Multistep nucleation and growth mechanism of aluminosilicate gel observed by cryo-electron microscopy

Although sodium aluminosilicate hydrate (NASH) gel governs the physicochemical properties of geopolymer concrete, its nucleation and growth mechanism remain poorly understood. Here, time-resolved cryogenic transmission electron microscopy (cryo-TEM), in addition to other characterization techniques, were applied to probe the nanostructural evolution of NASH gel from early age up to 7 days. We found that the nucleation and growth of NASH gel is not a one-step process and that intermediate phases precede the formation of the geopolymeric network. More specifically, we detected the formation of prenucleation clusters (~1–2 nm) in the first minute of geopolymerisation, which then agglomerated into partially polymerized globules (~15 nm). As geopolymerisation proceeded, these globules transformed into the more stable, highly polymerized NASH gel. These findings suggest the nucleation pathway of NASH can be more accurately depicted by non-classical nucleation theory, which paves the way for the development of more robust nanoscale models of geopolymer binders.