Recycling of industrial wastes in ceramic manufacturing: State of art and glass case studies

Nowadays, ceramic tile are manufactured at zero emissions permitting to recycle all by-products and part of residues derived from depuration treatments (exhausted lime, glazing sludge and polishing sludge). In addition to this environmentally friendly tendency, in the last years an increasing number of scientific studies demonstrated the feasibility to use alternative raw materials in substitution of different component of the ternary clay-feldspar-quartz system. In the first part of the paper is reported the state of the art of industrial waste recycling in the ceramic sector, with the focus on review studies related to both ceramic tiles and bricks..In the second part of the work are reported two case studies conducted by the authors with the aim to formulate ceramic bodies using alternative raw materials. New tailored compositions were obtained replacing clays, flux and/or inert compounds (higher than 60 wt%) by scraps from packaging waste glass in tiles, and cathode ray tube glasses and packaging waste glass up to 20 wt% in the brick compositions.     

Synthesis and characterization of porous ceramics from spodumene tailings and waste glass wool

Glass wool waste remains a challenging waste fraction with relatively little utilization prospects. The present study investigated the development of porous ceramic materials from glass wool waste and spodumene tailings mainly made of quartz feldspar sand (QFS), with 0.05–0.5% silica carbide (SiC) as a pore-forming agent. The formulated compositions were sintered at 950 °C and analyzed in terms of mechanical properties, phase composition, and microstructure using X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray micro-computed tomography. The results showed that a synergetic effect of glass wool and SiC started to be significant from 15 wt% glass wool and 0.05 wt% SiC, the strength reducing and the porosity increasing with the increase of SiC. The porous ceramics were largely amorphous, with compressive strength ranging from 5 to 30 MPa while the water absorption and apparent density ranged from 2 to 10% and 0.7–1.2 g/cm³, respectively. The total porosity varied between 20 and 75%, and the wall thickness between 62 and 68 μm; besides, most of the prepared materials floated in water. These results are of interest for the repurposing of glass wool waste in the development of non-flammable lightweight materials for potential filtering or high-rise building applications.     

Preparation of lightweight hollow glass microsphere ceramics by gel casting

Owing to low density, hollow glass microspheres (HGMs) float on common media such as water, ethanol, and tertiary butanol. As a result, HGMs are not suitable to prepare slurries and ceramics by gel, slip, and freeze castings. In this paper, polyacrylamide (PAM) was used as dispersant and thickener agent to prepare homogeneous HGM aqueous slurries with controlled solid loadings and subsequent lightweight HGM porous ceramics were prepared by gel casting. Effects of PAM content on the stability of aqueous slurries as well as effects of solid (i.e., HGM) loading on density, porosity, pore size distribution, and compressive strength of porous ceramics were investigated. Increasing the viscosity of the slurry resulted in HGMs with significantly lower floating rates and more stable HGM aqueous slurries. HGM porous ceramics with densities and compressive strengths of 0.127–0.219?g/cm³ and 0.74–1.71?MPa, respectively, were prepared by gel casting.     

碎粒烧结法玻璃陶瓷的气孔问题探讨

认为碎粒烧结法玻璃陶瓷的气孔率主要由玻璃液的澄清、粒度级配、杂质、烧成制度和化学组成决定。采用严格合理的澄清制度,以酸洗等方式剔除杂质或予以掺杂,建立适宜的烧结模型和碎粒紧密堆积所要求的粒度级配,优化配方,加人一定的Na2O等助熔剂,可显著减轻玻璃陶瓷的气孔率。     

Sinterability,microstructure and compressive strength of porous glass-ceramics from metallurgical silicon slag and waste glass

Porous glass-ceramics have been prepared by the direct sintering of powder mixtures of metallurgical silicon slag and waste glass. The thermal behavior of silicon slag was examined by differential thermal analysis and thermogravimetry to clarify the foaming mechanism of porous glass-ceramics. The mass loss of silicon slag below 700 °C was attributed to the oxidation of amorphous carbon from residual metallurgical coke in the silicon slag, and the mass gain above 800 °C to the passive oxidation of silicon carbide. The porosity of sintered glass-ceramics was characterized in terms of the apparent density and pore size. By simply adjusting the content of waste glass and sintering parameters (i.e. temperature, time and heating rate), the apparent density changed from 0.4 g/cm³ to 0.5 g/cm³, and the pore size from 0.7 mm to 1.4 mm. In addition to the existing crystalline phases in the silicon slag, the gehlenite phase appeared in the sintered glass-ceramics. The compressive strength of porous glass-ceramics firstly increased and then decreased with the sintering temperature, reaching a maximal value of 1.8 MPa at 750 °C. The mechanical strength was primarily influenced by the crystallinity of glass-ceramics and the interfaces between the crystalline phases and the glassy matrix. These sintered porous glass-ceramics exhibit superior properties such as light-weight, heat-insulation and sound-absorption, and could found their potential applications in the construction decoration.     

