Valorisation of high-alumina ferroalloy slags into high-performance geopolymer composites within a circular economy approach for sustainable construction materials

This study was conducted to address the critical environmental and economic challenges of metallurgical waste accumulation and rising CO₂ emission costs by valorising highly contaminated, high-alumina ferroalloy slags into sustainable, high-performance geopolymer composites. To achieve this, the present work investigates the synthesis, characterization, and performance evaluation of geopolymer composites based on ferrovanadium and aluminum-chromium slags with controlled variation of SiO 2 content. The experimental procedure included mechanical activation and particle size reduction of slag precursors, followed by physicochemical characterization using X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetric analysis (TG), and scanning electron microscopy (SEM). Geopolymer formulations were prepared using potassium silicate solution as an alkaline activator, with subsequent incorporation of SiO₂, graphite, carbon fibers, and silica sand. Hardened specimens were cured under ambient conditions for 28 days prior to testing. Phase analysis of high-alumina ferroalloy slags confirmed the presence of crystalline constituents including Grossite (CaAl 4 O 7 ), Hibonite (CaAl 12 O 19 ), Corundum (α-Al₂O₃), Spinel (MgAl 2 O 4 ), and Ye’elimite (C 4 A 3 S), which governed the reactivity and structural evolution of the geopolymer matrix. Mechanical testing revealed a systematic increase in compressive strength with increasing SiO 2 content, from 3.53 MPa to 7.23 MPa in case of geopolymers based on ferrovanadium slag (no. 1). A corresponding increase in thermal conductivity was observed, ranging from 1.29 W/m·K to 1.43 W/m·K. Compressive strength for geopolymer composites based on aluminum-chromium slag (no. 3) was in the range of 2.37 MPa to 12.54 MPa, meanwhile, the influence of sand additives was also determined. While the compressive strength of alkali-activated geopolymers based on aluminum-chromium slag (no. 2) was increased with the growth of binder content, namely from 2.99 MPa to 56.4 MPa. Microstructural observations indicated progressive densification and reduction of porosity with increasing silica content, consistent with the mechanical and thermal results. The obtained results demonstrate that ferroalloy slags are suitable precursors for geopolymer synthesis, while SiO 2 content serves as a critical parameter controlling the balance between mechanical strength, thermal transport properties, and microstructural compactness.