Treatment of red mud with blast furnace slag followed by calcination: impact on the chemical reaction and leaching after association with Portland cement

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Abstract
04-049 Roberto Cesar de Oliveira Romano Romano, R.C.(University of São Paulo); Bernardo, H.M.(University of São Paulo); Maciel, M.H.(University of São Paulo); Mesquita, J.A.(University of São Paulo); Roschel, B.P.(University of São Paulo); Vieira-Coelho, A.C.(University of São Paulo); Pileggi, R.G.(University of São Paulo); Cincotto, M.A.(University of São Paulo);

The discharge of red mud (RM) by the alumina and aluminium industry from bauxite is a huge environmental problem, but economic reasons, energy savings, transport cost, management and productivity reasons favours this disposal option. Currently, the residue generation is estimated to be over 2.7 billion tons, with an annual growth rate of over 120 million tons of discharged material. Only a small fraction of this waste is being used in a few countries, the remaining not being utilized. An alternative use of the red mud is in association with Portland cement or with another kind of binder. Nevertheless, the tendency for agglomeration is increased, and the particle charge surface is changed, affecting the rheological properties. Trying to reduce the impact of RM in the properties of fresh Portland cement, two alternatives were evaluated: i). calcination and ii). use of superplasticizer. In the first case, the free-sodium content decreases considerably in the suspension, which, per se, is a good alternative to reduce leaching, but did not reduce the water content required to prepare the mixing of Portland cement and RM. On the other hand, even with the water demand reduction promoted using superplasticizer, the chemical reaction is delayed. In this work another alternative is being evaluated: the combined use of blast furnace slag (BFS) and RM, acting as an alkaline activator, in order to accelerate the slag hydration reactions. The generated product was associated with Portland cement to evaluate the chemical reaction and alkali leaching. It is confirmed that the free sodium/potassium leached is reduced, but the chemical reaction still remains. In the second part of this work the BFS-RM compound was calcinated and again mixed with Portland cement. Heat release during the hydration reaction is not changed. It is observed that the reduction of leached alkali was higher as compared with non-calcinated products.