Michele Dondi

Effect of soda-lime glass on sintering and technological properties of porcelain stoneware tiles

The feasibility of waste glass recycling in ceramic tile production was assessed with special reference to fully vitrified products (porcelain stoneware). Soda-lime float or container glass was introduced, in replacement of sodic feldspar, in typical porcelain stoneware bodies (up to 10 wt.%) that underwent a laboratory simulation of tilemaking process, with a technological and compositional characterization of both fired and unfired tiles. Soda-lime glass had no significant effect on semi-finished products, but it influenced remarkably the firing behaviour, increasing shrinkage and closed porosity, decreasing open porosity and bulk density, and lowering mechanical and tribological performances. Waste glass promotes a more effective melting of quartz and a partial dissolution of mullite, leading to a more abundant and less viscous liquid phase, which accelerates the sintering kinetics. In conclusion, soda-lime glass can be used in small amounts (5% or less) with tolerable modifications of technological behaviour and performances of porcelain stoneware tiles.

Recycling PC and TV waste glass in clay bricks and roof tiles

Disposal of PC monitors and TV sets is a growing problem, with over 40% of the weight of these systems comprised of waste glasses with high Pb (funnel) or Ba-Sr concentrations (panel), making them unsuitable for recycling and manufacturing new glass. A possible way to re-use these glasses is in the manufacturing of clay bricks and roof tiles. This possibility was appraised by laboratory simulation of the brickmaking process and technological characterization of unfired and fired products. The recycling of both funnel and panel glasses into clay bodies is technologically feasible, resulting in a substantially reduced plasticity behaviour during shaping-drying (implying a reduction of mechanical strength), and a promotion of sintering during firing. No significant release of Pb, Ba, and Sr was observed during the firing and leaching test for the carbonate-poor body; in contrast, some Pb volatilization during firing and Sr leaching were observed for the carbonate-rich body. Additions of 2 wt.% appear to be practicable, while 5 wt.% glass induces unacceptable modifications of technological properties. The recommended amount is within 2 and 4 wt.%, depending on the characteristics of the clay bodies. The main constraint is that the glass must have a particle size below the limit of the pan mills used in brickmaking (<1mm).

The influence of shaping and firing technology on ceramic properties of calcareous and non-calcareous illitic–chloritic clays

Abstract Two very different illitic–chloritic clays (calcareous and non-calcareous), both currently used in the Italian brickmaking industry, were used. Technological testing consisted in a simulation of the industrial processing performed at a laboratory scale. The pieces were obtained by three shaping techniques, pressing, extrusion and moulding, and fired at 1000 and 1100 °C, in fast and slow heating cycles. In each case their technological properties were studied. The microstructure and technological parameters of the ceramic pieces vary greatly, depending on the clay composition and the processing techniques. Pressing always produces pieces made from non-calcareous clay with the lowest drying and firing shrinkage, open porosity and water absorption. As it regards the calcareous clay, the lowest values of drying and firing shrinkage and water absorption are obtained by extrusion and moulding, respectively. The mechanical resistance, pore size distribution and critical pore diameter do not reflect clearly the influence of shaping techniques. Independently from the shaping technique adopted, all non-calcareous bodies show higher Maages indexes than the calcareous ones. In any case, the Maages durability factor is higher in extruded samples.

Influence of zeolites on the sintering and technological properties of porcelain stoneware tiles

Low-cost zeolitic rocks are promising substitutes for feldspathic fluxes in ceramic bodies, since their fusibility, modest hardness and high cation exchange capacity (CEC) should improve grinding and sintering. Five large-scale Italian deposits of natural zeolites with different mineralogy were characterised and tested in porcelain stoneware bodies. Their behaviour during processing was appraised and compared with that of zeolite-free bodies. Zeolites increased the slip viscosity during wet grinding, causing a coarser grain size distribution and consequently some drawbacks in both unfired and fired tiles. After overcoming this hindrance by dry grinding of zeolite rocks, the technological behaviour of zeolite-bearing tiles appear to be similar to that of current porcelain stoneware, though with larger firing shrinkage and residual closed porosity.

Clay materials for ceramic tiles from the Sassuolo / District Northern Apennines, Italy . Geology, composition and technological properties

Abstract In the Sassuolo area (northern Italy) there is the largest tilemaking district in the world, which has practically served over the past years as an industrial scale laboratory for assessing the technological properties of clays and their suitability for the production of wall and floor tiles. The local clays are recovered from different geological units and distinguished in two principal types, with clearly differentiated compositions and technological properties: “marly clays” and “red shales”. These local clays, which 20 years ago constituted the sole mineral resource of the Sassuolo District, now supply only 40% of the demand. This trend is largely connected with both process innovation (wet grinding, fast firing, etc.) and product changes (from red to white bodies). Overall, approximately 2 million tpa of clay materials are used in coloured bodies: marly clays for wall tiles (majolica, “birapida”, “monoporosa”) and red shales for both floor tiles (glazed red stoneware) and wall tiles (“monoporosa”).

