Mechanical characteristics of clay structural ceramics containing coal fly ash

In this work, the mechanical characterization of ceramic products processed from red clay with different amounts of added coal fly ash was investigated. Coal fly ash produced by power plants is a waste material that constitutes an alternative source of minerals for the production of traditional building ceramics, as it is a complex mixture of several oxides such as SiO₂, Al₂O₃, CaO, Fe₂O₃, Na₂O, TiO₂, which are usually present in the composition of such ceramics. A powder technology and firing route was followed for the processing of the clay and coal fly ash based ceramics. Different proportions of waste (10, 25 and 50%, by weight) were added to red clay, and then the mixed powders were pressed to form compacts that were fired at a selected temperature in the range 850–1,150°C. The effects of waste content and of heating conditions on the microstructure and mechanical characteristics of the obtained materials were investigated. The density, porosity, water absorption, flexural strength, hardness and fracture toughness of the produced materials were evaluated. A comparison was made between the properties of the produced ceramics with those of traditional ceramic materials used in construction, e.g. floor or wall tiles, and it was observed that the clay based products with coal fly ash additions may be used in similar applications.     

Microwave sintering of zirconia ceramics

The potential of a microwave heating technique for the sintering of 3Y-TZP ceramics is demonstrated. High density samples were obtained by short duration firing in a domestic microwave oven. The ultrafine and monomodal size distribution of grains resulting from the process has important implications in terms of mechanical properties. The hardness and fracture toughness values compare very well with long duration, conventionally fired 3Y-TZP ceramics.     

Reutilization of waste inert glass from the disposal of polluted dredging spoils by the obtainment of ceramic products for tiles applications

The vitrification treatment has been successfully exploited as a solution for the disposal of polluted dredging spoils from the industrial area close to the Venice Lagoon. The addition of 20% by wt. of glass cullet to the calcined sediments in the vitrification batch provides a suitable chemical composition for the production of an inert glass, despite the compositional variations of the sediments. The obtained waste glass, after being finely ground, has been employed (i) as a raw material for the manufacture of sintered glass-ceramics, by cold pressing and single-step sintering at about 940^∘C, and (ii) as sintering additive (the maximum addition being 10% by wt.) for the manufacture of traditional red single firing ceramic tiles, with a maximum firing temperature of 1186^∘C. Both applications have proved to be promising: in the first case, the sintered glass ceramic product exhibits notable mechanical properties (bending strength > 130 MPa, HV ≈ 6.5 GPa); in the second case, the addition of waste glass does not modify substantially the investigated physical and mechanical properties of the traditional product (water absorption, linear shrinkage, bending strength, planarity).     

Process design for the production of a ceramic-like body from recycled waste glass

The influence of fabrication variables on the sintering behaviour of a ceramic-like body containing 90% recycled waste glass was inferred from measurements of some of the fired properties (moduli of rupture and elasticity, firing shrinkage, bulk density and porosity). Interpretation of these results in terms of viscous sintering theory indicates the relative influence on the sintering behaviour of factors such as particle size and distribution, clay binder content and plasticity, pressing pressure, heating/cooling rate, firing temperature and time, thus enabling the fabrication variables to be optimized. Comparison of the physical properties of the resulting glass-based bodies with those of commercial ceramic tile bodies indicates that the glass-based bodies are very comparable with the best ceramic tiles tested, and considerably better than several commercially-produced clay-based bodies.     

Porous supports for conducting ceramic membranes

We describe glass-ceramic materials produced using several kaolins for the fabrication of inert porous supports by a casting process. Optimizing the slurry composition and preparing quality slurries, we obtained a support material which had an open porosity of ?34% after firing at 1380°C. Modification of the support material fired at 850–950°C through leaching, impregnation, and treatment with oxide sols insures effective control over its sintering shrinkage curve, which enables a reduction in the shrinkage mismatch between the support and ceramic layers to an acceptable level. To improve the chemical compatibility between the support material and ceramics, we formulated a number of effective solutions within the proposed approach.     

