A case history of a severe degradation of concrete foundation plinths and piers of an about-35-year-old building located in Northern Italy is described. Significant amounts of gypsum, near ettringite ...and/or thaumasite were detected by X-ray diffraction analyses performed on ground concrete samples. Large gypsum crystals were mainly located at the interface between the cement paste and aggregates, as observed by scanning electron microscopy coupled to energy-dispersive spectroscopy microanalysis. The degradation effects increased with decreasing the distance of concrete structures from an absorbing well located in the courtyard of the building. The well was recognized as the sulfate source due to the microorganism metabolism of sulfur compounds present in the sewage. Consequences of this attack were a very poor bond strength between cement paste and aggregates and a severe cracking of the concrete cover of the steel reinforcement.
The sinterability of two industrial mullite powders, in the presence of MgO as a sintering aid, was investigated. A glassy phase, which was generated during preparation, was present in both powders; ...this glassy phase had a strong influence on sintering, depending on its content, composition, and spatial distribution. MgO promoted sintering in the presence of a liquid phase, both in the as‐received materials and in samples washed with HF, in which most of the pre‐existing glassy phase was eliminated. Investigations using transmission electron microscopy, coupled with energy‐dispersive spectroscopy, as well as dilatometric measurements and X‐ray diffraction data, on washed and unwashed materials and on quenched and slow‐cooled samples allowed a better understanding of the influence of MgO and the glassy phase on the sintering behavior and the formation of new phases. Most of the phases, in fact, can be explained by using the MgO–Al2O3–SiO2 phase diagram, even in such complex systems.
Recent progress in the development of a beta-alumina gas sensor for automotive applications is reported. The sensing device consists of two solid electrolytes (namely a thin film of sodium sulfate ...deposited on beta-alumina by appropriate treatment in a gaseous atmosphere), with two electrodes having different catalytic properties, one made of platinum, the other of gold. The beta-alumina component was prepared as sintered pellets by pressing and as thick films by screen-printing. The electrical response of these materials at different temperatures in the range 300° to 800°C and under various gases (CO, NO, NOx) with dilution in pure air in the range 5 ppm to 5 vol% was investigated and studied as a function of the preparation technique. The dense ceramic and the thick film designs behave similarly. A sensor prototype based on the thick film design was then developed and tested. All the experimental results prove that the sensor can be successfully used for selective detection of CO and NOx. The selectivity results from an appropriate choice of the working temperature of the sensing element. At low temperatures (300 to 400°C) NO2 can be selectively detected in the presence of CO; the opposite happens in the higher temperature range (550 to 650°C).
The possible chemical interactions which occur between sodium (as sulphite), iron and cerium (as oxides) coming from pollutants or additives, and the materials which make up three commercial filters, ...in the working temperature of a Diesel soot filter, were considered. The commercial filters were respectively a cordierite monolith filter, a wound-fiber filter made by continuous AI₂O₃-B₂O₃-SiO₂ fibers, and a sintered Chrome-Nickel stainless steel filter plate. Some evidence of the chemical interactions was detected coupling X-ray diffraction (XRD) and Hg porosimetry analyses, scanning electron microscopy (SEM) observations and microprobe investigations. The extent of chemical etching, the involved reaction mechanisms and the possible effects on durability, mechanical performances and filtration efficiency of the filters were also investigated. From the experimental results it was concluded that sodium leads to a diffuse etching of the ceramic filters starting from low working temperatures, and in the case of the metallic filter it induces a negative modification of its porosity. Cerium oxide does not react with the three considered filters: the only interaction consists in a positive effect on the porosity of the metallic filter. Also iron oxide does not give rise to any significant interaction with the three filters.
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En vue d'utiliser tes roches calcaires comme granulats pour la fabrication des bétons de ciment alumineux, tes auteurs ont étudié l'interaction, en milieu aqueux, entre l'aluminate monocalcique ...CaAl₂O₄ et le carbonate de calcium CaCO₃, en utilisant la calorimétrie isotherme et 1e couplage calorimétrie˗conductimétrie. Les échéances de formation des hydrates (aluminates et carboaluminate) sont nettement plus courtes que dans le cas de l'hydratation de l'aluminate monocalcique seul et dépendent des caractéristiques physiques des carbonates.
In anticipation of the use of calcareous rocks as aggregates for the production of aluminous concretes, the authors studied the interaction occurring, in aqueous medium, between monocalcium aluminate CaAl₂O₄ and calcium carbonate CaCO₃, using most of all the isothermal calorimetry and conductivity directly connected to calorimetry. The times for formation of hydrates (aluminates and/or carboaluminate) are definitely shorter than for hydration of pure CA and in relation to the physical characteristics of carbonates.
En vue d'utiliser tes roches calcaires comme granulats pour la fabrication des bétons de ciment alumineux, tes auteurs ont étudié l'interaction, en milieu aqueux, entre l'aluminate monocalcique ...CaAl₂O₄ et le carbonate de calcium CaCO₃, en utilisant la calorimétrie isotherme et 1e couplage calorimétrie˗conductimétrie. Les échéances de formation des hydrates (aluminates et carboaluminate) sont nettement plus courtes que dans le cas de l'hydratation de l'aluminate monocalcique seul et dépendent des caractéristiques physiques des carbonates.
In anticipation of the use of calcareous rocks as aggregates for the production of aluminous concretes, the authors studied the interaction occurring, in aqueous medium, between monocalcium aluminate CaAl₂O₄ and calcium carbonate CaCO₃, using most of all the isothermal calorimetry and conductivity directly connected to calorimetry. The times for formation of hydrates (aluminates and/or carboaluminate) are definitely shorter than for hydration of pure CA and in relation to the physical characteristics of carbonates.
Negro Alfredo, Bachiorrini Alessandro, Murat Michel. Interaction, in aqueous medium, between calcium carbonate and monocalcium aluminate at 5 °C, 20 °C and 40 °C. In: Bulletin de Minéralogie, volume 105, 3, 1982. pp. 284-290.
