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- Designing particle sizing and packing for flowability and sintered mechanical strengthPublication . Silva, Abílio P.; Pinto, Deesy G.; Segadães, A M; Devezas, TessalenoIn this work, alumina powders in five different commercially available size ranges were used to prepare various refractory castable mixtures, defined using the statistical design of mixture experiments (STATISTICA, StatSoft Inc.) and the EMMA 3.3 software (Elkem Materials). Those mixtures were characterized for packing density, Andreasen particle size distribution modulus (q), flowability and after sintering properties, in order to investigate the relationships between these variables. The optimization of matrix and aggregate sizes and matrix-aggregate proportion, subjected to different property requirements, brought to light the relationships between q, specific surface area (SSA) and maximum paste thickness (MPT). Those relationships were investigated for three fundamental processing steps, namely, dry powders, fresh paste and consolidated dried and sintered bodies. The optimized all-alumina castable was found to require 47.5wt.% of a fine size matrix with high flowability, which provides the necessary flow bed for 52.5wt.% of coarse aggregates, resulting in a gap-sized particle size distribution, and presented a fresh paste flowability index above 130% with minimum added water (28mg/m2) and sintered modulus of rupture above 50MPa.
- Thermomechanical evaluation of self-flowing refractory castables with and without the addition of aluminate cementPublication . Pinto, Deesy G.; Silva, Abílio P.; Segadães, A M; Devezas, TessalenoMuch has been discussed about the advantages regarding the thermomechanical performance of self-flow refractory castables (SFRC), with very little or no addition of calcium aluminate cement (CAC) when compared to conventional composites with cement. Previous works demonstrated that a 100% alumina SFRC with optimized particle size distribution simultaneously results in high-flowability fresh paste and high mechanical strength. In this work, the thermomechanical behaviour of zero-cement ceramic matrix is compared to that of the equivalent composite with 1% CAC content. The results show that the differences in performance can be related to differences in hydration mechanisms, namely the formation sequence of calcium aluminate hydrates, which results in increased open porosity. With lower dried strength, the all-alumina castable presents lower porosity, higher mechanical strength and uncompromised thermal shock resistance, being a valuable option when fast drying or rough green handling can be avoided.