I. R. de Oliveira

Processo de hidratação e os mecanismos de atuação dos aditivos aceleradores e retardadores de pega do cimento de aluminato de cálcio

One of the main aspects for the development of refractories castables is to master the knowledge regarding calcium aluminate cement (CAC), as this binder is the most applied in these products. The objective of this work was to analyze the available information in the literature in order to explain the understanding regarding the actions of retarder and accelerator additives in the setting mechanisms of CACs. The analysis of the compiled information pointed out that the retarder additives, in a general way, act in two ways: 1) inhibiting the process of cement dissolution, by the build up of insoluble barriers around the cement particles, and 2) enhancing the formation of more soluble hydrates, which increase the time necessary for the hardening beginning. On the other hand, the accelerators can induce the formation of less soluble hydrates, diminishing the time necessary for the setting or speeding up the beginning of the hydrate nucleation. The overall analysis pointed out that the action of some retarder and accelerators additives takes place in different periods of the setting time of the CACs. This aspect allows the combination of additives confering a good workability associated to a short demolding time.

Review article: stabilization of graphite-containing aqueous suspensions

The application of graphite-containing refractories has increased enormously over the latest ten years, especially in the case of refractory castables. Graphite in such refractories contributes to the reduction of the material thermal expansion and improvement on its thermal shock resistance, fracture energy and slag resistance. However, the low wettability of graphite in water and organic liquids represents a barrier to its processing. Therefore, a surface treatment of graphite particles is required to improve its wettability and enable its dispersion in aqueous suspensions. The main structural and surface characteristics responsible for the low wettability of graphite, as well as the most common surface treatment methods applied are discussed in the present review article. Such surface treatment methods include the surface covering of graphite by hydrophilic species and the adsorption of active agents on the surface, such as surfactants and polyelectrolytes.

Agente espumante e seus efeitos nas propriedades físicas de alumina porosa

Porous ceramics have been applied in the refining process, micro filtration of liquids and emulsions and, also, as thermal insulators for high temperatures. Nevertheless, there are few studies in the literature concerning the production of ceramics using foam as the porosity precursor and its effect on the final properties of the material. Thus, the objective of this work comprises the evaluation of the different cationic and anionic foaming chemical additives, and a thickening agent on the production and stabilization of the water based foams. The evaluating criteria of these agents involved the foam volume generated, the time for the coalescence of the air bubbles and the chemical nature of the surfactants. The properties of the porous ceramic samples were carried out considering the porosity, the density and the distribution of pore size.Porous ceramics have been applied in the refining process, micro filtration of liquids and emulsions and, also, as thermal insulators for high temperatures. Nevertheless, there are few studies in the literature concerning the production of ceramics using foam as the porosity precursor and its effect on the final properties of the material. Thus, the objective of this work comprises the evaluation of the different cationic and anionic foaming chemical additives, and a thickening agent on the production and stabilization of the water based foams. The evaluating criteria of these agents involved the foam volume generated, the time for the coalescence of the air bubbles and the chemical nature of the surfactants. The properties of the porous ceramic samples were carried out considering the porosity, the density and the distribution of pore size.

Dispersão e comportamento reológico de concretos refratários ultra-baixo teor de cimento no sistema Al2O3-SiC-SiO2-C

Refractory castables in the Al2O3-SiC-SiO2-C system have been extensively used as linings for blast furnace runners, due mainly to their improved resistance to thermal shock damage and to slag and metal corrosion, respectively. In this work, ULC refractory castables in the Al2O3-SiC-SiO2-C system were prepared in order to evaluate the efficiency of different sort of additives on their dispersion and setting behavior. Although citrate ions are known to be efficient chelating agents, they were not able to properly control the working time of the castable studied, contradicting the general idea that citrate ions are necessary for controlling castable setting. On the other hand, the sodium polymethacrylate additive was found to be more efficient for the simultaneous optimization of the castable dispersion state and working time. This may be attributed to a retardation effect imparted by polymethacrylate molecules on the dissolution of ions from the surface of cement particles.

