F. A. O. Valenzuela

Alternative gelling agents for the gelcasting of ceramic foams

Abstract In recent years, a new class of ceramic foams with porosity levels up to 90% was developed as a result of the association of the gelcasting process to the aeration of a suspension containing foaming agents. The gelation of foamed suspensions results from the in situ polymerization of water-soluble monomers. Nearly spherical pores and highly dense struts characterize the structure of these foams, which results in unprecedented properties, such as high permeability, low thermal conductivity, high specific surface area and high mechanical strength. The main drawbacks of this process are the inhibition of the polymerization in the presence of oxygen and the toxicity of the monomers. This work investigates two harmless alternative gelling agents that do not require atmosphere control to set foamed suspensions. The first route consists in the crosslinking of a previously dissolved polyvinyl-alcohol with the addition of an organotitanate. The other approach assessed the use of gelatin as a setting agent. This polymer gels the suspension due to changes in the structure of polypeptidic chains induced by temperature reduction. Gelatin-based systems presented higher storage modulus (G′) than the systems with crosslinked polyvinyl-alcohol chains. This characteristic prevented the formation of cracks during drying of gelatin-based samples, which constituted a limitation of PVAl-based systems. The results point out gelatin as a promising gelling agent to produce ceramic foams without the disadvantages of monomeric systems, such as the toxicity and the necessity of atmosphere control.

Drying behavior of hydratable alumina-bonded refractory castables

Abstract The drying behavior of a hydratable alumina-bonded (HAB) refractory castable was evaluated by thermogravimetric tests and compared with an ultra-low cement composition (CAC). The key properties of monolithic refractory dry-out performance, such as fluid permeability and mechanical strength, were also determined. The results showed significant differences among the drying profiles ascribed to the distinct hydrated binding phases, which affect the castables’ physical properties to a differing extent. The consequences of these features on the explosive spalling tendencies of the compositions are discussed.

Setting behavior of ultra-low cement refractory castables in the presence of citrate and polymethacrylate salts

The effect of citrate and polymethacrylate salts on the setting behavior of ultra-low cement (ULC) refractory castables was addressed in this paper, with special attention to the interactions of such chemical additives with the cement particles in the aqueous medium. Rheological experiments, electrical conductivimetry, temperature measurements, SEM, X-ray diffractometry and mechanical tests were performed to characterize the cement-based compositions prepared in this work. In spite of its strong inhibiting effect on the precipitation of cement hydrated phases, sodium citrate was found to induce the earlier setting of ULC refractory castables, due most likely to the formation of a gelled phase between the citrate anions and the ions originated from the dissolution of cement particles. The most probable reasons for the formation of this unexpected gelled phase are discussed in this paper.

Propriedades termo-mecânicas de concretos refratários ligados por sílica coloidal

Calcium aluminate cements (CAC) are the most used hydraulic binders in refractory castables. However the presence of CaO in the castable composition reduces the refractoriness in the Al2O3-SiO2 and Al2O3-MgO systems. In this context, colloidal silica has been pointed out as an important alternative for the CAC use because promotes, among another advantages, easier drying behavior and higher reactivity to the system. Previous works of the authors have indicated that the microsilica and hydratable alumina addition to colloidal silica containing castables can optimize their performance. Nevertheless, researches in these systems at high working temperatures (above 350 oC) are scarce in the literature. In this work, the colloidal silica bonded castables behaviors at different working temperature were analyzed through the fired linear shrinkage, apparent porosity, mechanical strength and hot modulus of rupture. The obtained results were compared to the CAC containing castable compositions.

Efeito da técnica de aeração sobre a macroestrutura e a permeabilidade de espumas cerâmicas produzidas via "gelcasting"

The production of porous ceramics through the aeration of suspensions associated with the gelcasting process provides cellular porous ceramics with excellent properties and a wide range of densities. Nonetheless, the use of this material in applications such as filtering, thermal insulation, or catalyst supports requires a macrostructural tailoring in order to favor specific properties, such as permeability, thermal conductivity or specific surface area. These properties are determined by characteristics such as density, pore size distribution and connectivity between neighboring cells, which are defined during foam formation. The aim of this work was to assess the influence of the aeration process on the macrostructure of ceramic foams. A planetary mixer and a stirring vessel with a bubbling ring at the bottom surface were studied in this work. Alumina suspensions containing different foaming agent concentrations were submitted to mixing cycles in both foam generators. After gelling, samples were sintered and had their pore size distribution evaluated using image analysis and mercury porosimetry. The air permeability was also evaluated. The results showed that the aeration process may strongly influence the density and the mode pore size, having a significant effect on the materials permeability. It was observed that mercury porosimetry and image analysis lead to different pore size distributions. However, both results are useful, depending on the aim of the analysis.