A. Negro

Sulfate attack of concrete building foundations induced by sewage waters

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.

Interaction between some oxides and cordierite

Abstract Today, cordierite plays a very important role in the reduction of noxious substances emitted from internal combustion engines. Honeycomb structures with parallel channels offer a good solution as supports for catalysts at an acceptable counterpressure. Porous ceramic materials permit an efficient mechanical filtration of carbonaceous particles contained in the exhaust gases of diesel engines, which are forced to flow through the walls. Interest in these components is increasing due to their potential effectiveness; it is therefore important to study the problems sometimes encountered during actual applications. This research considers the interaction between some oxides encountered in practical use and the material itself in order to understand the behaviour of cordierite as a basis for better utilisation; some observations and comments are reported.

Lithium metazirconate for nuclear application: physical and mechanical properties

Lithium metazirconate is a candidate in fusion reactor designs as a tritium-breeding material. An amorphous powder was synthesized by gelling zirconium propylate and lithium acetate: after heat treatment at 900 °C, pure Li2ZrO3-l was obtained. Preliminary tests made on sintered pellets, obtained from this powder, demonstrated an appropriate tritium release at low temperature, when the sample porosity was about 20% and the mean pore radius 3 μm. The subject of this paper was to realize a good compromise between mechanical properties and the spread porosity, necessary for tritium diffusion. Dilatometric investigations were performed on compacts of powders treated at 800, 900 and 1000 °C to identify the best temperature and time of sintering. The calcined powders were also uniaxially pressed at 200, 300 and 500 MPa: the green bodies were sintered at 1200, 1250 and 1300 °C for 2 or 4 h, in order to produce ceramics having different porosity. On the sintered bodies, the pore size distribution, the mean pore radius and mechanical properties were evaluated.

Electrochemical sensors for CO/NOx detection in automotive applications

Recent progress in the development of a β-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 β-alumina by appropriate treatment in a gaseous atmosphere), with two electrodes having different catalytic properties, one made of platinum, the other of gold. The β-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).

Action de CaCO3, CaSO4 et CaSO4 · 2H2O sur l'hydratation de C3S

Abstract The accelerating effect of limestone and gypsum fillers on the hydration of C 3 S is shown by calorimetry and X-ray diffraction. The Ca/Si ratios of C-S-H directly measured on polished sections by EDAX are not very different in mixtures with and without calcium sulfate (gypsum or anhydrite). On the contrary they are lower in samples with limestone. It seems possible to substitute 50% gypsum for 50% limestone in mixtures containing 10% mineral additive.

Non-conventional syntheses of beta-alumina powders

Abstract Because of its high ionic conductivity β-alumina is currently used as a solid electrolyte for gas sensors. In order to control the morphological properties and sodium contents of powder used in such applications, it has been produced by two types of synthesis: by thermal denitrification and by the use of a gel. The various β-alumina crystallization mechanisms have been characterized. They illustrate the role played by certain precursors which, when present, are capable of affecting the ratio of β and β″ phases. Electrical conductivity measurements by AC technique at 300–500°C were carried out. Ageing tests confirmed the much higher stability of the gel-prepared products.

Stabilite thermodynamique des phases dans le systeme CaOAl2O3CO2H2O

Abstract The paper presents a theoretical investigation on the nature of thermodynamical stable precipitating phases in the CaOue5f8Al 2 O 3 ue5f8CO 2 ue5f8H 2 O system when monocalcium aluminate (CA) with or without an excess of lime or alumina is mixed with water in equilibrium with gaseous carbon dioxide. Basing on the nature of ions in solution (A10 2 − , Ca ++ , CO 3_ −− , OH − and H + ). and supposing that only C C , AH 3 , CH, CAH 10 , C 2 AH 8 and C 4 ACH 11 are the possible stable phases, the system was mathematically described taking into account the values of solubility products, the equilibrium between atmospheric carbon dioxide and the solution, ionic dissociation constant of water, matter balance of calcium and aluminate ions and condition of the solution electroneutrality. For a given system of initial parameters, computer calculation has allowed to determine the only possible solid-liquid system among the 22 theoretically possible cases (zero, one or two solids in equilibrium with the solution). So, varying the initial parameters has led to draw isobaric and isosteric phase diagramms, what shows that, in the conditions of the study, CAH 10 is the only stable phase at 21°C. The result suggests that the precipitation of C 2 AH 8 and C 4 A C H 11 can only be a consequence of saturations of the solution with respect to CAH 10 .

Durability of cordierite honeycomb structure for automotive emissions control

Abstract Ceramic (cordierite) honeycomb structures are employed in the automotive industry for trapping and incineration of particulate material from the exhaust gases of diesel-powered cars. The most diffuse pollutants present in the particulates are sodium, lead, iron, calcium, zinc and vanadium: they can react with cordierite during regeneration of the filter. In this paper, the short-term interaction of these pollutants as oxides with cordierite filter was studied to examine the consequences on the ceramic characteristics, using differential dilatometry, coupled to X-ray diffraction (XRD) and scanning electron microscopy (SEM). These investigations demonstrate that pollutant ions diffuse into the cordierite lattice, modifying its thermal expansion behaviour, often leading to surface etching and formation of new phases. Some of these new phases present a thermal expansion coefficient that is higher than that of pure cordierite one: different local dimensional variations in the ceramic structure lead to the appearance of microcracks. Sodium and lead oxide are the pollutants that most affect the lifetime of cordierite filters.

Sintering behaviour of gel-derived powders

Abstract85Al2O3-15ZrO2 (wt%) powders were synthesized by gel precipitation starting from AlCl3 · 6H2O and ZrCl4 solutions and dried by two different methods: (i) by spray-drying and (ii) via a sol-gel route by n-octanol in a pilot plant. The particles by process (i) were spherical granules of diameter 15 μm, and those by process (ii) were microspheres of 25 μm. The powders were characterized in terms of morphology, particle size distribution, surface area, weight loss and crystallization behaviour at different temperatures. Shrinkage, microstructure, density and pore size distribution were evaluated on compacts at different temperatures to study the sintering kinetics. Experimental observations suggest that (a) the use of controlled-geometry powders allows one to obtain high green densities; and (b) the sintering of gel-derived powders develops in two steps: during the first one, sintering mainly takes place inside each microsphere (or granule), and during the second step, mainly between the microspheres (or granules). At temperatures > 1100°C, sintering produces shrinkage of the microspheres (or granules) leading to pore formation between them, which prevents the achievement of high densities. Only by using hot pressing is it possible to obtain theoretical densities and high mechanical properties.

Hydratation de l'aluminate monocalcique I — influence des ecarts de composition

Abstract The more favourable experimental conditions of cooling and grinding was defined for calcium aluminates obtained by solid-solid reaction with C/A ratio in the range 0.96 – 1.04. The composition of the samples was related to the apparition of different phases : “substoichiometric” solid including CA and CA 2 , with a calcium deficiency “super-stoichiometric” samples including C 12 A 7 with an excess of calcium as well as mixtures of sub and super-stoichiometric solids could be obtained. “Super-stoichiometric” samples presented ionized anionic vacancies. During hydration, “super-stoichiometric” aluminate led to the formation of a first hydrate. The hydration kinetics depended on the ratio C 12 A 7 /CA 2 of the samples.