Sintering mechanism of blast furnace slag–kaolin ceramics

A general ceramics processing scheme by cold uniaxial pressing and conventional sintering process have been used to prepare ceramics from mixtures of blast furnace slag (BFS) and kaolin (10%, 30% and 50% kaolin). The properties of the ceramics were studied by measuring linear shrinkage, bulk density, apparent porosity and mechanical properties of samples heated at temperatures from 800 °C to 1100 °C. The formed crystalline phases were characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM). Slag melt formed at relatively low temperatures (800–900 °C) modified the sintering process to liquid phase sintering mechanism. Combination of BFS with 10% kaolin gave the highest mechanical properties, densification and shrinkage at relatively low firing temperatures. The crystalline phases were identified as gehlenite (Ca₂Al₂SiO₇) in both BFS and BFS with 10% kaolin samples. Anorthite (CaAl₂Si₂O₈) phase increased with increasing kaolin contents. In the case of kaolin-rich mixtures (30% and 50% kaolin), increased expansion took place during firing at temperatures in the range 800–1000 °C. This effect could be attributed to the entrapment of released gases.     

Microstructure effect on the properties of a commercial low-fusing dental porcelain

The aim of this study was to investigate the effect of firing cycle on a dental porcelain microstructure in order to correlate microstructure changes with mechanical and thermal properties. A commercial low-fusing dental porcelain powder (Omega 900, Vita) was investigated for this purpose. The powder was treated at different temperatures in the range 750–1000 °C. The fired samples were characterized in terms of their morphology and microstructure, and their mechanical and thermal properties were evaluated. The results showed that firing temperature affects porcelain microstructure influencing significantly in this way both the mechanical properties and the thermal expansion coefficient of the fired objects. Firing at 800 °C led to a homogeneous structure. After treatment at this temperature, the leucite crystals exhibit their maximum concentration and they are well dispersed into the glassy phase. As a consequence the optimum mechanical strength and the maximum thermal expansion coefficient are observed in these samples.     

Preparation of novel ceramics with high CaO content from steel slag

Steel slag, an industrial waste discharged from steelmaking process, cannot be extensively used in traditional aluminosilicate based ceramics manufacturing for its high content of calcium oxide. In order to efficiently utilize such solid waste, a method of preparing ceramics with high CaO content was put forward. In this paper, steel slag in combination with quartz, talcum, clay and feldspar was converted to a novel ceramic by traditional ceramic process. The sintering mechanism, microstructure and performances were studied by scanning electron microscope (SEM), X-ray diffraction (XRD) techniques, combined experimenting of linear shrinkage, water absorption and flexural strength. The results revealed that all crystal phases in the novel ceramic were pyroxene group minerals, including diopsite ferrian, augite and diopsite. Almost all raw materials including quartz joined the reaction and transformed into pyroxene or glass phase in the sintering process, and different kinds of clays and feldspars had no impact on the final crystal phases. Flexural strength of the ceramic containing 40 wt.% steel slag in raw materials can reach 143 MPa at sintering temperature of 1210 °C and its corresponding water absorption, weight loss, linear shrinkage were 0.02%, 8.8%, 6.0% respectively. Pyroxene group minerals in ceramics would contribute to the excellent physical and mechanical properties.     

Utilization of steel melting electric arc furnace slag for development of vitreous ceramic tiles

Steel melting through electric arc furnace route is gaining popularity due to its many advantages, but generates a new waste, electric arc furnace slag, which is getting accumulated and land/mine filling and road construction are the only utilization. This slag has been tried to be value added and utilized to develop vitreous ceramic tiles. Slag, to the extent of 30–40 wt% with other conventional raw materials, were used for the development in the temperature range 1100–1150°C. The fired products showed relatively higher density with shorter firing range and good strength properties. Microstructural and EDAX studies were also done to evaluate the developed products.     

Influence of Microstructure on The Resistance to Salt Crystallisation Damage in Brick

In the article we study the variation of brick durability and, more specifically, its resistance to salt crystallisation produced by changes in its microstructure during firing. For this purpose, the evolution of both mechanical and pore structure properties are studied within a wide range of temperatures (700–1100^∘C). An increase in the firing temperature produces a more homogeneous and resistant brick, measured using ultrasound velocity and uniaxial compressive strength. This result is obtained thanks to the vitrification process and changes in the brick's pore structure: larger, rounder pores, which are quantified by their roundness and fractal dimension. As a result of these changes, an excellent durability is achieved in the bricks studied when fired at temperatures above 1000^∘C. Considering that few differences are noted in pore structure and brick strength between 1000 and 1100^∘C, the recommended firing temperature is, for raw materials with a similar composition and production process, 1000^∘C, as this involves a lower production cost than firing at 1100^∘C.     

