Chiara Zanelli

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).

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.

Tetrahedrally coordinated Co2+ in oxides and silicates: Effect of local environment on optical properties

Abstract The Co2+ ion in fourfold coordination provides d-d electronic transitions with the strongest optical density among oxides and silicates. For this reason, it is widely used in pigments and dyes to get blue shades detectable down to a very low cobalt concentration. Such a low-detection limit turns the Co2+ ion into a suitable probe to disclose the local ligand environment in a wide range of materials by means of optical spectroscopy. Even if extensively studied in organometallic complexes, an in-depth investigation of optical properties of Co2+ in tetrahedral coordination into oxidic structures is limited to some case-study in minerals and synthetic analogs (spinel, zincite, gahnite, willemite, calcium cobalt selenite). The present study represents an attempt to outline crystal structural (long-range metal-oxygen distances, O-T-O bond angles, and distortion parameters by XRD) and optical parameters (10Dq, Racah B and C, band splitting by EAS) in 13 samples of oxides and silicates providing a wide set of different local fourfold coordination around Co2+ added as a dopant. Subtle variations of crystal field strength and interelectronic repulsion can be appreciated in gahnite, Ca-Sr-hardystonite, Ca-Sr-Baåkermanite, willemite, Ba2MgSi2O7 melilite-related (where Co2+ substitutes Mg2+ or Zn2+ by 0.25-0.3 apfu) as well as in gehlenite and fresnoite (where Co2+ substitutes Al3+ and Ti4+, respectively, by 0.2 apfu due to charge mismatch). Results are compared with literature data about hibonite, spinel s.s., staurolite, yttrium garnets, and zincite. Spectral interpretation is not straightforward owing to the occurrence of different Co2+ bands: spin-allowed and spin-forbidden electronic transitions, two- or threefold split due to both lowering of point symmetry at the tetrahedron and spin-orbit coupling plus presumably vibronic transitions. Optical spectra vary significantly even for apparently small changes in the long-range CoO4 arrangement as measured by XRD. The expected relationship between 10Dq and the mean Co-O distance is fulfilled, but the accommodation into small AlO4 sites in gehlenite (YAG and hibonite) implies a significant structural relaxation around the Co2+ ion. The threefold splitting of the spin-allowed 4T1(F) and 4T1(P) bands can be related to the angular distortion of the CoO4 tetrahedra. Overall, changes of spectral features of tetrahedrally coordinated Co2+ can be attributed to different local arrangement of ligands with an effect correlated to the second nearest neighbors by the bond valence theory. This was disclosed contrasting 10Dq with the ratio of the observed and ideal bond valence sum for the polyhedra sharing oxygen with the Co-centered tetrahedron.

Influence of Strengthening Components on Industrial Mixture of Porcelain Stoneware Tiles

