Enhancing densification of metakaolin-based geopolymers via the cold sintering process

• New densification mechanisms observed beyond geopolymerization • NaOH enabled cold sintering of metakaolin with flake exfoliation • Flake shearing and pressure-solution drove unique densification • Cold sintering yielded crack-free samples confirmed by µ-CT • Cold sintered samples showed 30% higher strength than conventional samples Clay-based materials typically require high-temperature processing (>900 °C), resulting in high energy consumption. This study explores cold sintering of metakaolin (MK) powders to achieve high mechanical strength at significantly lower processing temperatures. By applying uniaxial pressure of 400 MPa and heating to only 240 °C in the presence of 15 mol/L NaOH solution, successful densification of structurally sound and dense samples with an average density of 2.16 g/cm³ and a biaxial flexural strength of ∼35 MPa was achieved. This strength surpasses that of conventionally sintered MK (1470 °C) by ∼30%. Densification was found to be governed by synergistic mechanisms involving MK exfoliation, conformal sintering around aggregates, and the precipitation of an amorphous sodium aluminosilicate hydrate phase. Crack formation was avoided through process optimization and verified using X-ray micro-computed tomography. These findings offer new insights into the chemo-mechanical mechanisms of cold sintering and demonstrate its potential as an efficient route for producing structural materials.