Papers by Author: Ana M. Segadães

Abstract: This study investigates the use of an industrial refractory brick waste in the refractory based silica and alumina ceramic body. From the characterization of the waste, a mixture containing a clay, silica, alumina and waste was processed via wet route and sintered at 1400 °C. The specimens were evaluated in dry (density) and after sintering (linear shrinkage, density and modulus of rupture).Microstructural characterization by scanning electron microscopy (secondary electron) revealed a microstructure unevenly and a presence of big porous which shows also small amounts of vitreous phase, which can be related to sintering process not fully finished. The results for the linear shrinkage (about 6 %) and modulus of rupture at three points (about 63 MPa) showed that the mixture containing the refractory brick waste and the processing conditions were satisfactory for obtaining a suitable refractory material for the manufacture of bricks for melting furnaces.

Abstract: In Brazil, the majority of construction and demolition waste materials (CDW) is sent to waste dumps or landfill sites. Having low cost applications in mind, this work has the purpose of investigating the effect of the incorporation of fired ceramic rubble reclaimed from CDW obtained directly from the building construction industry on the final properties of compressed earth blocks, which are especially interesting in low-income and marginalized communities. To this aim, clay-based mixtures containing up to 5 wt.% of ceramic rubble were prepared. Lime and cement were added as binders (6, 8, 10 and 12 wt.%). Cylindrical test pieces were produced by uniaxial compression and left to harden at ambient conditions for 7, 28 and 56 days. The hardened specimens were characterized in terms of microstructure (SEM), compressive strength, water absorption and wear resistance. The results obtained in physical and mechanical evaluation tests demonstrated that small contents of ceramic rubble from the building construction industry can easily be incorporated into compressed earth blocks without degradation of typical properties, enabling savings in cement addition.

Abstract: Alumina, with high melting point (2050°C), high hardness and mechanical strength, and excellent abrasion resistance, is one of the most common raw materials used in self-flow refractory castables (SFRC) for monolithic linings and is commercially available in various fine to coarse size classes. However, the performance of the refractory lining depends not only on the properties of its ingredients but also on its easy installation (good flowability). The aim of this work was to evaluate the relationship between the flowability index (FI) of fresh castable and the specific surface area (SSA) of its particles, which is mostly determined by the finer particles content. The results obtained showed that, by controlling the proportion between matrix and aggregate, it is possible to control the SSA of the refractory castable and find a mathematical relationship between the specific surface area and the minimum flowability index required to obtain a self-flow refractory castable. It is, thus, possible to optimize the refractory castable size composition and obtain an estimate for FI as a function of SSA. Using a minimum 45 wt.% matrix content in the castable mixture, a SSA value above 2.215 m2/g is obtained, which leads to FI ≥ 80%, the recommended value for self-flow.

Abstract: The setting reaction of magnesium phosphate cements starts with water addition to a mixture of ammonium phosphate and magnesium oxide, is very fast and takes place at room temperature (cold setting). Literature shows that controversy is still going on about the reaction mechanism, hence, about the effect on the setting time of factors such as the water/cement ratio or the magnesia specific surface area. This work is focused on the magnesia powder characteristics, which were varied by calcining MgO at temperatures ranging from 900 to 1050°C, for periods of 30 to 60 min. Cement pastes were obtained by mixing MgO and diammonium phosphate with water and setting aids. The pastes were left to set in air and were characterized in terms of mineralogical composition (XRD) and microstructure (SEM). The results obtained show how the combined effect of the magnesia characteristics can be used to control the workability and adjust the setting time.

Abstract: Brazil has one of the world’s most important Bauxite deposits, the raw material for the aluminium extraction metallurgy. This work is focused on finding a suitable application for the white dross residue (WDR), a second-generation waste material produced during the metal recovery from the slag left after the primary extraction of aluminium from the ore. A commercial lime-silica based glass frit was used, to which WDR additions were made (up to 30 wt.%), aimed at studying the devitrification process of the glasses produced. Such mixtures were melted at temperatures varying from 1100 to 1500°C and the resulting fritted glasses were heat treated at 900°C. The starting materials and the mixtures thereof were characterized before and after thermal treatment by differential thermal analysis, X-ray diffraction and fluorescence, and scanning electron microscopy. The results obtained showed that the WDR is easily incorporated into the glass matrix and causes easy devitrification after short heat treatment periods at low temperature.

