The role of hematite in aluminosilicate gels based on metakaolin

International audienceThis paper investigates the role of iron oxide in natural clay and its contribution to polycondensation reactions. Iron oxide was added to pure kaolin in the same proportion as Medenine clay with different amounts of potassium hydroxide. The physicochemical and structural properties of these materials were determined by infrared spectroscopy, scanning electron microscopy, thermal analyses (DTA-TG) and X-ray diffraction. The amorphous character seen in the diffractograms obtained from the materials and the displacement of the IR wavenumber are indicative of a polycondensation reaction. The representative characterization of the microstructure of different samples using electron microscopy highlights the nonreactivity of iron exhibited during consolidation. In general, the contribution of iron oxide seems to be small under the conditions described here

Produtos de hidratação em argamassas geopoliméricas à base de argila da Tunísia para reparação de estruturas de concreto

A reparação de estruturas degradadas de concreto representa uma oportunidade para a indústria da construção mas também um desafio para a comunidade científica. O desenvolvimento de novas argamassas de reparação constitui por isso uma importante área de investigação. Os geopolímeros são ligantes inovadores alternativos ao cimento Portland pelo que as argamassas à base destes materiais, geopolíméricas, apresentam algumas potencialidades no campo da reparação das estruturas de concreto. O presente artigo apresenta resultados de uma investigação sobre o desenvolvimento de argamassas geopoliméricas à base de uma argila da Tunísia sujeita a tratamento térmico. É incluída uma análise da argila e também dos produtos de hidratação da argamassa os quais apresentam fases geopoliméricas típicas

Effets des paramètres de cuisson sur la distribution des alcalis et sur l'hydratation des ciments

Dans les fours de cimenterie, on cherche généralement à fabriquer le clinker du ciment Portland en atmosphère oxydante pour une question de qualité. Toutefois, il est souhaitable, qu'à la sortie du four, que l'oxygène en excès soit le plus faible possible pour optimiser l'utilisation du combustible dont le coût croît sans cesse. Dans le travail que nous présentons, nous avons étudié, d'une part, l'effet de la nature de l'atmosphère du four sur la microstructure des clinkers produits et surtout sur la distribution des alcalis et du soufre dans les différentes phases du clinker en jouant sur la position du brûleur à l'intérieur du four. D'autre part, nous avons essayé de corréler la microstructure des clinkers obtenus avec le comportement rhéologique et les propriétés mécaniques des ciments correspondants. En effet, en atmosphère oxydante, l'enfoncement du brûleur dans le four favorise l'élimination du soufre avec les gaz et les poussières, par conséquent, le clinker correspondant contient moins de sulfates alcalins par suite du déplacement du cycle des volatiles; par contre, les alcalis se combinent aux phases du clinker pour stabiliser la variété orthorhombique du C₃A et la forme α' du C₂S. En atmosphère réductrice, les sulfates alcalins se décomposent pour libérer de l'oxygène, des alcalis libres et du S0₂. Le S0₂ s'échappe du four si bien que les alcalis se trouvent en excès par rapport au soufre contenu dans le clinker. Une partie de ces alcalis se combine alors à la bélite pour stabiliser la forme a du C₂S, l'autre partie reste sous forme d'alcalis libres. L'hydratation des clinkers cuits en atmosphère oxydante a été normale quoiqu'un peu plus rapide dans le cas du clinker le plus riche en sulfates alcalins. Par contre, l'hydratation du clinker produit en atmosphère réductrice a été très rapide par suite de la production de la syngénite qui se forme par la réaction entre alcalis libres qui sont très rapidement mis en solution et le gypse. Le C₃A ainsi que le C₄AF qui a un rapport A/F élevé, ne trouvent pas d'ions S0₄²⁻ s'hydratent sous forme d'hydrogrenats. La morphologie de ces derniers et de la syngénite sont responsables du mauvais comportement rhéologique des ciments fabriqués à partir du clinker cuit en atmosphère réductrice. La cinétique d'hydratation très rapide du ciment fabriqué avec du clinker réduit explique les bonnes propriétés mécaniques des mortiers à jeune âge, en outre la présence de la variété α du C₂S est responsable des bonnes résistances à la compression à long terme comparativement aux ciments fabriqués avec des clinkers cuits en milieu oxydant

Investigation of Dealumination in Phosphate-Based Geopolymer Formation Process: Factor Screening and Optimization

