This study addresses the environmental challenges of carbon dioxide (CO2) emissions from Portland cement by investigating the valorization of crystalline black slag (CBS) as an eco-sustainable precursor for geopolymer mortars (GPMs). A novel thermal activation process was employed, where CBS was calcined at 800°C and rapidly water-quenched to produce granulated black slag (GBS) with an amorphous structure. The research evaluates the impact of this thermal treatment and the subsequent chemical activation using sodium hydroxide (NaOH) and sodiumsilicate. Furthermore, the effect of substituting GBSwith metakaolin (MK) at various levels (20%, 50%, 80%, and 100%) was analyzed. Characterization of raw materials was performed using X-ray diffraction (XRD), XRF, FTIR, and laser granulometry. Six GPM blends were synthesized: two using 100% GBS with varying precursor/activator (P/A) ratios (1.5 and 2.0) and four incorporating MK at a constant P/A ratio of 1.5. All specimens were cured at 60°C for 24 h. Analytical techniques including thermogravimetric (TGA), differential thermal analysis (DTA), and XRD confirmed the successful structural transformation of CBS into a reactive granulated slag. Mechanical testing showed that compressive strength (CS) and flexural strength (FS) were significantly influenced by the MK content. The optimal performance was achieved with an 80% MK substitution, yielding 27.34MPa in CS and 16.52MPa in FS at 28 days. Microstructural investigations via scanning electron microscopy (SEM)-EDS and XRD revealed that this specific blend promoted a denser matrix and enhanced pore distribution. These findings demonstrate that thermally activated black slag is a viable geopolymer precursor and that its synergy with MK offers a highperformance, sustainable alternative for the construction industry and the development of low-carbon binders.
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