The effect of firing temperature on properties of natural steatite and pyrophyllite

The flexural and compressive strength, Knoop hardness, microstructure, shrinkage, and phase transtormations of naturally occurring pyrophyllite and steatite were studied as a function of firing temperature. Both materials were fired in hydrogen and air, over the temperature ranges of 950 to 1100° C and 1000 to 1150° C for the steatite and pyrophyllite, respectively.After firing the pyrophyllite at 1025° C or higher in hydrogen and 1050°C or higher in air a distinct change was noted in the microstructure which corresponds with significant increases in the strength and hardness. A similar correlation was observed for steatite following firing.The strength and hardness of the hydrogen-fired pyrophyllite was higher than its air-fired counterpart. The opposite result was observed in the steatite.This work was conducted under the partial sponsorship of funds made available through National Aeronautics and Space Administration grants NGR-37-003-026 and NGL-37-003-026.     

Fabrication of hydrothermal ageing resistant of 3 mol % yttria-stabilised tetragonal zirconia polycrystalline via microwave sintering

The influence of microwave sintering on the densification, mechanical performances, microstructure evolution and hydrothermal ageing behaviour of pure 3 mol % yttria-stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramics was compared with conventional sintered samples. Green bodies were sintered via conventional pressure-less and microwave sintering method between 1200 °C to 1400 °C with dwelling time and firing rate at 120 min, 10 °C/min and 1 min, 20 °C/min. Result showed that reduced processing temperature and holding time is possible with microwave sintering technique for fabricating good resistant zirconia sample with bulk density, Young's modulus, and Vicker's hardness that are comparable to samples sintered with conventional method. However, the microwave sintered samples suffered from hydrothermal ageing where their average grain size is above critical size. The enhancement of hydrothermal ageing resistance of the sintered samples is associated with the decreasing grain size of the sintered samples instead of sintering method.     

Using mathematical methods for designing optimal mixtures for building bricks prepared by solid industrial waste

This study evaluates the use of solid industrial waste generated by a water treatment plant (WTP) at a pulp mill in Brazil for manufacturing building bricks. The sludge from the WTP was mixed with three different wastes generated by the same factory (dregs, grits, and lime mud) and other wastes generated from crushing and grinding granite rock (granite fines). Mathematical and statistical methods are proposed for designing mixtures that satisfy the material properties for ceramic processing and the mechanical properties for the end product. A method for solving a linear system of equations using fewer equations than variables was required, as the number of materials exceeded the number of reference grain size distributions. This type of system is generally compatible and indeterminate. To obtain feasible solutions, a combination-of-variables method is proposed to optimize the mixture design. The formulation of the mixtures was based on the grain size distribution of the residues and the proportions of calcium, sodium, and potassium oxides in the mixtures. The nonparametric Kruskal–Wallis test and the median test were applied for all mixtures, followed by a multiple comparison test of classes. The mechanical properties of the specimens were evaluated after drying (linear shrinkage and flexural strength) and firing (ignition loss, linear shrinkage, and flexural strength) according to the Brazilian Association of Technical Standards and Brazilian Standards for red ceramics.     

A comparative study on the use of fly ash and phosphogypsum in the brick production

Over 15 million tons of fly ash (FA) and 3 million tons of phosphogypsum (PG) are produced in Turkey every year. The utilization of these industrial by-product materials is important in terms of environmental and economical issues are concerned. The main purpose of this study is to evaluate the technical possibilities of incorporating FA and PG in production of building blocks. Various mixtures were prepared by incorporating these industrial wastes by replacing clay with seven different weight proportions (0%, 5%, 10%, 15%, 20%, 25% and 30%). All specimens were fired at 1000°C peak temperature. The physical and mechanical properties of all specimens such as; unit weight, compressive strength, flexural strength, dimensional stability and water absorption values were recorded. The effect of PG incorporation on the properties of samples seems to be more dominant than the effect of FA incorporation. The test results showed that; PG incorporation increased the unit weight and mechanical strength values while lowering the water absorption values. Utilization of these wastes additives is not only for conservation of clay resources, but also an alternative solution to a difficult and expensive waste disposal problems.     

