Calcination Effects on 3D-Printed Phosphate-Activated Geopolymer Lattices: Structure, Strength, and Stability in Acidic Media.

Phosphate-activated geopolymers (PAGPs), a class of acid-activated rather than alkali-activated binders, are promising monolithic materials, yet optimizing porosity while maintaining stability in highly acidic waters remains challenging. Here, a metakaolin-phosphoric acid geopolymer ink containing laponite and Pluronic F127 (PLU) was formulated for direct ink writing (DIW) and printed as lattice monoliths (15 × 15 × 8 mm). A calcination window (350-550 °C) was applied to remove the PLU template and to elucidate how thermal severity governs phase/bond evolution, pore features, mechanical integrity, and performance in a multicomponent synthetic acid water (pH 2.96) containing Fe, Al, Zn, Mn, and Cu. XRD/FTIR indicated progressive template removal, dehydration, and phosphate-network consolidation with increasing temperature, consistent with structural evolution reported for acid-based geopolymers. Texturally, calcination strongly increased bulk porosity (45% to 61-63%) and promoted mesopore "unblocking" (largest BJH pore diameter and pore volume at 450 °C), whereas the specific surface area (SSA) increased only modestly (4.89 to 6.53-6.87 m2 g-1), highlighting that the dominant porosity changes occurred at larger length scales not fully captured by N2 physisorption. Mechanically, a pronounced strength-porosity trade-off was observed: 350 °C increased compressive strength (13.72 ± 0.33 → 27.16 ± 0.89 MPa), while ≥450 °C produced highly porous lattices but reduced strength to ∼5-6 MPa. In acidic conditions, calcination did not uniformly reduce leaching/enhance removal for all target metals; however, it consistently reduced the release of framework/trace species (notably Al, Ni, and V), evidencing improved chemical stability under acidic exposure. Overall, calcination can be tuned either toward higher robustness (350 °C) or higher porosity with reduced leaching (≥450 °C), depending on the intended operating constraints in acidic remediation.