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‐Al 2 O 3 ) via the intermediate NaAlO 2 . Formation of β‐Al 2 O 3 disrupts the reaction path of calcia with alumina, delaying crystallization of calcium hexaluminate, CaO·6Al 2 O 3 , from 1350°C to 1500°C. β‐Al 2 O 3 is also shown to reduce Young's modulus and delay sintering. The presence of NaAlO 2 and β‐Al 2 O 3 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‐Al 2 O 3 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.