Abstract: The success of a refractory castable is largely due to the quality of its properties and ease of application. Self-flow refractory castables (SFRC), with high flowability index (>130%), can be easily accommodated in a mould without the application of external energy, being ideal for the manufacture of monolithic linings. SFRC castables without cement require a matrix of very fine particles, which guarantees improved rheological behaviour and performs the role of the binder in the absence of the refractory cement. The presence of the aggregate (coarse particles) hinders the flowability index, but improves the castable mechanical strength and reduces firing shrinkage, and also contributes to the reduction of the castable costs. The control of the maximum paste thickness (MPT) allows the reduction of the coarse particles interference, minimizing the number of contact points among the grains and avoiding the formation of an aggregate skeleton that impairs the flowability of the mixture. In the present work, 100% alumina SFRCs without cement were produced with a fixed matrix of fine particles, whose particle size distribution was optimized using statistical techniques (mixtures design and triangular response surfaces). Different aggregate particle size distributions were used, with several MPT values, with the objective of evaluating which was the mean distance that maximized the flowability index, simultaneously ensuring good mechanical strength for the refractory castable. Ensuring a minimum surface area of 2.22m2/g, the mixtures reach the self-flow turning point with a minimum water content and the maximum flowability is obtained for an aggregate particle size distribution modulus of q=0.22, and consequently an optimized MPT value. SFRC with high mechanical strength (>60MPa) were obtained.

Abstract: In this work, commercial alumina fine powders were used as raw materials, namely two tabular alumina fractions (–500 mesh and –230 mesh) and a reactive alumina. Statistical modelling and the Response Surface Methodology (Statistica, Mixtures Designs and Triangular Surfaces module) were applied to three-component mixtures and used to calculate the various property-composition surfaces. To that aim, the various mixtures were prepared, cast, dried, fired and characterised. The particle size distribution modulus, q, was determined for all mixtures using the software LISA. The various response surfaces were then combined, so that the water content in the mixture could be minimised and the matrix flowability maximised. The properties of the resulting test-bricks (linear shrinkage, mechanical strength, apparent density and porosity) were also modelled and response surfaces were obtained. Combined results enabled the definition of an optimised particle size composition range, which guarantees the presence of a low water flow-bed that enables the aggregate self-flow.

Abstract: The vitrification process is an attractive route for the inertization treatment of hazardous industrial wastes. The corrosion resistance of this kind of materials is one of the most important requirements to ensure the long term retention of the toxic metals. In this work, silicate glasses with various waste concentrations were obtained using a galvanic sludge from metallurgical activities and glass forming rejects from ceramic activities. Glasses with several galvanic waste concentrations were obtained. The corrosion behaviour of the vitrified materials under various pH media was evaluated. The FTIR technique was used to investigate the glass structural modifications. Glasses containing 40 wt.% galvanic waste additions show higher resistance to corrosion media than those without waste additions.

Abstract: A self-flow refractory castable (SFRC) without cement requires a matrix of fine particles and a broad size distribution of coarse particles (aggregate). The matrix ensures the rheological behaviour and performs the binding role of the absent refractory cement. The presence of the aggregate coarse particles hinders the flowability index (FI), but improves the castable mechanical strength and reduces firing shrinkage. A new methodology of SFRC particle distribution design was developed, using response surface statistical modelling and commercial alumina powders (reactive and tabular). First, the composition of the fine matrix was optimised, seeking minimum water content and maximum IF. To this matrix, various aggregate distributions, combining six tabular alumina size fractions and with different Andreasen distribution modulus, q, between 0.18 to 0.28, were added, to identify the composition with maximum FI. The results obtained show that a minimum specific surface area (SSA) of 2.22m2/g is necessary to reach the self-flow turning point, after which the largest FI requires the maximisation of the aggregate maximum paste thickness (MPT), corresponding to a distribution with q=0.22. The optimised castable composition presents high mechanical strength (>60MPa) and low shrinkage.

Abstract: This work describes the studies carried out with various industrial wastes and natural byproducts based on the SiO2-Al2O3-CaO system, aimed at extruding ceramic products of industrial interest. Four waste materials were selected and characterised, namely, (i) Al-rich anodising sludge (A-sludge), (ii) sludge from the filtration/clarification of potable water (W-sludge), (iii) sludge generated in marble sawing processes (M-sludge), and (iv) foundry sand (F-sand). Two different formulations were prepared and bodies were shaped by extrusion, after its detailed optimisation. Screw extruded rods were then fired at several temperatures and characterised in terms of relevant functional properties (shrinkage, density and mechanical strength). Their electrical insulating properties were assessed by impedance spectroscopy. The design of interesting technological properties, such as high electrical and mechanical resistance, refractoriness, etc, was found to be easily achievable by controlling the initial batch formulation and/or the sintering schedule.