Time-resolved reaction kinetics and printability evolution of one-part alkali-activated materials for 3D printing

Abstract

One-part alkali-activated materials (OPAAMs) are a promising low-carbon alternative for 3D printing, though the coupling between their reaction kinetics and time-dependent printability remains unclear. This study establishes a six-phase kinetic framework (Phases I-VI) directly linked to printability evolution. A key finding is that the non-linear printability was governed by Phase II (Pre-gelation), where early gel formation transiently decreases and subsequently restores fluidity and pumpability-a distinctive OPAAMs feature. The developed OPAAMs achieved a 90-min pre-printing period and a 210-min stable printing window, up to tenfold longer than conventional AAMs. Mechanistically, nano clay accelerated precursor dissolution in Phase I and gel nucleation in Phases IV/V, enhancing mechanical strength, while a retarder extended the Phase III induction period, synergistically prolonging workability. With the two additives, the 3D-printed specimens exceeded the cast reference (9.18 MPa) in flexural strength in all three directions (X, Y, Z), reaching 12.28 MPa in the Y direction. This improvement is attributed to shear and compaction during printing, which suppresses large pores and reduces porosity. The printed OPAAMs also exhibited marked anisotropy in mechanical performance, arising from pore flattening during deposition and interfaces. Nano clay effectively mitigated this anisotropy by up to 92.41%. Multiscale characterization (XRD, FTIR, SEM-EDS, TG-DTG) tracked microstructural evolution from 15 min to 28 days, elucidated the activation kinetics, and substantiated the six-phase framework by clarifying the underlying reaction mechanisms. These findings offer fundamental insights into the alkali activation mechanism and provide practical guidelines for optimizing the 3D printing process of OPAAMs.