Decomposition of a concrete floor and ignition of gaseous products due to spillage of molten aluminium

Spillage of molten aluminium onto the concrete floor of the Bone Island Aluminium Smelters in Queensland, Australia has caused several serious fires which have decomposed the floor and sometimes partially melted the pot assembly, resulting in enormous repair costs and delays in production. A study of the concrete showed that it was made with limestone aggregate. The molten aluminium at about 1050°C formed ponds about 2 m in diameter and 200 mm deep. It decomposed the limestone and produced CO₂ gas which was reduced by the molten aluminium to CO gas which ignited. This strongly exothermic reaction produced further heat and melted the reinforcing bars in the concrete. The moisture in the concrete formed steam and agitated the system so that the molten metal was more efectively in contact with the concrete and caused further decomposition of the limestone. Laboratory experiments with limestone and non-calcareous aggregates, and concretes made with them, showed that the limestone was the primary cause of the fires. Based on computer modelling, it was determined that a concrete capping of 75 mm thickness made with non-calcareous aggregate and applied on the existing floor would protect it from decomposition and ignition.