Valorization of Ferrosilicon-Derived Silica Fume as a Functional Nanostructured Extender in Magnetite (Fe₃O₄)-Based Black Pigment Composites: Thermomechanical Deagglomeration and Colorimetric Performance

Ferrosilicon-derived silica fume (FSF), an amorphous nanostructured by-product of ferroalloy smelting, accumulates in open storage at the Uzbekistan Metallurgical Combine JSC (Bekabad) at approximately 3000 t per annum without an established commercial outlet. Because magnetite (Fe₃O₄)-based black pigments are comparatively expensive and carry a substantial manufacturing footprint, this study evaluates FSF as a functional nanostructured extender enabling partial substitution of the primary pigment with an inexpensive industrial residue. Three principal findings are reported. First, the apparent laser-diffraction median of the as-received material (D50 ≈ 50 µm) proved to be an agglomeration artefact: transmission electron microscopy confirmed discrete spherical primaries of 40–150 nm, whose liberation by thermomechanical processing collapsed D10 from 8.5 to 0.4 µm and raised the specific surface area roughly fifteen-fold. Second, an optimal processing window of 25–30 min at 250 rpm and 42–46 °C delivered a mixing index of 100 % and a tinting strength of 99.9 %, beyond which frictional heating drove re-aggregation and colour degradation. Third, CIELAB colorimetry established 20 wt.% FSF as the substitution ceiling, at which the total colour difference (ΔE*ab = 0.11–0.15) remains imperceptible; paving-stone trials confirmed full colour retention at 63 days. A cradle-to-gate assessment indicates that each tonne of composite pigment diverts 0.20 t of FSF from landfill, avoids approximately 300 kg CO₂-eq and 7 GJ of primary energy, and reduces raw-material cost by around 20 %, establishing FSF as a viable low-cost extender for black-pigment manufacture.