In spinel-forming castables, additive additions can promote the development of the spinel phase at elevated temperatures. Here we study the effect of small additions of titania and carbon with respect to the amount of in-situ formed spinel. The samples used for the investigation were casted from an aqueous model mass containing beside fine-grained alumina and magnesia also a certain amount of the additive. The phase compositions of the burned samples were determined in quantitative phase analyses employing measured X-ray powder diffraction data. Further investigations were carried out to scrutinize the effect of the additives on changes in the microstructure and some crucial key properties of the castables.
Compared to additive-free reference samples, the castables containing titania were found to exhibit an improved sinterability and a higher cold crushing strength after burning. In parallel, the TiO2-containing samples formed significantly higher amounts of spinel during the heat-treatment.
For the carbon-containing samples, the grain size of the carbon source proved to be an important parameter controlling the influence on the spinelization. While the addition of carbon black (particle size d90 < 1 µm) clearly facilitated the spinel formation, the influence of fine-grained graphite (particle size d90 ~ 75 µm) was less pronounced. The use of coarse-grained graphite (particle size d90 ~ 180 µm) finally even hampered the spinelization. This latter effect is attributed to the presence of large graphite flakes which presumably inhibit the chemical reaction by blocking the direct contact between alumina and magnesia grains.