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

Abstract

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.