Repurposing Bauxite Residue into Construction Materials through Hydrothermal and Carbonation Curing

Promovendus/a: Fábio Cabral de Oliveira

Promotor(en): Prof. dr. Yiannis Pontikes, De heer Tobias Hertel

Bauxite residue (BR), also known as red mud, is a major industrial waste produced during the extraction of alumina, the raw material used to make aluminum. Every year, more than 150 million tons of this highly alkaline waste are generated worldwide, and most of it is stored in large disposal areas. These storage sites pose environmental risks such as dust emissions, groundwater contamination, and accidental spills. Finding safe and sustainable ways to reuse this material is therefore an important global challenge. This PhD research explores how bauxite residue can be transformed into construction materials, aiming at its holistic valorization. Two main approaches were investigated: autoclaving and carbonation.

In the first approach, untreated BR was mixed with a sodium silicate solution, pressed into monoliths, and then subjected to hydrothermal treatment in an autoclave at temperatures between 150 and 250 °C. This process increased the reactivity of BR by dissolving mineral phases that then reacted with the alkaline activator to form a binder phase, producing dense and mechanically stable blocks. These blocks showed improved environmental performance, with a lower release of potentially hazardous elements, as well as enhanced mechanical strength comparable to benchmark products such as lime-silica bricks.

The second approach focused on the carbonation of de-alkalized BR (DABR) to produce either carbonated monoliths or a supplementary cementitious material (SCM). During carbonation, the hydrogarnet phase present in DABR reacted with CO₂ to form calcium carbonate (CaCO₃) and an amorphous gel. In monoliths, this reaction densified the microstructure, significantly improving compressive strength and reaching values of up to 60 MPa. As an SCM, carbonated DABR showed limited reactivity, and blended cement mortars containing the material performed similarly to those produced with limestone. The maximum CO₂ uptake was about 90 kg of CO₂ per ton of DABR, achieved under relatively mild conditions of 50 °C and 20% CO₂ concentration.

Together, these approaches provide effective alternatives for the valorization of bauxite residue, addressing both waste management and climate challenges. The results highlight new pathways for integrating this industrial by-product into the construction sector, reducing reliance on Portland cement and supporting circular economy principles.