Can a Polymeric Fiber Crack a Refractory Castable? On the Role of Microcracking on the Permeability Enhancement of Castables with Polymeric Fibers

Hydraulic bonded refractories can be prone to explosive spalling during its heating process if the rate of mass transfer towards the environment is insufficient to counterbalance the vapor generation within the pores of the castable. In that case, the complex interaction of evaporation, pressurization and thermal gradients can lead to mechanical loads that surpasses the material's strength. One of the most common approaches adopted to mitigate the likelihood of facing explosions during water removal is by using polymeric fibers as drying additives, which have the role of increasing the material's intrinsic permeability. Although their effectiveness has been widely evaluated by both empirical laboratory observations and their widespread successful use in the industry, the underlying mechanism responsible for this mass transport improvement remains unknown. Thus, to leverage the latest advances in polymer science and engineering, and to unlock more effective, cheaper, or even greener options, it is a key aspect to critically evaluate the existing hypothesis that tries to explain the effectiveness of these drying additives. Thus, the current work investigates the proposed microcracking mechanism, which describes that the thermal expansion mismatch between the refractory castable and the polymer is responsible for the creation of cracks that act as microchannels capable of improving the percolation of the moisture through the refractory. Thermomechanical calculations were carried out for systems comprising different polymers including polyethylene, polypropylene, polyethylene terephthalate and polyamide. The results revealed unexpectedly large mechanical stresses arising from the differential thermal expansion. Although the current findings do not conclusively validate the microcracking hypothesis, they highlight that this aspect should not be overlooked when selecting potential polymer candidates for drying additives. These insights provide a starting point for advancing the polymer selection strategies to optimize the refractory castable drying process.