Effect of Sodium on Microstructures and Thermoelastic Properties of Calcium Aluminate Cement-Bonded Refractories

The effect of sodium on refractory phase formation in a model Calcium Aluminate Cement–bonded refractory was investigated from 700°C to 1500°C. Sodium reacts with α‐alumina to form sodium β‐alumina (β‐Al2O3) via the intermediate NaAlO2. Formation of β‐Al2O3 disrupts the reaction path of calcia with alumina, delaying crystallization of calcium hexaluminate, CaO·6Al2O3, from 1350°C to 1500°C. β‐Al2O3 is also shown to reduce Young's modulus and delay sintering. The presence of NaAlO2 and β‐Al2O3 result in an increase in internal friction. Increased linear expansion of up to 47% is observed when 1 wt% Na is added. The expansion is shown to scale with the amount of dopant with only 0.3 wt% Na leading to an additional 31% linear expansion. On cooling, the presence of β‐Al2O3 can be demonstrated by a peak in internal friction between 1200°C and 1000°C which could be caused by Na+ ion hopping along the spinel‐like planes.