Effect of replacement ratio and calcination temperature of red mud on the strength, pore structure, and electrical resistivity of alkali-activated slag mortars

Cement manufacturing is estimated to account for approximately 7–9% of global anthropogenic CO 2 emissions, while red mud (RM), a highly alkaline residue of the Bayer alumina process, is generated at a rate of 1.5–2.5 tons per ton of aluminum and is predominantly disposed of in storage impoundments with associated environmental risk; addressing these two problems jointly, through the valorization of RM in low-carbon alkali-activated binders, remains a critical objective for reducing the environmental footprint of the building sector. Although RM has been explored as a supplementary material in cementitious systems, a research gap exists regarding its combined structural and functional role in alkali-activated slag (AAS) composites, particularly concerning the necessity of energy-intensive thermal treatment and its impact on electrical properties. This study provides an integrated evaluation of the mechanical, microstructural, and electrical performance of AAS mortars in which slag is partially replaced with raw and calcined RM (600 °C and 800 °C) at 10%, 20%, and 30% by weight. The novelty of this work lies in jointly resolving the strength, pore-structure, and resistivity responses of RM-modified AAS within a single experimental framework, and in proposing RM as a potential functional precursor whose internal alkalinity may promote slag dissolution rather than acting solely as an inert filler. A 10% RM replacement increases 28-day compressive strength by 26%, reaching 42.33 MPa, consistent with Mercury Intrusion Porosimetry evidence of pronounced nano-scale pore refinement. Scanning Electron Microscopy reveals a dense matrix at low replacement levels and a more granular, agglomerated morphology at 30% RM. Electrical resistivity decreases systematically from 55.5 to 15.0 Ω m as RM content increases, suggesting potential relevance to self-sensing applications. At the 30% level, calcination provides no mechanical benefit over untreated RM, indicating that, within the screening-level comparison performed here, raw RM offers a more energy-efficient processing route than calcined RM to multifunctional AAS binders. These findings provide the literature with an integrated performance dataset and the construction industry with a practical, evidence-based dosage-selection guideline for incorporating RM into alkali-activated mortars without mandatory thermal pretreatment.