Influência de aditivos dispersantes nas propriedades de concretos refratários de Al2O3 e Al2O3-MgO

The state of dispersion of the castables matrix particles presents a direct influence in their rheological behavior, determining the techniques that can be used for their installation. Different additives have been used aiming an efficient dispersion, which constitutes an important requirement in the optimization of the particles packing and mechanical resistance of castables. However, the influence of those additives in the castables properties has not been extensively studied. In this work, the effect of 3 dispersants was investigated in the properties and drying behavior of different refractory compositions. The presence of sodium polyacrylate resulted in castables with higher permeability and larger porosity. On the other hand, in Al2O3-MgO compositions the presence of sodium hexametaphosphate provided extremely low permeability castables resulting in a slower drying, whereas the citric acid presented an intermediate behavior. Such effects in the castables properties were correlated to intrinsic characteristics of the additives.

Tendência à hidratação do pó de alumínio em concretos refratários contendo elevado teor de carbono

The benefits promoted by the addition of carbon in refractory castables resulted the increase use of these materials in steel making industry. Nevertheless, carbon-containing refractory castables faces the problem of oxidation at temperatures above 600°C. One of the alternatives to overcome this drawback is the use of antioxidants. Among them, aluminum powder has shown good results in carbon containing refractory bricks. However, its application in refractory castables is restricted by the strong tendency of aluminum powder to react with water. Some surface treatments, based on Sol-Gel process, have been employed to generate an oxide coating in order to inhibit the hydration reaction when aluminum is exposed to water. However, the efficiency of these coatings has not been examined yet in refractory castables. The present research aimed to evaluate the efficiency of SiO2 or TiO2 based coatings on inhibiting the aluminum-water reaction when added to high-carbon-containing refractory castables. These coatings were generated by metallic alkoxides or a reactive polymer, both using the Sol-Gel technique.

Heterodefloculação no sistema Al2O3 - SiC

A study of the rheological behaviour of alumina and of silicon carbide in aqueous suspensions was carried out with the purpose of evaluating processing conditions that enable simultaneous dispersion of both raw materials. The dispersing agents used in this work were polyelectrolytes, including a salt of polymethacrylic acid and polyethyleneimine. The isoelectric points (iep) of powders were determined in aqueous suspensions either in the presence or not of deflocculants. The deflocculant concentration that promotes best dispersion, as well as the influence of pH and of solids content on the stabilisation were evaluated through viscosity and zeta potential measurements. It was verified that alumina and silicon carbide can be simultaneously dispersed at pHs where high surface charge of same sign is obtained for both powders, preferably using the same type of dispersant.

Quelantes como aditivos anti-hidratação da magnésia

A magnesia confere aos concretos excelentes propriedades refratarias aliadas a uma elevada resistencia a escorias basicas. Entretanto, na presenca de agua, ela se hidrata em uma reacao expansiva formando o hidroxido de magnesio. A expansao gerada, se nao for controlada ou reduzida, danifica a estrutura do material podendo ate levar a sua completa desintegracao. Neste trabalho, aditivos conhecidos como quelantes foram adicionados a suspensao de sinter de magnesia, com e sem cimento de aluminato de calcio, a fim de evitar a reacao de hidratacao da magnesia. Sob o ponto de vista quimico, foram realizadas medidas de potencial zeta e grau de hidratacao, correlacionando a mudanca da carga superficial da magnesia gerada pela presenca dos aditivos a quantidade de hidroxido formada. Sob o ponto de vista mecânico e fisico, foram realizados o monitoramento da expansao volumetrica aparente e o registro de imagens das amostras. O melhor aditivo anti-hidratacao para as amostras com aluminato de calcio foi o acido citrico. A adicao de apenas 0,3%-p foi capaz de quase anular a quantidade de hidroxido formada e adicionalmente zerar a expansao.

Projeto da reologia de concretos refratários zero-cimento no sistema Al2O3-SiC-C

Refractory castables in the Al2O3-SiC-C system have been used mainly as linings for blast furnace runners. The association of these raw materials has enabled the development of high quality refractory castables due to improvements on their resistance to thermal shock, slag and metal corrosion. Nevertheless, the scientific knowledge regarding particle dispersion in this system is quite limited. In this work, representative suspensions of the matrix of Al2O3-SiC-C refractory castables were prepared in order to evaluate the efficiency of additives on the dispersion state of different raw materials and analyze the matrix rheological behavior as a function of pH and additive content. The polyacrylic and citric acids and a non-ionic surfactant were utilized as dispersants. The apparent viscosity and yield stress data obtained from the suspension rheological evaluation were used to elaborate matrix stability maps. Initial pH and flowability results for castables with different additive contents revealed that initial pH conditions were not coincident with the optimum dispersing region established by the stability maps. A shift of the castable pH value towards the optimum dispersing region was shown to be an effective approach to increase castable flowability.