Effect of sodium on the microstructure and properties of calcium aluminate cement bonded refractories

Additions of 0.1 - 1.0 wt% Na on Al2O3-CaO and Al2O3-CaO-SiO2 model refractory castable systems were investigated with respect to the effect on formation characteristics, and mechanical properties of the consolidated refractory. In the Al2O3-CaO system, sodium is shown to form sodium β-alumina (NaβA) via the intermediate NaAlO2. Formation of NaβA disrupts the reaction path of calcia (CaO) with alumina (Al2O3), delaying crystallisation of calcium hexaluminate (CA6) from 1350 to 1500 ◦C. The linear expansion associated with NaβA is quantified and is shown to scale with the amount of dopant added: addition of 1 wt% Na leads to up to 47% additional expansion. The preferential formation of NaβA, rather than CA6, delays sintering and reduces the elastic modulus of systems with > 0.3 wt% additional Na as a consequence of a reduction of particle cohesion. With regards to the consolidated (i.e. heat treated) model castable, the system was able to tolerate addition of 0.3 wt% Na without a significant reduction in stiffness. Addition of 0.3 wt% Na had no negative effect on flexural strength of the formulation and up to ≤ 0.5 wt% Na does not affect creep resistance of the consolidated samples. NaβA formation was shown to result in enhanced internal friction, likely caused by Na+ ion hopping through the spinel-like planes of the NaβA. During long exposures to 1500 ◦C, sodium is lost from the samples as shown by quantification of NaβA and the internal friction peak associated with this phase. In the system Al2O3-CaO-SiO2 sodium leads to formation of nepheline (Nep, C2AS) and soda-anorthite (Na-An, Na-CAS2). Up to 1100 ◦C the system can tolerate the addition of 0.5 wt% Na with regards to elastic modulus and linear thermal expansion. At higher temperatures, the formation of a viscous phase leads to a significant deterioration of the creep resistance.Open Acces