In this case, above 12008C the ultralow-cement castable goes from being the weakest to the strongest in a very short temperature span.
This illustrates a physical property effect
due to proper formulation. In this case a correct balance of cement to silica fume results in an interesting bond phase development where mullite was able to crystallize from a liquid phase that was formed between 11008C and 13008C (24).
This crystallization of mullite led to an increase in hot strength at 14008C that would not have been predicted and is not experienced in low- or ultralow-cement castables based on alumino-silicate aggregates even when the
same cement/silica fume ratio is maintained.
All these advanced castables discussed contained silica fume.
There are specially reduced cement and cement-free systems for high-temperature and/or slag/metal contact applications that are based on high alumina and magnesium
aluminate spinel aggregates and don’t use a silica fume addition.
These specialty castables can yield some very strong elevated temperature strengths (up to
15–20 MPa at 14508C) though low temperature strengths are not remarkable.