Open Ceramics (Dec 2023)
The in-situ spinel formation in a magnesia alumina castable and the effect of selected additives on the properties of the castable
Abstract
Four additives (a sodium polyphosphate, microsilica, titania and zirconia) were tested in a magnesia alumina castable to investigate their influence on the spinel formation at high temperatures. The impact of the additives is studied with respect to both the amount of spinel formed in the burned castable and the microstructure established by the spinalization.Dilatometric measurements conducted on the castables allowed to determine the onset temperature of the spinel formation, which was indicated by a strong increase of the linear thermal expansion. For phosphate-containing samples, the expansion associated with the spinelization occurred already ∼100 °C below the temperature at which the corresponding expansion was observed for the additive-free reference sample.Furthermore, by a heat-treatment of the castables at 1200 °C or 1500 °C, the phosphate-containing samples developed notably higher amounts of spinel than the reference sample. The phosphate addition in these samples also results in a remarkably larger increase of the sample volume during the heat treatment and comparatively lower mechanical strengths of the burned samples. The SiO2, TiO2, or ZrO2 additions influenced the physical properties of the castable in a substantially smaller degree than the addition of sodium phosphate, and they also exerted a smaller effect on the spinelization in the castable than the latter one.