Effect of composition and sintering process on mechanical properties of glass ceramics from solid waste

Sintered diopside glass ceramics were successfully prepared from mixtures of blast furnace slag, fly ash and mining tailing. Results showed that sample C2 with relatively low iron oxides and mass ratio of CaO/SiO₂ possessed the highest bending strength value among samples. A low content of iron oxide enhanced densification degree because pores were developed by reduction of ferric oxide into ferrous oxide. Moreover, a low CaO/SiO₂ mass ratio also greatly promoted the densification process by prolonging the sintering time and delaying the crystallisation. In addition, sample C1 developed by one-stage sintering had a worse mechanical performance than that obtained by two-stage sintering although they had the same crystals. For all samples, despite of different compositions and sintering processes, the main crystal phases are augite and diopside ferrian.     

Elaboration and characterization of porous granules based on waste glass

The evolutions of the world today and the population increase have resulted in pollution and human wastes, which contaminate the environment and adversely affect the human's quality of life.All forms of food and drink packages, made of paper, plastic foam, aluminum and glass, are designed for the consumers' convenience. After being used, these packages are discarded as garbage and have today become a social problem.It is in this context that our work for obtaining a building material (foam glass granules) that is much lighter is registered with the properties of heat insulation and acoustic improved based on cullet in order to recycle it and for improving the present laws about the waste products in closed circuit (finished products → waste products → finished products).Investigations have shown that grinding waste glass to particle size less than 0.1 mm and adding 1% of Ca CO₃ content provide the production of material with the following properties: particle density of 0.5 g/cm³, strength of 17.50 MPa and water adsorption of 95%, with the temperature for foaming ranges determined at 850 °C. The microstructures are homogenous, with pore sizes of up to 2 mm. The foam glass is counted among the new glass products meeting certain requirements on comfort, in particular in the building industry (thermal and acoustic insulation). The product obtained presents excellent thermal (λ = 0,031 W/m°C) and acoustic (R = 15 dB) properties.     

Amorphous silica wastes for reusing in highly porous ceramics

The paper analyzes a solution in green manufacturing of foamed or cellular ceramics. The objective of this study was to determine the technical solution for rice husk ash and “tales” of mixed glass cullet reusing based on the specific properties of these materials for creation of spherical holes inside ceramic using the process of coalescence of cellular glass. The paper reports on experimental results obtained from the production of lightweight cellular glass granules produced using glass cullet and rice husk ash. Lightweight cellular glass granules were mixed with clay, pressed and fired in air at 920°C. Clay sintering and the formation of ceramic were followed with the coalescence of cellular structure of glass granules and with the formation of spherical hollows inside the matrix. Density and strength of the fired ceramic bodies were determined. It is observed that the lightweight ceramics with density 900 ÷ 920 kg/m³ possess a compressive strength of about 5 MPa that is acceptable for bricks or tiles manufacture. The utilization of amorphous silica waste for lightweight ceramics manufacture helps in reducing waste disposal concerns and costs associated, and also transforms the waste into an alternative raw material with added value, moreover making the final product cheap.     

Recycling photovoltaic silicon waste for fabricating porous mullite ceramics by low-temperature reaction sintering

In this study, porous mullite ceramics with coral-like structures were fabricated at a low temperature of 900 °C by using photovoltaic silicon waste (PSW) as the silicon source directly. The effects of additive content and sintering temperature on the mullitization reaction of green bodies were studied. The results showed that ammonium molybdate tetrahydrate molybdenum (H₂₄Mo₇N₆O₂₄·4H₂O) as an additive could reduce the reaction temperature for mullitization from 1100 °C to 900 °C. The research on the influence of catalyst on material properties showed that porous mullite ceramics with a flexural strength of 52.83 MPa, a 41.78 % porosity, a sintering expansion rate of 0.49 % and an average pore size of 0.23 μm could be fabricated by introducing 7.5 % H₂₄Mo₇N₆O₂₄·4H₂O at the sintering temperature of 1000 °C. This study develops an environment-friendly recycling method of PSW and provides a new idea for the low-cost preparation of porous mullite ceramics with high purity.     

