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Al2O3–SiO2 Nano-Bonded Castables Containing a Boron-Based Sintering Additive

This work addresses the evaluation of the thermomechanical behaviour (up to 1000 °C) of three vibratable refractory castables, mainly comprising tabular alumina, mullite or fused-silica aggregates and containing a boron-based mineralizing agent and colloidal silica as a binder. Hot modulus of rupture, in situ hot elastic modulus and XRD measurements were carried out in order to characterize the samples’ behaviour. Additionally, a complementary analysis based on equilibrium phase diagrams of Al2O3–SiO2, Al2O3–B2O3 and Al2O3–SiO2–B2O3 systems was also performed to explain the interaction of the selected boron-based compound with the different raw materials. The attained results indicated that the selected mineralizing additive led to the generation of a liquid phase at intermediate temperatures (>400 °C), which interacts with the castables’ components, resulting in different reaction products. The Al2O3/SiO2 ratio in the formulations’ matrix fraction affected these phase transformations. Despite the presence of a liquid phase in the castable structure, the SiO2-rich compositions did not reduce their mechanical strength at 1000 °C, which is associated with the fact that silica may be incorporated by the liquid, changing its properties (i.e., increasing the liquid viscosity). The designed refractories are suitable options to be used in petrochemical and aluminium production

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