MPT Influence on the Rheological Behaviour of Self-Flow Refractory Castables

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.

Designing particle sizing and packing for flowability and sintered mechanical strength

“…In an earlier work, 11 aggregates were prepared by combining coarser size classes, so that the distribution modulus, q, varied between 0.18 and 0.28. Using the MPT concept, the best proportion between matrix and coarse aggregate for maximum FI was found to be near 50% of each, corresponding to q = 0.22.…”

“…11 When working with commercial size classes, however, the finest sizes of the aggregate overlap part of the matrix sizes, meaning that the matrix content can be accordingly reduced. Details of the optimization of the matrix/aggregate proportion can be found elsewhere.…”

“…Small variations in the composition can create significant changes in the model line slope and the properties of the particle size distributions. 11 However, the work carried out with discrete narrow size classes 6,11,12 showed that maximum particle packing translates into minimum flowability and brought to evidence the effect of the two major factors that promote flowability, namely, the lubricating effect of a high amount of water and the optimization of the PSD. Moreover, the optimization of the PSD for high flowability with minimum added water was shown to be associated with the self-flow character of the castables, which presented a "turning point", and an ideal Andreasen q value near 0.22, which corresponds to a higher maximum paste thickness (i.e.…”

Effect of particle size distribution and calcium aluminate cement on the rheological behaviour of all-alumina refractory castables

“…However, maximum particle packing translates into minimum flowability and the ideal Andreasen q values should be near 0.22, which corresponds to a reduced interference between coarse particles (i.e. higher maximum paste thickness, MPT) [3,6,[8][9][10][11].…”

“…With limited water content, the flowability role has to be played by a matrix of fine powders, frequently called glidants, that increases the distance between coarse particles and reduces their interference, thus being responsible for the paste-like behaviour of the fresh castable [6,8,9,11]. Although the presence of the aggregate (coarse particles) hinders the flowability of the fresh castable, it improves the sintered castable mechanical strength and reduces firing shrinkage, contributing also to the reduction of the castable cost [10].…”