Mg(OH)₂ shells drive reactive sintering to form spinel network and stabilize heavy metals in secondary aluminum dross

Secondary aluminum dross (SAD) contains soluble halide salts, reactive Al/AlN, and multi-metal impurities that jointly drive high ionic releases and heavy metal mobilization. While MgAl₂O₄-based ceramics offer a promising stabilization route, conventional MgO blending and reactive sintering often form impurity-enriched intergranular phases that can become highly connected through the microstructure, degrading strength and increasing leaching vulnerability. Here we identify a MgAl₂O₄ connectivity‑control mechanism by transforming Mg delivery from stochastic mixing to interface‑directed enrichment. Mg was introduced as MgCl₂·6H₂O and converted by pH‑controlled in situ precipitation (pH ≈ 9) into nanoscale Mg(OH)₂ shells on mechanically activated SAD particles, followed by filtration and washing for desalination before identical forming and sintering. Thermal and microstructural analyses indicate that the Mg(OH)₂-derived MgO preferentially reacts at particle contacts, accelerating interparticle spinel necking to build a spinel-rich load-bearing framework while reducing the connectivity of impurity-rich intergranular phases. Consequently, at 1400 °C the ceramics achieved 3.12 g cm⁻³ density, 6.7% open porosity, 86 MPa flexural strength, and 1.58 MPa·m¹ ᐟ² fracture toughness, with markedly lower salt and metal releases in TCLP including crushed-sample leaching (<200 mesh) (e.g., Cl: 3.4→0.01 mg L⁻¹; Zn: 1.2→0.02 mg L⁻¹). A washed mixed-powder control confirmed that desalination improves performance, but the additional gains arise from interface-directed Mg delivery and phase-connectivity regulation. These findings identify connectivity programming at the precursor/interface level as a general lever for converting complex hazardous residues into mechanically reliable and environmentally robust ceramics.