Al2O3–CaO macroporous ceramics containing hydrocalumite-like phases

Abstract

A mechanism to explain the lower onset strengthening temperature induced by CaCO3 in alumina-based macroporous ceramics is proposed, which relies on hydrocalumite-like phase formation during processing. Close to 600 °C, such phases are decomposed to lime and mayenite (12CaO·7Al2O3), where the latter, due to its intrinsic nanoporosity and high thermal reactivity, generates bonds between the ceramic particles at ~700ºC, resulting in microstructure strengthening. Based on this premise, the authors concluded that other Ca 2+ sources could act similarly. Indeed, compositions containing Ca(OH)2 or CaO showed the same effect on the onset strengthening temperature, which reinforces the proposed mechanism. The results attained indicated that macroporous insulators could be thermally treated at lower temperatures, just to acquire enough mechanical strength for installation, finishing in-situ their firing process. Besides that, lower sintering temperatures could be used to produce macroporous ceramics that would be applied in low thermal demand environments, e.g. aluminum industries.

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.…”

“…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.…”

Fabrication of Energy-Efficient Carbonate-Based Cementitious Material Using Sodium Meta-Aluminate Activated Limestone Powder

“…The new combination of hydration product phases builds a weaker microstructure, so it is not conducive to the development of strength. 46 Hydrated Products and TG-DTG Analysis. The XRD patterns of the measured paste with various LP/SMA ratios at 3 and 28 days are exhibited in Figure 5.…”

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

“…Macroporous samples were produced using a standard composition (see Table 1) and a direct foaming method [11], which consisted of incorporating a previously prepared surfactant-based foam into an alumina suspension, as described in [12]. Alumina CL370 and CT3000SG (Almatis, Germany) were used in the appropriate ratio to obtain optimized packaging, according to Andreasen's model (q=0.37).…”