa) were recorded, reflecting the porosity of the different specimens. [ABSTRACT FROM AUTHOR]