Synthesis and Applications of Inorganic Materials Derived from Aluminium Smelting Waste

Red mud is the highly alkaline, toxic residue of the aluminium ore bauxite after extraction of the aluminium by the Bayer process. The storage and utilization of red mud present significant environmental problems. The possibility of producing viable inorganic polymers (geopolymers) from red mud and its precursor mineral bauxite was investigated, using sodium hydroxide and/or sodium silicate as the activator, and adjusting the composition of the mixture by the addition of fine silica or ρ-alumina. The compressive strengths of the samples were measured after curing for 21 days. Although all the samples showed drying cracking, the strengths were very encouraging, the highest strength being 58 Mpa from a red mud sample containing additional silica, and the highest strength from bauxite samples being 28 MPa; the compositions of these samples also being adjusted by the addition of fine silica. These strongest samples were prepared from red mud and bauxite that had been calcined at 500°C given by RMGP4, and 28 MPa from BS2. They were made from calcined red mud and bauxite, which therefore seem to be more reactive to alkali than the as-received materials. XRD, SEM/EDS and solid-state NMR spectroscopy were used to study the microstructure and compositions of the end products. XRD revealed that iron occurs as hematite (Fe₂O₃) in the red mud, bauxite and most of the red mud geopolymers, and is present as other crystalline minerals in the other geopolymer samples. SEM spectroscopy shows that the red mud and bauxite were relatively highly porous and non-crystalline. EDS confirms that iron is present as one of the major elements in the material as well as in the geopolymers. ²⁷Al NMR spectroscopy revealed that Al is present in more tetrahedrally coordinated sites than in octahedral. ²⁹Si NMR is greatly affected by the presence of iron, resulting in very noisy spectra and in some cases no signals were obtained. These results suggest that iron does not necessarily interfere with geopolymer formation, and thus the utilisation of red mud to produce usefully strong geopolymers on a larger scale is feasible, provided the problem of cracking can be solved.