In this study, phosphate-based geopolymers obtained from two aluminosilicate precursors with different mineralogical compositions were investigated. The used experimental methods were X-ray fluorescence, X-ray diffraction, atomic absorption measurements and experimental designs. Nine factors influencing the dealumination process during geopolymer formation were screened based on a Plackett–Burman design. The results show that the control factors are the P/Al molar ratio, curing temperature and curing time. These significant parameters were selected for further optimization using a central composite design. It was found that regardless of the used P/Al molar ratio, curing temperature and curing time increases generate an increase in the response. However, the P/Al molar ratio variation strongly affects Al removal only at low curing temperatures (between 22 and 50 °C) and with short curing times (between 0.2 and 3 h). The curing time parameter is the most significant factor. In addition, the same percentage of liberated Al can be achieved either by increasing the curing temperature in the earliest steps of geopolymer formation or by prolonging the curing time, even at low curing temperatures. Finally, the optimal conditions allowing maximum aluminum release are P/Al molar ratio = 2.0, curing temperature ≈ 70 °C and curing time = 4.76 h

Investigation of Dealumination in Phosphate-Based Geopolymer Formation Process: Factor Screening and Optimization

In this study, phosphate-based geopolymers obtained from two aluminosilicate precursors with different mineralogical compositions were investigated. The used experimental methods were X-ray fluorescence, X-ray diffraction, atomic absorption measurements and experimental designs. Nine factors influencing the dealumination process during geopolymer formation were screened based on a Plackett–Burman design. The results show that the control factors are the P/Al molar ratio, curing temperature and curing time. These significant parameters were selected for further optimization using a central composite design. It was found that regardless of the used P/Al molar ratio, curing temperature and curing time increases generate an increase in the response. However, the P/Al molar ratio variation strongly affects Al removal only at low curing temperatures (between 22 and 50 °C) and with short curing times (between 0.2 and 3 h). The curing time parameter is the most significant factor. In addition, the same percentage of liberated Al can be achieved either by increasing the curing temperature in the earliest steps of geopolymer formation or by prolonging the curing time, even at low curing temperatures. Finally, the optimal conditions allowing maximum aluminum release are P/Al molar ratio = 2.0, curing temperature ≈ 70 °C and curing time = 4.76 h

Effect of the reactivity of the alkaline solution and the metakaolin on the geopolymer formation

International audienc

Synthesis and characterization of cement clinker based on Tunisian dam sediments

International audienc

Synthesis and Characterization of Wood Rigid Polyurethane Composites

Incorporating biodegradable reinforcement, such as wood particles, into rigid polyurethane foams (RPUFs) is among the alternatives to reduce their environmental impact. This study aims to assess the effect of different wood particles as reinforcement in RPUFs. Reinforced rigid polyurethane foams are synthesized with milled wood particles of various forms and sizes and commercial polyol and isocyanate. The effect of fiber treatments and mechanical stirring on foams’ properties is also studied. Additional tests on polyisocyanurate foams (PIR) were undertaken to assess the effect of reinforcement on their properties. Mechanical properties are measured to investigate the impact of wood particle reinforcement on the foam. Confocal microscopy and Fourier-transform infrared spectroscopy (FTIR) showed the interaction between the wood fibers and the matrix. Despite the adhesion observed for some fibers, most of the cell walls of RPUFs were punctured by the rigid wood fibers, which explained the decrease in the compressive strength of the composites for manually mixed foams. Mechanical stirring proved to be an efficient method to enhance the reinforcement power of untreated fibers. RPUF foams’ properties showed similar changes when untreated wood flour was introduced to the formula, increasing compressive strength significantly

effect of the reactivity of alkaline solution and metakaolin on geopolymer formation

International audienc

Defining existence domains in geopolymers through their physicochemical properties

International audienceGeopolymeric material is a new type of cement that is synthesized through the alkali activation of aluminosilicates. To improve the properties of the aluminosilicates, the composition is greatly modified. However, through numerous possible synthetic routes to form geopolymers, the determination that the final material has a geopolymer network cannot be ensured, despite the apparent or partial consolidation of material. This work proposes a definition that a domain in the geopolymeric material exists that can be correlated using different techniques to quantify the amount of silicon, potassium and aluminum. Two sources of potassium silicate and various clays with different degrees of amorphous character were compared to determine the importance of the starting materials. The existence domain is an important tool to determine if the synthesized material is a geopolymer network independent of the raw materials used