Binders and Monoliths from Bauxite Residue (Red Mud)

Bauxite residue (BR), the alkaline waste produced in the Bayer process for the production of alumina is generated in vast amounts and is essentially landfilled on disposal sites. An increasing lack of storage area, several accidents as well as potential risks for health and environment have led to a rethinking in the bauxite residue management. Currently, the reuse rate in the construction sector is approximately 1 %, which is mainly due to the low reactivity of the residue considering a use as a binder material; hence, low value applications prevail. Multiple suggestions have been made to increase the reactivity of the residue, however, a cutting edge technology has not been identified yet. In most cases, other reactive solid precursors are added, which are eventually responsible for the performance.

This thesis investigates the potential of using the residue in high volumes in novel alkali-activated or hydraulic binders and monoliths in which the residue or the BR-derived material is contributing to the performance. Several processes are suggested that fit the characteristics of the residue, for instance, making use of the inherent alkalinity of the residue. In most processes, the viewpoint of the alumina industry is considered, i.e. additions and infrastructure are chosen, which are in potential reach of an alumina plant.

Starting from a low-temperature process, hydrothermally cured alkali-activated monoliths were produced that contained a high content of BR and minor additions of bauxite or silica-rich materials. In this process, phases from BR and the additives dissolved and formed eventually water-insoluble monoliths.

Several high-temperature processes were developed, in which BR is transformed into either an alkali-soluble or hydraulic precursor. Reducing conditions during a heat treatment and additional silica (and calcia) to BR promoted the formation of a liquid phase below 1200 °C, which solidified as a semi-vitrifed material upon quenching. Alkali-activation thereof led to the formation of dense inorganic polymers with satisfactory performance. In-depth investigations suggested that the reactivity of the precursors can be linked to the structure of the amorphous phase and that a higher reactivity was observed for slags that had a less polymerised amorphous phase. The occurrence of some crystalline phases was found to be beneficial as they encapsulated potentially problematic elements.

An increase in temperature to 1300 °C during the thermal treatment and the addition of alumina, calcia and sulfate to BR led to the formation of hydraulic, fast-reacting Fe-rich calciumsulfoaluminate-ferrite clinker, which obtained satisfactory properties.

Using life cycle assessment, the thermal treatment was identified as the major environmental impact of the proposed thermal processing route towards inorganic polymers. Hence, several options to mitigate the overall environmental footprint were devised. A replacement of the vitrified BR precursor with about 70 wt% dry BR, followed by alkali-activation and pressing, led to comparable mechanical properties as cast inorganic polymers consisting of 100 % treated residue as solid precursor. Alternative heat treatment technologies were suggested, for instance, the use of a sinter pot furnace, a robust process used for iron ore sintering, or a microwave-assisted furnace in which reactions take place significantly faster. In both cases, BR was transformed into semi-vitrified precursors, which were prone to dissolution in an alkaline environment.

In another approach, hybrid binders, which are considered as a compromise technology between alkali-activated and hydraulic systems, were produced incorporating BR slurry in its as-is state. The alkalinity of the liquid fraction of the BR slurry was highly beneficial, as it activated a mix consisting of mainly semi-vitrified BR slag, minor cement and additives. Satisfactory compressive strengths of about 50 MPa were reached after 28 d and the leaching of potentially problematic elements was below legislative thresholds.

Integrated, multi-stage processes leading to Fe recovery and reactive slag production were achieved using either a smelting process or a raw material feed that included boron oxide as flux.

Going beyond the production of binders, alkali-activated, highly porous inorganic polymer monoliths were prepared and used as adsorbents for pollutant from wastewater. A high uptake of methylene blue and a satisfying recyclability were achieved, demonstrating the high-added value potential of these functional materials.

The results obtained in this thesis demonstrate that BR can be transformed in high volumes into binders and monoliths, in which BR or BR-derived materials play a significant role concerning the reactivity and performance. The conceptualisation of robust valorisation strategies might be an impulse for future works and pilot scale efforts as well as a first base considering an industrial implementation. The valorisation options can be potentially transferred to other, rather underutilised and low reactive, residues or tailings with a similar chemistry and hence contribute to the achievement of some of the sustainable development goals set by the United Nations.