Valorization of Red Mud and Bottom Ash as Precursors for the Synthesis of Alkaline Activation in Metakaolin-Based Binder

Red mud, a by-product of the aluminum industry, and bottom ash, a residue from coal combustion, present major challenges for environmental disposal. By incorporating these waste materials as binders in alkali-activated materials, this study aims to reduce waste and potentially enhance the binder’s properties. The objectives of this research were to consider the valorization of red mud and bottom ash waste for the formation of alkali activation pastes without thermal curing and to evaluate the physical and microstructure properties. The results show that the alkali activation of bottom ash and red mud was not satisfactory to achieve minimum strength (2 MPa at 1 day). Thus, a new combination with metakaolin was used with bottom ash and red mud contents up to 70%. When 50% metakaolin was used with 50% bottom ash, the paste exhibited strengths of 49.8 and 72 MPa at 1 and 28 days, respectively, and the use of bottom ash up to 50% yielded a strength of 37.4 MPa. The pastes with 50% red mud exhibited compressive strengths of 55.4 and 78.1 MPa at 1 and 28 days, respectively. Fourier transform infrared analysis revealed the Si–O–T band, which is typical of the geopolymerization process associated with the formation of the N–A–S–H gel. Additionally, a notable increase in the heat flow and total heat of the bottom ash and red mud was observed, indicating an increase in the degree of reaction of these materials. Bottom ash and red mud can be utilized effectively as raw materials in the synthesis of geopolymers with metakaolin to produce alkali-activated materials. Using contents of up to 50% of these residues enables the production of alkali-activated matrices with properties that are equivalent to or even superior to those created using only metakaolin.