Ohmic heating curing of iron ore tailings mortars at subzero temperature: Strength development and microstructural evolution

Subzero temperatures severely suppress cement hydration, posing a critical barrier to utilizing iron ore tailings (IOT) in cold-region construction. This study investigates whether a two-stage ohmic heating curing (OHC) schedule can enable early-age strength formation of IOT-based mortars in a −20 °C ambient environment, and how this response depends on the IOT utilization route. Two strategies were evaluated: utilizing IOT powder (IOTP) as cement replacement and IOT sand (IOTS) as fine aggregate. The OHC schedule enabled early-age strength formation, but the response differed between the two systems. In IOTP mortars, O2 specimens were 24.14%–39.07% higher than the corresponding S3 specimens in the stated O2-versus-S3 cross-age comparison; however, high IOTP replacement still impaired later-age development because reduced clinker content and limited IOTP reactivity produced a loose matrix. In IOTS mortars, O2–100 was 22.82% higher than S3–0 in the same stated cross-age comparison, whereas later-age strength remained limited by higher porosity and initially coarser pores induced by IOTS. A binder containing 10% IOTP and 30% fly ash (FA) showed a favorable response among the tested mixtures; for the IF10–30 mixture only, O2-IF was 113.08% higher than S3-IF in a cross-age comparison between 2 d OHC at −20 °C and 3 d standard curing; XRD showed curing-related differences in peak intensities, and SEM showed that this mixture developed a denser matrix and a tighter fiber–matrix ITZ. Overall, the adopted −20 °C OHC regime enabled early-age strength formation in IOT-based mortars, whereas long-term performance was governed by tailings reactivity and pore-structure constraints.