Novel drying additives and their evaluation for self-flowing refractory castables

Abstract

The drying step of dense refractory castables containing hydraulic binders is a critical process, which usually requires using slow heating rates due to the high explosion trend of such materials during their first thermal treatment. Thus, this work investigated the performance of alternative additives to induce faster and safer drying of self-flowing high-alumina refractory castables bonded with calcium aluminate cement (CAC) or hydratable alumina (HA). The following materials were analyzed for this purpose: polymeric fibers, a permeability enhancing compound (RefPac MIPORE 20) and an organic additive (aluminum salt of 2hydroxypropanoic acid). The drying behavior and explosion resistance of the cured samples were evaluated when subjecting the prepared castables to heating rates of 2, 5 or 20°C/min and the obtained data were then correlated to the potential of the drying agents to improve the permeability and mechanical strength level of the refractories at different temperatures. The collected results attested that the selected additives were more efficient in optimizing the drying behavior of the CAC-bonded compositions, whereas the HA-containing castables performed better when the aluminum-based salt was blended with a small amount of CAC (0.5 wt.%), which changed the binders hydration reaction sequence and optimized the permeability level of the resulting microstructure. Consequently, some of the designed compositions evaluated in this work showed improved drying behavior and no explosion was observed even during the tests carried out under a high heating rate (20°C/min).

Aluminum lactate role in improving hydration and drying behavior of MgO-bonded refractory castables

“…However, as highlighted in Fig. 8, the obtained permeability results were higher than the ones commonly observed for calcium aluminate cement (CAC) or hydratable alumina (HA)-bonded refractory systems [27,37]. Hence, AL addition to the designed compositions induced the generation of a greater number of permeable paths, which is a key aspect for safe and fast drying, as the castable's mechanical strength itself may not be enough to withstand the high thermomechanical stress.…”

Crack-free caustic magnesia-bonded refractory castables

“…According to some studies [33,36,37], Mg6Al2(OH)16(OH)2.4.5H2O and Mg6Al2(OH)16(CO3).4H2O compounds present the release of their interlamellar structural water and a simultaneous dehydroxylation and decarbonation of the layered double hydroxide framework around 110-250°C and 300-400°C, respectively. The permeability results obtained after firing at 250 and 450°C were up to two orders of magnitude higher than the ones commonly collected for calcium aluminate cement (CAC)-bonded refractory systems [42,43], when using the same testing conditions. Thus, AHL addition to CM-1.0AHL castable induced the generation of a greater number of permeable paths, which is a key aspect for safe and fast drying.…”

Electrospun fibers as drying additive in cement‐bonded alumina castables

“…. However, some studies showed that these additives may also negatively affect the flowability, setting time, and mechanical strength of the castables 12,13 . In a recent publication, Barakat et al 14 .…”

“…However, some studies showed that these additives may also negatively affect the flowability, setting time, and mechanical strength of the castables. 12,13 In a recent publication, Barakat et al 14 proved the effects of PEAC on the boiling front temperature of refractory castables. Two drying stages were reported: "(1) a rapid, externally limited drying process that depends on the surface boundary conditions of the material and (2) internally limited drying, which is intrinsically determined by the pore structure of the materials."…”