Rheological Behaviors of Calcium Aluminate Cement-Hydratable Alumina Bonded Corundum-Spinel Castables Suspensions with Different Dispersants

Calcium aluminate cement (CAC) and hydratable alumina (HA) are the most widely used binders for refractory castables. Nevertheless, CAC as the binder for refractory castable generates some low melting point phases when coupled with SiO2, which deteriorate the high temperature performance of refractory castables. Although hydratable alumina (HA) bonded castables overcome the drawbacks of CAC bonded ones, they demand a longer mixing time and higher water contents due to HA's high specific surface area, as well as increasing the shear stress and viscosity of HA bonded castables. Therefore, in this paper, CAC combined with HA was used as the binder for corundum- spinel castables to obtain better performance. Sodium tripolyphosphate (STPP), dispersing alumina (ADS) and polycarboxylate ether dispersant (FS60) were used as dispersants, and their effects on the rheological behaviors of CAC-HA bonded corundum-spinel castables suspensions had been investigated. According to the viscosity-shear rate and stress-shear rate curves of suspensions with different dispersants, it was suggested that STPP, ADS, FS60 were all helpful for rheological performance of suspensions. Suspensions achieved much better rheological behavior with 0.3 wt.% of STPP, 1.0wt.% of ADS and 0.1wt.% of FS60, respectively. Suspensions with ADS and FS60 presented to be pseudo plastic fluid, which was helpful for the installation process of CAC-HA bonded corundum-spinel castables. And FS60 could simultaneously impart electrostatic and steric stabilization to the given suspension. Considering the actual rheological performance of CAC-HA bonded corundum-spinel castables, FS60 was much better than ADS.

Porogenic Behavior of Water in High-Alumina Castable Structures

Direct casting is a processing method that can produce large parts of complex geometry. It uses dispersions of ceramic particles in water and a hydraulic binder for consolidation. Studies on castable structures have reported the generation of a significant fraction of pores due to the presence of water. This effect occurs because drying preserves part of the interparticle pores originally occupied by water. The fraction of pores obtained in these cases can reach levels higher than 50%, affecting the properties of the structure. However, the porogenic behavior of water in these materials was not investigated in depth. In this work, aqueous suspensions of calcined alumina and hydratable alumina (hydraulic binder) of different water contents (40–60 vol.%) were prepared in a paddler mixer. For compositions of lower (5–35 vol.%) or higher (66–80 vol.%) water amounts, casting and curing steps were assisted by external pressure and a rotating device, respectively, to produce homogeneous microstructures. The total porosity after drying was similar to the initial volumetric content of water in the compositions when side effects, such as sedimentation and air entrapment, were prevented. Below water content of 25 vol.%, particles packing flaws became the main pore generator and the hydroxylation reaction of the binder no longer occurred effectively.