Binder effect on ZnAl2O4-containing high-alumina refractory castables

Phase Transformations and Properties Evolution of Alumina-Based Refractory Castables Containing ZnO and SiO2

“…AZCS composition, on the other hand, contained 6 wt.% of calcium aluminate cement (CAC, Secar 71, containing Al 2 O 3 > 68.5 wt% and CaO < 31.0 wt%, SSA = 1.1 m 2 /g, Imerys Aluminates, France), which should favor the generation of ZnAl 2 O 4, calcium aluminates and silica-containing phases in the castable's microstructure after firing. A fixed amount of zinc oxide or a mixture of zinc and magnesium oxides were selected to give rise to a total of approximately 21 wt% of spinel phases (MgAl 2 O 4 and/or ZnAl 2 O 4 ) in the evaluated compositions, which is similar to the previously reported formulations taken as reference 16 .…”

“…AZAS and AZMAS castables required higher liquid content (5.0-5.2 wt.%) during their preparation when compared with the cement (CAC)-bonded material (AZCS), which is explained by the high specific surface area (187 m 2 /g) and reactivity of the selected binder (hydratable alumina) 32 . Besides that, 8.3 up to 10% of additional water were incorporated into the mixtures containing 1wt.% of silica fume to obtain vibratable flow values close to the ones measured for the equivalent silica-free formulations 16 .…”

“…Besides that, E decay was noticed above 1200 °C mainly for AZAS and AZMAS castables [Figure 2, region (ii)] due to the formation of SiO 2 -rich liquid phase in the samples' microstructure, as indicated in Table 5. Such behavior (softening at high temperature) was not detected for the equivalent silica-free compositions 16 , which highlights the influence of this oxide in the thermomechanical performance of the refractories.…”

“…Table 6 points out the influence of silica in the dimensional stability of the refractories, comparing the PLC results obtained in this study with the ones collected for the equivalent silicafree castables 16 . As this additive leads to liquid generation at high temperature in the evaluated systems (Table 5), in general, this phase contributed to counterbalance the expansion associated with the spinel and calcium aluminates formation, reducing the overall expansion (negative values presented in Table 6) measured for the ZnO-containing refractories.…”

“…Additionally, this phase may affect the diffusion pathway between Al 2 O 3 and CA (CaO.Al 2 O 3 ) phase, hindering its reaction to form CA 2 (CaO.2Al 2 O 3 ) and, subsequently, CA 6 (CaO.6Al 2 O 3 ) grains at higher temperatures, thereby slightly reducing the permanent linear change (PLC) of the samples at 1550 °C11, 13 . However, the in situ generation of MgAl 2 O 4 and calcium aluminates in refractories is accompanied with a large volume expansion (e.g., up to 8.4 vol% for MgAl 2 O 4 formation) 10,14,15 . As pointed out by some authors 11,13,16,17 , ZA generation in small amounts in alumina-based castables gives rise to micropores, whereas functionally optimum quantities of 21 wt% create undesirable expansion and porosity, increasing vulnerability to slag infiltration. Knowing that Zn 2+ diffusivity is much higher than Al 3+ , ZA formation will take place mainly by a one-way migration of ZnO (or Zn 2+ ions) to Al 2 O 3 creating an extreme scenario of Kirkendall effect 18,19 .…”