Composition and ceramic properties of tertiary clays from southern Sardinia (Italy)

Abstract The Sardinian ceramic industry produces structural clay products (common and hollow bricks). The raw materials are obtained from Tertiary continental sediments (Cixerri, Ussana and Samassi Formations) found in the south-western part of the island. These clays have different mineralogical, chemical and grain size compositions and a different technological behaviour in the brick and tile production cycle. The Cixerri clays (Eo-Oligocene) are fine-grained and consist of illite, interstratilied I/S, quartz, kaolinite and high percentages of carbonates (19–24%). Overall, they are suitable for the production of high-quality common bricks, while they could also be part of mixes for roofing tiles and wall tiles. The Ussana clays (Oligo-Miocene) consist of illite-chlorite-kaolinite with high contents of quartz and variable percentages of carbonates (3–20%). In terms of grain size, they can be classified as silty clays and clayey silts. The behaviour in ceramic production is suitable for high-quality bricks, including roofing tiles. In addition, they could be the base for bodies to manufacture porous wall tiles. The Samassi clayey silts are rich in quartz and calcite, with a clay fraction consisting mostly of illite, smectite and chlorite. They are characterised by high dry shrinkage and excessive porosity of the fired samples, for which they are suitable only for mixes for common bricks.

Technological behaviour and recycling potential of spent foundry sands in clay bricks

The feasibility of recycling spent foundry sand in clay bricks was assessed in laboratory, pilot line and industrial trials, using naturally occurring sand as a reference. Raw materials were analyzed by X-ray fluorescence, X-ray diffraction, particle size distribution, and leaching and combined to produce bodies containing up to 35% wt. sand. The extrusion, drying and firing behaviour (plasticity, drying sensitivity, mechanical strength, bulk density, water absorption, and shrinkage) were determined. The microstructure, phase composition, durability and leaching (EN 12457, granular materials, end-life step, European Waste Landfill Directive; NEN 7345, monolithic materials, use-life step, Dutch Building Material Decree) were evaluated for bricks manufactured at optimal firing temperature. These results demonstrate that spent foundry sand can be recycled in clay bricks. There are no relevant technological drawbacks, but the feasibility strongly depends on the properties of the raw materials. Spent foundry sand may be introduced into bricks up to 30% wt. Most of the hazardous elements from the spent foundry sand are inertized during firing and the concentrations of hazardous components in the leachates are below the standard threshold for inert waste category landfill excepting for chromium and lead; however, their environmental risk during their use-life step can be considered negligible.

Orimulsion fly ash in clay bricks—part 2: technological behaviour of clay/ash mixtures

Abstract In order to appraise the technological feasibility of the Orimulsion ash recycling in clay bricks, a laboratory simulation of the brickmaking process was carried out with various clay/ash mixtures up to 6% waste. Two different clays were selected and mixes were characterized by XRF, XRPD, TGA–DTA, TDA and PSD analyses. Plasticity, extrusion and drying behaviour, and mechanical strength, were determined on unfired mixtures, while shrinkage, water absorption, bulk density, modulus of rupture, pore size distribution, microstructure and phase composition were measured on fired bricks. Orimulsion ash caused some detrimental changes of technological properties of both unfired and fired products, concerning particularly plasticity, drying rate and drying sensitivity, porosity and colour. These effects were slightly different on the two raw materials, the carbonate-rich clay being less sensitive to the presence of ash with respect to the carbonate-free clay. In all events, drawbacks appeared to be tolerable, in technological terms, for low waste additions, approximately 1–2% ash.

Green and easily scalable microwave synthesis of noble metal nanosols (Au, Ag, Cu, Pd) usable as catalysts

A green synthesis process was developed for the production of PVP-coated noble metal nanoparticles in the form of stable nanosols. Water is the environmentally benign solvent; glucose serves as a mild, renewable and non-toxic reducing agent and microwave irradiation is an effective and fast heating technique. The same green process has been optimized to obtain several metal nanoparticles (Au, Ag, Cu, Pd), and therefore encourages the easy preparation of bimetallic nanostructures. Nanosols were characterized by dynamic light scattering DLS, HR-TEM, UV-Vis spectroscopy, XRD and ICP-AES. The total reaction yield for all the samples was assessed, the prepared nanoparticles were spherical shaped with an average diameter ranging from 3 to 20 nm. Nanosols with excellent stability over several months, achieved even for high solid contents, were prepared. Additionally, it is shown that all of the synthesized nanoparticles can act as effective catalysts for the reduction of 4-nitrophenol (4-NP) in the presence of NaBH4 (which is otherwise unfeasible without a metal catalyst). This reduction was spectrophotocally followed and the rate constants were determined by measuring the change in absorbance at 400 nm (the wavelength typical of 4-NP) as a function of time. The following ranking of decreasing efficiency of the catalyst was found: Pd > Au > Ag > Cu.

Structural relaxation in tetrahedrally coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution

Abstract The structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1-yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm-1 with increasing cobalt amount, while the Racah B parameter is in the 744-751 cm-1 range (C ∼3375 cm-1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.