Preparation and characterization of tubular porous ceramics from natural zeolite

Tubular porous ceramics with more than 40% open porosity and about 6 μm mean pore size were fabricated from natural zeolite powder with starch as pore-forming agent. The optimized processing parameter was 1100–1150 °C with a holding time of 1 h. Permeability of nitrogen and water flux of the tubular specimens was measured and discussed, and the obtained optimized values are 2480 m³m⁻² h⁻¹bar⁻¹ and 26 m³m⁻² h⁻¹ bar⁻¹, respectively.     

Recycling of waste red mud for fabrication of SiC/mullite composite porous ceramics

SiC/mullite composite porous ceramics were fabricated from recycled solid red mud (RM) waste. The porous ceramics were formed using a graphite pore forming agent, RM, Al(OH)₃ and SiC in the presence of catalysts. The influence of firing temperature and the pore-forming agent content on the mechanical performance, porosity and the microstructure of the porous SiC ceramics were investigated. Optimal preparation condition were determined by some testing. The results indicated that the flexural strength of specimens increased as a function of firing temperature and a reduction in graphite content, which concomitantly decreased porosity. The ceramic prepared under optimal conditions having 15?wt% graphite and sintered at 1350?°C, demonstrated excellent performance. Under optimal preparation conditions the flexural strength and porosity of the ceramic were 49.4?MPa and 31.4%, respectively. Scanning electron microscopy observation result showed that rod-shape mullite grains endowed the samples with high flexural strength and porosity. X-ray diffraction analysis indicated that the main crystallization phases of the porous ceramics were 6H-SiC, mullite, cristobalite and alumina. This work demonstrates that RM can be sucessfully reused as a new raw material for SiC/mullite composite porous ceramics.     

Processing and properties of silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity

Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity were prepared by sintering nano-SiC powder-carbon black template compacts at 600–1200 °C for 2 h in air. The microstructure of the silica-bonded porous nano-SiC ceramics consisted of SiC core/silica shell particles, a silica bonding phase, and hierarchical (meso/macro) pores. The porosity and thermal conductivity of the silica-bonded porous nano-SiC ceramics can be controlled in the ranges of 8.5–70.2 % and 0.057–2.575 Wm⁻¹ K⁻¹, respectively, by adjusting both, the sintering temperature and template content. Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity (0.057 Wm⁻¹ K⁻¹) were developed at a very low processing temperature (600 °C). The typical porosity, average pore size, compressive strength, and specific compressive strength of the porous nano-SiC ceramics were ∼70 %, 50 nm, 2.5 MPa, and 2.7 MPa·cm³/g, respectively. The silica-bonded porous nano-SiC ceramics were thermally stable up to 1000 °C in both air and argon atmospheres.     

Low-temperature preparation and microwave dielectric properties of ZBS glass–Al2O3 composites

The preparation and microwave dielectric properties of ZnAl₂O₄-based glass–ceramic composites were investigated. Using zinc borosilicate (ZBS) glass and Al₂O₃, glass–ZnAl₂O₄ composites with high quality factor was successfully prepared at temperatures below 950 °C. The linear shrinkage for 50 vol% ZBS glass–ZnAl₂O₄ composite showed a steep increase up to 650 °C and a plateau between 700 °C and 950 °C, implying that one-stage densification process occurred. The crystallization of ZnAl₂O₄ was observed above 700 °C and an insufficient densification occurred due to the consumption of the glass. As the sintering temperature increased, the quality factor (Q × f₀) showed an increase with an S-type curve whereas the dielectric constant was almost constant. The formation of ZnAl₂O₄ might correspond to the increase of Q × f₀; a high value of 17,757 GHz (1415 at 12.6 GHz) was obtained for the specimen sintered at 900 °C.     

Recycling of CRT panel glass as fluxing agent in the porcelain stoneware tile production

In the present work, the feasibility to substitute feldspar raw material in a porcelain stoneware body with Panel Cathode Ray Tube (CRT) glass was investigated. A standard batch and a composition, where 35 wt.% Na-feldspar was substituted by CRT glass, were sintered at different temperatures in the range of 1000–1250 °C. The degree of the densification was studied by evaluation of the closed and total porosity, while the sintering rate was estimated by non-isothermal dilatometric measures. The variation of the crystalline phase composition was evaluated by XRD analysis. From the preliminary study other ceramic samples with different percentages of CRT glass (i.e. 2.5, 5 and 10 wt.%) were prepared and fired in industrial kiln. The sintering parameters, the microstructure and the mechanical properties were measured and compared with the standard composition.     