Drying shrinkage of expansive cements

Drying shrinkage behaviour of expansive cement pastes were studied and compared with those of portland cements. Results indicate that the shrinkage behaviour of these two cements is significantly different from each other. Generally, expansive cements shrink more than portland cements, and especially more so if they have not been adequately cured, i.e. curing period of at least 7 days is necessary to ensure good performance against shrinkage. Internal damage caused by large amounts of expansion leads to a large magnitude of shrinkage. In that event curing does not seem to have any beneficial impact on shrinkage performance. Steel reinforcement also seems to decrease shrinkage magnitude, but it has no effect on the shrinkage characteristic. Much of the research on expansive cements has so far been focused on the expansion behaviour rather than on the shrinkage behaviour. More research on shrinkage is needed to improve its field performance.     

Effect of quartz sand replacement by agate rejects in triaxial porcelain

The ceramics industry, given the high volume of materials processed, stands as one of the largest consumers of natural raw materials but has also the capacity and potential to make significant contributions to solving environmental problems associated with other industries rejects. This work investigates the effects of quartz sand replacement by agate rejects (scrap) in a traditional triaxial porcelain composition. The study was carried out using the design of experiments (DoE) method. Characterization results were used to calculate statistically significant and valid regression equations, relating dried and fired body properties with clay, feldspar and agate scrap contents in the unfired mixture. The regression models were then discussed against X-ray diffraction and scanning electron microscopy results and used simultaneously to delimit the combinations of those three raw materials most adequate to produce a porcelainized stoneware floor tile with specified properties. Thus, an alternative use of an otherwise waste material is proposed, which can be translated into economic benefits and an important and welcome relief on environmental and waste disposal concerns.     

Compaction pressure effects in the firing of self-vitrifying ceramic materials

The firing behaviour of two self-vitrifying ceramic materials, coming from a magmatic rock of the aplite type consisting basically of quartz, feldspar and kaolin, both in the raw and purified (iron-free) state, were investigated as a function of the compacting pressure over the range 14 to 280 MN m^–2. The materials exhibit a linear relationship of firing shrinkage versus the logarithm of the cold compacting pressures in the temperature range where vitrification processes occur, while the linearity falls down outside this range. The iron oxide content lowers the vitrification temperature as well as the vitrification range. The observed linear relationship is quite similar to that exhibited by other self-vitrifying ceramic materials for wall and floor tiles previously investigated, and it does not seem to depend on the compacting characteristics of the powders. It has therefore to be related to the gradually growing glassy phases which arise during firing at increasing temperatures. This relationship may thus be considered as a general characteristic of the firing behaviour of strongly self-vitrifying ceramic materials when subjected to green density changes.     

Valorization of olive mill wastewater by its incorporation in building bricks

This investigation deals with the possibility of incorporating the effluent resulting from olive oil extraction activity, known as olive oil mill wastewater (OMW), in the brick-making process. It was undertaken at an important Tunisian brickworks company. In this study, the OMW was mixed with clays following the same brick-making procedure used at the collaborative brickworks in Tunisia. The samples containing OMW were found to be comparable in forming/extrusion performance to a control product that used fresh water. The experimental products produced were tested for their comparative physical properties (volume shrinkage, water absorption, tensile strength of bricks, after firing at 920 degrees C and paste plasticity) in the unfired and fired states against a control representing the commercial product in the same factory. The results showed a significant increase in the volume shrinkage (10%) and the water absorption (12%), while the tensile strength remained constant. The maximum plasticity index value was found when incorporating 23% of OMW. This rate either maintained the physical and mechanical properties of bricks or improved them. The incorporation of OMW in bricks can represent a promising way to valorize this effluent, to rid the environment of a highly polluting wastewater and to save huge and precious amounts of water in a country where water shortage is a serious problem. This newly-prepared material has a double positive impact: it protects the environment and allows water economy.     

Ceramic-matrix composites

Ceramic materials often exhibit a combination of useful physical and mechanical properties, including high refractoriness, but their applications are restricted due to their brittle behaviour; in an attempt to improve the strength, and particularly the toughness, of brittle ceramics particle-strengthening and fibre-reinforcement have been utilized, with limited success. The factors which affect the mechanical properties of these composite systems are discussed, and the various experimental systems that have been investigated are reviewed. It is concluded that, although the potential applications of such materials are extremely diverse, several important aspects, particularly the effect that matrix microcracking may have on the mechanical and thermal stability of reinforced-ceramic systems, must be fully evaluated before ceramic-matrix composites can seriously be considered as useful replacements for more conventional materials.