Investigating the impact of red mud substitution on physicochemical properties of carbonated cement composites

This study investigates the potential of red mud as a partial replacement for cement and sand in carbonation-cured cementitious composites, focusing on the physicochemical properties. Cement pastes and mortar specimens were prepared with 5 %, 10 %, and 15 % red mud replacement and subjected to carbonation curing for 72 h post-casting. Fresh and hardened properties were evaluated, while microstructural characterization was conducted using Thermogravimetric analysis (TGA), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy - Energy Dispersive Spectroscopy (SEM-EDS). Results indicate an initial strength enhancement in red mud-incorporated mortar specimens compared to control specimens, though a decline was observed at 28 days under normal curing. Carbonation curing increased compressive strength by up to 13.3 % in paste specimens and 20.6 % in mortar specimens with 0 % red mud, while specimens with 10 % red mud replacement showed 13.3 % strength gain compared to non-carbonated controls. Thermogravimetric analysis revealed a CO₂ uptake of up to 14.2 % by mass in 15 % red mud specimens. Carbonation also reduced drying shrinkage by up to 22 % compared to control mixes and helped maintain pore solution pH above 12.5, ensuring passivation potential for reinforcement. FTIR and XRD confirmed increased calcite and reduced portlandite content, while SEM with EDS identified a hybrid matrix of calcite and C-S-H. These findings suggest red mud improves carbonation efficiency, structural stability, and sustainability of cement composites.