Can a laboratory experiment express the size effect on the drying of refractory castables?

Abstract

Refractories play an important role in societies’ progress as they enable extensively consumed materials to be processed such as steel and cement. Because of the high demand of these supplies, refractory-lined equipment comprises massive structures that have large dimensions and weighs several tons. Conversely, laboratory samples, used to study the behavior of these materials, are orders of magnitude smaller and often found weighing less than 1 kg. Within the context of refractory castable drying, this work aims to investigate the impact of size effects on water removal dynamics during thermogravimetry tests. The analysis of mass loss and its derivative with respect to temperature indicates that increasing the heating rate applied in the small samples can lead to a better reproduction of the evaporation and ebullition stages of the drying of large ones, due to the compensation effect of the faster temperature increase on the smaller sample sizes leading to similar thermal gradients. Furthermore, the presence of polymeric fibers and their influence on the size effect are also examined, showing that the increased permeability induced by such additives reduces the intensity of the scale effect. Additionally, it was found that for cubic samples with sizes equal to or larger than 10 cm, a kink in the temperature evolution measured at the center of the samples took place. A hypothesis based on the results and on observations of recent studies is proposed to explain this phenomenon, attributing it to the rapid evaporation of accumulated liquid water at those positions. This study also highlights the importance of considering the size-dependent nature of drying dynamics in refractory castables, as conventional thermogravimetry on laboratory scale samples may overlook important phenomena. Understanding the mechanisms occurring at real structural scales during water removal is crucial for predicting their behavior upon heating and enabling faster and safer drying processes for refractories.