Performance and mechanisms of calcium carbide slag as a low-cost alkaline activator for sustainable fly ash-red mud geopolymers

This study investigates the synergistic enhancement of red mud-fly ash geopolymers (RFG) by incorporating calcium carbide slag (CCS) as a partial substitute for conventional alkali activators. We evaluated the impact of replacing 0–100 wt% of NaOH-based activator with CCS on the compressive strength, efflorescence suppression, and microstructural evolution. Results indicate that a 40 wt% CCS substitution achieves optimal performance: 3-day and 28-day compressive strengths reach 7.68 and 26.8 MPa, respectively, representing increases of 102% and 13.5% over the control RFG. Incorporating CCS significantly reduces efflorescence by limiting free alkali migration and promoting early gel formation. Microstructural analyses indicate that CCS-derived Ca(OH) 2 facilitates the simultaneous formation of C-(A)-S-H and N-A-S-H gels, which densifies the matrix and enhances early strength. This approach enhances RFG properties while reducing costs and environmental burdens by utilizing industrial waste. The findings provide a cost-effective and low-carbon alternative alkali activator, offering a sustainable strategy for advancing geopolymer technology in construction applications.