Polymer-derived porous ceramics from novel silicon-based preceramic/sucrose systems

Porous Si/C/O/(N) ceramic bodies were developed by the direct consolidation of novel liquid silicon-based preceramic polymer/porogen (methacryloxypropyl silsesquioxane/sucrose) systems, burnout, and N₂ pyrolysis (1300–1500 °C), and they were characterized via open porosity, volumetric shrinkage, and mass loss measurements. The evolution of phases as temperature increased was analyzed using XRD, TGA-mass spectrometry tests, and ²⁹Si NMR. The free carbon phase was characterized via Raman spectroscopy, and its content was determined using a carbon analyzer. Porous microstructures were analyzed by SEM/EDS and Hg-porosimetry, and by measuring the N₂ adsorption/desorption and specific surface area. The final ceramics exhibited a hierarchical porosity composed of large irregular pores that grew with temperature, together with a lower volume of meso- and micropores. β-SiC whiskers and faceted hexagonal crystals of α-Si₃N₄ were observed inside of cavities. The presence of meso- and micropores explained the high specific surface area achieved in the material pyrolyzed at 1500 °C.     

工业废料在耐火材料中的应用现状和进展

简单介绍了工业废料的现状,重点阐述了废料在耐火材料中的研究与应用现状,以及废弃料合成耐火原料的方法、再利用途径和发展前景。     

Bioactive fluorapatite-containing glass ceramics

Fluorapatite-containing glass ceramics were synthesized on the basis of the glass-forming system SiO₂–Al₂O₃–P₂O₅–CaO–CaF₂. The introduction of phosphorus and fluorine containing materials, as well as the specific regime of heat treatment of the glasses gave glass ceramic materials with crystalline phases of the apatite group—fluorapatite (Ca₁₀(PO₄)₆F₂), apatite (Ca₃(PO₄)₂), vitlokite (Ca₉P₆O₂₄), etc. The X-ray phase analysis showed that the main phase in all the glass ceramic samples was fluorapatite. The phase composition, structure and some of the basic properties of the glass ceramic samples were determined.     

Non-isothermal crystallisation behaviour of fluorophlogopite/nepheline glass ceramics

Abstract

Fluorophlogopite/nepheline glass ceramics were formed from the system of SiO₂–Al₂O₃–MgO–K₂O–Na₂O–F, and the thermodynamic, crystallisation behaviour and microstructure were investigated using differential thermal analysis, X-ray diffraction, and scanning electron microscopy. It was found that fluorophlogopite crystals and nepheline crystals crystallised simultaneously, and bulk nucleation was the main crystallisation mechanism. Crystal growth was prone to follow the two-dimensional direction and controlled by diffusion. Activation energy for glass transition was 797·43 kJ mol^?1, and crystallisation activation energy was 433·16 kJ mol^?1.     

Microporous glass ceramics from combination of silicate,borate and phosphate wastes

Abstract

Borate mineral wastes and phosphate ash resulting from the incineration of meat and bone meal represent two particularly abundant inorganic wastes. This paper is dedicated to the combination of such wastes, together with kaolin clay, focused on the development of highly porous ceramic bodies. Borate waste has a multiple effect, providing liquid phase at the sintering temperature (1050°C), gas release (from the decomposition of its calcite fraction) and a CaO source, which reacts with residues from clay and promotes the formation of anorthite crystals as a newly formed phase. Control of the heating rate, i.e. adoption of fast heating (20°C min^?1) and, above all, introduction of recycled soda–lime–silica glass as secondary additive, allowed obtaining lightweight microporous bodies (density below 0·45 g cm^?3) with uniform pore structure that could be useful for thermal and acoustic insulations.     

Mechanical strength and damage tolerance of highly porous alumina ceramics produced from sintered particle stabilized foams

Highly porous alumina particle stabilized foams were prepared by combining the concepts of particle stabilized foams and gelcasting, using sulfonate surfactants and poly vinyl alcohol (PVA) as the gelcasting polymer. The ceramic samples sintered at 1500 °C for 2 h had porosities from 65% to 93%, with pore sizes in two categories: “big pore” around 300 μm and “small pore”, around 100–150 μm, depending on the type and amount of surfactant added. The mechanical behaviour of the foams (axial and diametral compression) depended on the overall porosity and pore size. On average, tensile and compressive strengths around 5 and 16 MPa respectively were measured for samples with bigger pore sizes and larger porosities. Samples with smaller pore sizes and lower porosities produced average values of 12 and 57 MPa for tensile and compressive strengths, respectively. The elastic modulus reached a maximum around 3GPa for “small pore” size samples. The effect of increasing amount of PVA in the samples had a strong effect on the green mechanical strength, but it did not significantly affect the mechanical response of the sintered alumina foams. Large and complex shape sintered components produced using this route showed a remarkable damage tolerance, due to crack tip blunting.