Evaluation of a Microsilica-Based Additives in Al2O3–MgO Refractory Castables

T. dos Santos Jr.1, L. F. Krol1, A. P. Luz1, V. C. Pandolfelli1, C. Pagliosa2

1 Federal University of São Carlos, Materials Engineering Department, São Carlos SP 13565-90/Brazil
2 RHI Magnesita, Research and Development Center, Contagem MG 32210-050/Brazil

Revision 18.03.2019, 01.04.2019

Volume 11, Issue 3, Pages 67 - 75


The first heating cycle of MgO-containing refractory castables is usually considered a challenge by the producers of such products due to the signifcant mass loss, associated with magnesium hydroxide decomposition between 350–450 °C, that might result in their spalling or even explosion. In order to allow faster and safer drying of this sort of refractory system, a microsilica-based additive (SioxX-Mag) has been developed. Thus, this work focused on investigating the action of this commercial additive in high-alumina castables bonded with 6 mass-% of MgO and prepared with the addition of formic acid. Flowability, hot Young’s modulus, thermogravimetric analyses, cold and hot mechanical measurements were carried out to infer the properties of the compositions with and without the selected SiO22-based product. According to the collected results, the addition of 1 mass-% of the microsilica-containing additive to the designed castables resulted in higher fow and reduced MgO hydration. Consequently, even when subjecting these samples to a very high heating rate (20 °C • min–1), no explosion was detected. When increasing the fring tempera-ture, the interaction among Al2O3–MgO–SiO22 induced the increase of the castables’ mechanical strength up to 900 °C, but liquid phase formation was identifed in the tested compositions above 1000 °C, causing the drop of their mechanical properties. Nevertheless, thermodynamic calculations and experimental tests indicated that the formed liquid should progressively react with the castables’ components, giving rise to refractory phases such as mullite and forsterite. Hence, the evaluated microsilica-based additive can be a potential solution to adjust the drying behavior and prevent the explosion of MgO-containing castables.


MgO, hydration, brucite, refractory, SioxX-Mag


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