Macroporous high-temperature insulators physical properties by in situ CA6 formation: Does the calcium source matter?

Abstract

Aiming the in situ formation of CA6 (CaO•6Al2O3) at alumina-based macroporous insulators, distinct Ca 2+ sources and contents were used and their effect on some of the refractories properties were investigated. Adding CaCO3, Ca(OH)2 or CaO resulted in the decrease of the strengthening onset temperature (TS) and also of the linear shrinkage. However, a higher amount of Ca(OH)2 and CaO could not be used because of their effect on reducing the insulator total porosity. The composition prepared with 12.9wt% of CaCO3 was the most promising one, leading to an expansion of 0.81% after firing at 1600ºC for 5h, TS of 680ºC and low thermal conductivity. These results point out the potential reduction of sintering temperatures and to the possibility of in situ firing the ceramic insulator. These features enable the development of a macroporous refractory composition with a higher thermal insulating effectiveness, which can help industries to decrease their energy demand.

CA6-based macroporous refractory thermal insulators containing mineralizing agents

“…Previous research by the authors of the present paper on macroporous refractories produced by direct foaming and comprised by in situ formed calcium hexaluminate (CaAl12O19 or CA6) by the reaction of Al2O3 and CaCO3, resulted in the development of a macroporous insulator that can be thermally treated at lower temperatures [5], presents low effective thermal conductivity (keff) [6] and undergoes volumetric expansion after firing at 1600ºC for 5h [6]. However, because the two latter rely on CA6 formation, which was fully carried out after firing at 1600ºC, these 3 features could not be attained simultaneously.…”

“…Macroporous samples containing the precise ratio of Al 3+ and Ca 2+ to enable complete CA6 formation after thermal treatment were produced by the direct foaming method following the steps described in [6], using a standard composition (see Table 1).…”

“…Thus, a lower firing temperature could be applied to these compositions without decreasing CA6 formation, leading to reductions in energy cost, production time and gas emissions during the manufacturing of components made with these compositions. It is worth mentioning the lower strengthening temperature induced by adding CaCO3 (better discussed in [5] and [6]). Therefore, the mechanical strength should not be greatly affected if these compositions were thermally treated up to 1400 ºC for 5 h.…”

Enhancing semi‐insulating refractory performance with polymeric microspheres and distinct calcium aluminate cements

“…1,2 Refractory thermal insulators play a crucial role in supporting energy-intensive processes by minimizing heat losses and reducing overall energy costs. 3,4 Although fiber-based insulators are widely utilized in this context, macroporous refractory ceramics have gained significant attention due to their superior lifecycle (especially in prefired shapes), ability to withstand high temperatures, and the availability of diverse production techniques. [5][6][7] Four distinct processing strategies are commonly employed in the fabrication of porous ceramics: partial sintering, replica, direct foaming, and sacrificial templating.…”