A typical industrial formulation for porcelain stoneware tiles was modified by adding synthetic mullite, and mullite-precursors, such as kaolin and gibbsite in order to enhance the mechanical properties by increasing the mullite content. The change in the physical properties as well as the technological and mechanical behaviour were evaluated on the basis of the phase composition modifications. Introduction Porcelain stoneware tiles are primarily composed of clays, feldspars and quartz, heat-treated to form a mixture of glass and crystalline phases (new formed and residual ones), which are characterized by high mechanical, chemical and tribological properties, besides excellent aesthetic appearance. The current trend of producing ceramic tiles having larger and larger formats, for ever more severe applications, require to further enhance their mechanical behaviour, in terms of both flexural strength and fracture toughness. Many theories were proposed to explain the strength of porcelain ware, mainly based on the body mullite content [1,2], the existence of compressive or tensile stresses, caused by the thermal expansion coefficient differences [3], or the presence of dispersed strengthening particles [4,5]. However, only few papers in the literature deal with the effectiveness of these hyphotesis for porcelain stoneware bodies, leaving many opportunities to better understand the role of the microstructure and phase composition on the mechanical strength, fracture mechanisms and crack propagation [6, 7]. The microstructure of porcelain stoneware bodies, in terms of nature and amount of crystalline phases, can be achieved as a result of in-situ chemical transformations or by adding unreactive components as substitutes of the basic raw materials. This paper is focused on the possibility of improving the mechanical performances of porcelain stoneware tiles, through the addition to the starting powders of synthetic mullite and mullite-precursors (kaolin and gibbsite), in order to evaluate their strengthening and toughening role. For this purpose, the microstructural evolution, in terms of physical properties, as well as the mechanical strength and the fracture behaviour of the products were evaluated. Materials and Methods A typical industrial porcelain stoneware body (B) was modified adding mullite (M), kaolin (K) and gibbsite (G) in partial replacement of quartz. The addition amounts were 3-6-12 % for mullite and gibbsite, 5-10-20% for kaolin (Table 1). The production process was simulated at a laboratory scale by wet grinding, pelletization and humidification with 5-6 % water. Tiles (110 x 55 x 5 mm 3 ) were pressed at 40 MPa, dried at 105±5°C and fired in a roller kiln, with an industrial thermal-like cycle of 51 minutes, from cold to cold, at a maximum temperature of 1220°C. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 1491-1494 doi:10.4028/www.scientific.net/KEM.264-268.1491

Structural relaxation around Cr3+ at the Na(Al1-xCrx)P2O7 octahedral site: an XRPD and EAS study

Abstract The isovalent substitution of chromium for aluminium at octahedral site along the Na(Al1-xCrx)P2O7 pyrophosphate solid solution has been investigated by means of the combined application of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In agreement with the structural refinements, deconvolution of the optical spectra revealed a progressive decreasing of the crystal field strength parameter 10Dq moving toward the NaCrP2O7 end-member, meaning that the local chromium–oxygen bond distance increased along the join with the amount of chromium. The calculated structural relaxation coefficient around the substituent ion Cr3+ was ε=0.97 (i.e., very close to that predicted by the Hard Spheres model). A detailed comparison with compact crystal structures (i.e., perovskite, garnet, spinel, clinopyroxene, and corundum) as well as with other pyrophosphate compounds with II-NaMP2O7 structure-type (with M=Al, Cr, Fe, V, Ti, Mo and In) highlighted that compounds belonging to the Na(Al1-xCrx)P2O7 solid solution – characterized by a network connectivity which is basically a framework topology – undergo a structural relaxation that is confined within the first shell (i.e., occurs at the octahedral site), while P2O7 dimers act as rigid units.

The inverse high temperature/high pressure relationship in the monoclinic Ba 2 MgSi 2 O 7 melilite-related structure

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 155-16 - DOI: 10.2451/2011PM0013 Special Issue in memory of Sergio Lucchesi The inverse high temperature/high pressure relationship in the monoclinic Ba 2 MgSi 2 O 7 melilite-related structure Matteo Ardit 1 , Chiara Zanelli 2 , Michele Dondi 2 , and Giuseppe Cruciani 1,* 1 Dipartimento di Scienze della Terra, Universita di Ferrara, Italy 2 Istituto di Scienza e Tecnologia dei Materiali Ceramici (ISTEC - CNR), Faenza, Italy *Corresponding author: cru@unife.it Abstract High-temperature study of the synthetic melilite-related Ba 2 MgSi 2 O 7 (s.g. C2/c) was performed up to 1273 K. Linear thermal expansion coefficients along the unit cell edges and of the volume are α a = 8.7×10 -6 K -1 , α b = 11.0×10 -6 K -1 , α c = 8.5×10 -6 K -1 , and α V = 31.1×10 -6 K -1 , respectively, showing an anisotropic expansion behaviour characterized by α a ≈ α c O 7 reveals that the tetragonal polymorph of the barium compound (Ba 2 MgSi 2 O 7 ) should be a metastable phase favoured by high pressure conditions. Key words : melilite-related structures; melilite-type structures; “inverse relationship”; high-temperature; high-pressure; comparative crystal chemistry.