Synergistic effects of red mud incorporation and CO₂ curing on the microstructure, carbonation efficiency, and mechanical performance of mortars

The utilization of red mud (RM), a highly alkaline by-product of alumina production, together with CO₂ curing offers a promising approach for developing sustainable cementitious materials. This study investigated the combined effects of RM incorporation and CO₂ curing on the carbonation behavior, microstructural evolution, mechanical performance, CO₂ sequestration capacity, and cost efficiency of cement mortars. Five mortar mixtures were prepared with RM replacing Ordinary Portland Cement (OPC) at levels of 0%, 10%, 20%, 30%, and 40% by mass and subjected to either conventional water curing or CO₂ curing at 65 psi for 24 h. Mechanical performance was evaluated through compressive, splitting tensile, and flexural strength tests, while microstructural characterization was conducted using SEM, EDS, XRD, and TGA. RM incorporation slightly reduced fresh density and workability. CO₂ curing significantly enhanced matrix densification through the formation of stable carbonate phases, resulting in denser microstructures characterized by the coexistence of C-S-H and C-A-S-H gels with calcite and aragonite. TGA revealed that the 20RM mixture achieved the highest CO₂ uptake of 49.50 kg/m³. CO₂ curing substantially improved compressive strength, with the 10RM and 20RM mixtures exceeding 35 MPa at 28 days. Although high RM replacement levels reduced splitting tensile strength, all RM mixtures achieved flexural strengths above 4 MPa after CO₂ curing. The 20RM mixture provided the most favorable balance between CO₂ uptake, mechanical performance, and production cost, highlighting its potential for the development of sustainable and cost-effective low-carbon cementitious materials.