Studies on high-alumina colloidal silica-bonded refractory castable systems

• Ano de defesa: 2024
• Autor(a) principal: Martinatti, Isabela Santos
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: alumina; colloidal silica; concretos refratários; microestrutura; microstructure; mulita; mullite; refractory castables; reologia; rheology; sílica coloidal
• Link de acesso: https://www.teses.usp.br/teses/disponiveis/18/18158/tde-25042025-092954/

Energy-intensive industrial processes contribute significantly to greenhouse gas emissions. Thus, the search for energy efficiency is a priority among organizations. In this context, refractory materials are vital, focusing on developing systems with minimized heat loss, optimized combustion, and extended equipment service life. As a result, cement-free castables, particularly colloidal silica-bonded alumina refractories, are gaining attention for their high heating rates and durability. They offer cost effectiveness, safety, and reduced carbon footprint despite some issues with lower green mechanical strength and limited studies on their characteristics, which are needed for a deeper understanding of the use of colloidal silica in refractories. Accordingly, this work studied suspensions of alumina and colloidal silica, understanding how the microstructure and physical properties of colloidal silica-bonded refractory castable matrixes based on alumina systems evolve during heating, and how their properties are affected. It evaluated the rheology behavior of suspensions of alumina and colloidal silica. Refractory systems were prepared, cast, cured, dried, and fired). In addition to evaluating rheology, microstructure, and physical properties, the study compared different grades of colloidal silica. The rheological results highlighted the strong dependency of the suspension viscosity on the solid content of colloidal silica. The silica particles contribute to the solid fraction rather than acting as part of the liquid phase. Moreover, different products affect the rheology differently, necessitating careful consideration in system development. During the early drying stages, the thermogravimetric analysis indicated an efficient and rapid drying process, with all water evaporating up to 200°C. Comparing different products, it was observed that the green mechanical properties were enhanced based on colloidal silicas with smaller average particle sizes due to the greater reactivity and faster gelling mechanism. The microstructure and physical properties results demonstrate that the use of colloidal silica benefits the early sintering processes, improving densification and strength up to 1100°C, achieving strong performance at lower temperatures compared to other binders, while higher temperatures lead to reduced performance in silica-rich compositions due to mullite formation and coalescence of small pores into larger ones. Additionally, no significant differences in densification rate and mechanical properties up to 1500°C with different products were observed. These effects suggest a smaller impact on the colloidal silica average particle size and particle size distribution post-sintering.