HYDRATION BEHAVIOUR AND ANTIOXIDISING EFFECT OF ALUMINIUM POWDER ADDED TO OXIDE- AND CARBON-BASED CASTABLES

Hydration behaviour and antioxidising effect of aluminium (Al) powder has been investigated. Bayerite Al(OH)3 product layers formed on Al in pure water at 25-45°C were porous, so the hydration rate, although very slow at 25°C, increased rapidly with increasing temperature from 25 to 45°C. On further increasing temperature from 45 to 95°C, initial hydration rate increased, but changed little over long hydration periods due to formation of denser and more continuous product layers. At 100°C, due to rapid water evaporation, hydration product layers (composed of Al(OH)3 and a small amount of boehmite AlO(OH) became detached from the Al surfaces, so offering less protection, so that the hydration rate of Al increased markedly. The presence of MgO or calcium aluminate cement (CAC) in water did not change the hydration product, but greatly accelerate the hydration rate of Al. Addition of even a small amount (e.g. 0.25wt% of Al amount) of MgO or CAC to water accelerated significantly the hydration of Al, and with increasing level of MgO or CAC, the hydration extent increased markedly. Sol-gel SiO2 coatings on Al were useful in improving the hydration resistance of Al, and did not have a negative effect on the behaviour of Al as an antioxidant.